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Garage Diary : Sunbeam Motorcycle resto's..

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I was about 3 and a half years old and enjoyed a sunny Coronation Day with flag lined streets.  We were based in Malta - dad was in the RAF.  


Ah, now that makes sense, but back in Blighty, boy was it miserable...

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Sorry to put a dampener on your glorious picture - but it pissed it down on Coronation Day and the maximum temp was 12 deg c.


Are you the sort to piss on my parade ?  :lol:


However, you're almost right   ..it was cool, not least because of the brisk north-easterly blowing (Tropical Storm Alice ?)  ..there was just 1.5mm of rain recorded in London ..but also 2.2 hours of Sunshine  8)  

< met office > :P

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where I'm at ?


..well the bike is an ongoing project.  It was bought as a runner (just), but having been neglected, then was  unused & forgotten in the back of a Bristol garage for several years.  Being a 60 year old bike, run on a very tight budget and by a young lad (a junior chef) who didn't even have a motorcycle license - I knew from the outset that the mechanicals would need to be stripped to be checked, to replace perished oil seals and home-bodged gaskets, replace the cam-chain, and to clean out the oil-ways of sludge.


And aside from the mechanicals ; I've completely stripped the tinware to be repainted ..ostensibly because the rear mudguard was dented and otherwise in a poor state.  Even 'silver' gaffer tape didn't hide the fact that the hole surrounds for mudguard bracket and (removed) pillion seat were dented, rust frayed, and otherwise scabby & cracked.  Without a pillion seat (as I wanted this bike) the top curve of the rear mudguard is a real in-your-face feature of the bike's cool style. And being aged monochromatic paint, I couldn't simply hammer out, weld repair and neatly paint over it. 


I then tripped arse-over-tit down the slippery slope..  


Reasoning that it would have looked odd if the rear mudguard was beautifully repainted - but petrol tank, headlamp shell, and front mudguard were left sun crazed, faded, and just a little scabby (..truly a fine Autoshite patina which I wanted to keep and use as was)....  


And so the 'get it on the road and enjoy as is'  became.. 'yet another time consuming & expensive,  yet unfinished project'. :blink2: 


The headlamp (electrics) and fork shrouds were stripped down too. And then, on this particular bike many 'bits' had been chrome plated. The side boxes under the seat contain electrics on one side and the battery on the other. The battery had clearly leaked at some time, and so the boxes were removed to be re-chromed. Having discovered the bike's early and then a later celebrity* history - the 1970's Easy Rider style ape-hanger handlebars were not the style I wanted.  I chose to restore the bike to not as original (mist green ..same as every other) but as it was when used as a prop by Mike Prior - London photographer. 8)



^ American Rock - The Nelson Brothers (son of Ricky Nelson), photographed on this bike by Mike Prior, when they were on tour in London. The Bike was also used with Robbie Coltrane, David Hasselhoft, and Martin 'Youth' Gover (who subsequently bought, and frequently used the bike around London). 


..so I found a set of original bars, which also needed re-chroming.. And so it went on. ..and on.  And then those infamous words came to the forefront  "While you are at it / have gone this far " ..I decided that I might as well repaint the frame properly., and that meant every bracket, the centre & side stands, and foot pegs, engine mounts, rear suspension, brake linkages.., et all.    :o  


Beware all ye who follow this road..!


The alloy rimmed wheels now on the bike are not as it was back then, but I like so will keep 'em.. The paintwork and chroming I had professionally done.  And I had the frame blasted back to bare metal (more on these things anon).  So I now have a pile of a thousand (expensive) bits rather than a quirky and great fun motorcycle. :help:



The tale continues..  A few years ago I'd been made redundant and started a Sunbeam m/c restoration business. It was successful enough - that even after just 18 months, when I declared that I was going to retire early by dropping out of society to live on my old boat, a chap (from the owners club and based down near Guildford) wanting to buy it..  In exchange for a very modest sum, he had a list of my suppliers & parts lists, adopted my business template, and copied what he wanted off my website ..to set him up. With a little help & technical advice, and my passing on a few customers - his business got off the ground immediately.  He's now doing very nicely for himself and appeared grateful for what I'd done for him (over & above the agreement). 


So last October., seeing as I had one or two other projects on the go (one of which was abroad) and recognising that this pile of bits was not happening - I asked him (said purveyor of the business) "what it would cost to rebuild my engine and gearbox ?"  I explained that the engine gearbox was running/engaging fine, but needed cleaning inside & out, checking, any repairs & wear, seals, etc.  but as the intended future mileage of this bike wasn't going to be very much - blueprint engineering wasn't necessary.  We discussed a price for labour, and also that my timescale was 6+ months (ie., it was to be a fill-in job).


I dropped the motor assembly down to him, along with two spare cylinder heads (to choose & use the best), plus gaskets, seals, and other parts should they be needed.   Off  I head to Slovenia for a month, to try and get ahead with another project.  Upon my return I checked what's been done with my engine/gearbox ..not really expecting much to have happened, but found the bill already closing on three times the amount discussed ...with the engine is still in bits.!  :signs053:

I call him up immediately to halt any further work and arrange to collect as is. Nothing had been put in writing - as I thought we were friends (who I'd gone out of my way to help).  I paid the bill but cordial conversation was notable in its absence.  Inside., from top of my head to the pit of my stomach shouted  Fokker Wolf ! Fokker Wolf ! Fokker Wolf !  it's happened to me again..  I must be the dumbest Son of a Gun ever !   (NB. obtuse language has been moderated for the more genteel)


I got back and put the engine under a bench., not wanting to even think about it.  I just needed to pick myself up, grasp my bruised bollocks, re-evaluate my crumbling financial position, and try to move on with said other project as best I could.  And that's where we are now.


Perhaps it also helps explain why your supporting me through this website forum is bit of a lifeline.  


Thank you guys (..& any ladies) B)

- - - - - -



So, I've again now pulled this bike out ..and will do bits as and when I can between aches and other pending projects, the garden becoming overgrown, and bills pecking away at my very finite resources. 


For those not familiar with restoration work, the first and often crucially important task is to thoroughly clean and inspect what condition the bits are in  ..even where you cannot see..



^ The inside of the crankshaft is drilled length-ways (about 7/8" bore) as an oil way to the big ends, which as it swings exerts centrifugal force on the oil within.  As a result the heavier dirt particles (in oil suspension) are thrown to the outside of the plenum and stick to it (designed as a centrifugal filter - with high mileages and infrequent oil changes - it becomes almost completely clogged).  One can check the internal state without removing the end core plug, which in turn necessitates pulling the caged front main bearing off (which tends to destroy it). It's done by feeling the inside wall with stiff u-bent wire. If clean (like this was) you can even hear the metallic sound when tapping. But if it's clogged then the sound is dull and scraping will pull out some of the softer-packed sludge.


The crank pins look to have been reground to +0.025" and the shells are like new - so are good to go. The main bearings are likewise fine, albeit with very minor wear - they are well within tolerance. 


In the past I rebuilt my S8's engine ..and blue printed every bearing and the pistons to tightest minimum tolerances. Subsequently it was an absolute bitch to get started (..like brakes were on inside the engine !).   It's fine now but, despite utmost care, she seized half a dozen times while running in. Seems like everything being to a tight tolerance doesn't suit these old engines, so I'll not make that mistake on this motor. 


Personally I like to thoroughly clean everything oily, greasy and dirty, externally - before stripping it down.  It's very much easier to work with clean parts, hands and tools. And sockets/spanners fit better if things like the sump nuts are not encased in crud.  Apparently, said professional doesn't work in the same way.  They've got to be cleaned at sometime, so yesterday I systematically removed parcels of still filthy fastenings and cleaned them.  





^ I don't want surface rust, nor particularly aged staining and dirt inside the engine, so the crankshaft and other ferrous internal engine parts have been thoroughly wire brushed. 


The geared oil pump, built into the timing gear backplate (mid-rhs of photo) had a little too much end float for my liking and so I dismantled that and skimmed (on emery cloth) a couple of thou off its closing face.


Special fastenings (mainly Whitworth thread on this bike) like the cylinder-head nuts are surprising expensive to replace and so I also redressed their spanner flats. They weren't in bad condition just minor spanner round-offs / burrs. The oil filter /sump baffle plate (lhs of photos) had been home-modified (and that included drilled holes / countersunk bolts through the bottom of the oil sump) so I'm replacing those. 



^ The cam-chain tensioner had been chewed up & seen better days and so, although still serviceable after being redressed, I'll also replace that with a better one I have.


One of the things about thoroughly cleaning and closely inspecting parts early-on in the task is that you can see what needs replacing, or professionally welded/repaired in advance.  Then you have time to order / get it done.. before being needed in final assembly. :unsure:    


For example., in a packet of fastenings I found a coil of aluminium.  It was the two or three winds of stripped thread from where a gearbox mounting stud had been over tightened. Again it's a task (fitting a thread insert) which imo ought to have been done before the cases were cleaned - so as to not risk scratching the finish while working on bits again (as has already happened on this one) ! 


Disappointing, and a waste of time, effort and money, is to finish cleaning something before the work of rectification is done...



^ what does this show me.. Firstly are the dents to the gasket face, these can be seen when the cover is on and do nothing at all to help seal the cover.  Like every other defect or damage - It ought to have been redressed before the final blast cleaning.


Then, the tappet adjustment nuts are a different size ?  And.,   I happen to know these engines well enough, but just looking at the exploded diagram in the workshop manual tells anyone - there are washers fitted here where there were none. One washer I see is the wrong size.  The flats of the nuts haven't been redressed to remove their wrong-spanner-size burrs.  And aside from it all being stained (..which means inspection for hairline cracks is impossible) - I noted this.. 



^ should either lump of steel break away.. then really I prefer them not to be inside my professionally rebuilt engine.!


Every stud had been removed from this engine, but for those in the picture below ..imo for no good reason.  The cylinder head had been stripped, blasted and reassembled - and is ready to go ?  No., sorry not nearly good enough.


NB. to remove the studs - and then have the case cleaned means that all the dusty gritty shit goes into those drilled and tapped holes.  Then it's very time consuming to then clean out.  Blast cleaning of aluminium is gentle and doesn't touch the toughened steel of studs or their threads.. it just cleans them.


Incidentally the engine's crankcase and cylinder head were blast cleaned but not the sump pan, the rocker cover, nor the gearbox and bell-housing. With their texture being different - those bits no longer visually match each other !


.. here's another minor (albeit cosmetic) detail ..


^ this is the other side of the cylinder head to that shown above.  Aside from the dents in the top gasket face is the broken fin, and the fin above that is bent, cracked & with scars.  He had cut away other broken cooling fins (from under the inlet manifold), and then paid (my money) to have weld-repair yet another broken off corner..  So why didn't he just use one of the others I'd supplied (choice of two other cyl. heads without damaged fins).?


I could go on and on, but I suspect you've had enough by now, and there are just so many faults to document.  I don't know how much detail you guys want me to go into here..


Bottom line being that is ; what has been done needs to come apart again, and I have a lot of rework to do.  That of course includes more cleaning, measuring, welding, and re-blast cleaning before I can even start to put this engine together.


Never-the less she'll be a fantastic bike when done, and following the advice of RayMK of this parish - I'm trying to win over to the mindset  "..hope that you will fall in love with the car (and equally this motorcycle)  and keep it.  Sod the bank manager


Bfg :-D


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All too familiar overtones to me in your tale, BFG.   Suspect that deep down you realised you were going to be doing all this yourself at some point!    The rocky path back to the road is never easy, is it?    I have always liked these 'Beams but realise that they are not for everybody and that probably includes me.   How they never picked any of the Harley aura up is quite amazing (and probably  certainly for the best) as is the fact that they have so far escaped being copy-catted.     


Carry on with the cathartic therapy of documenting the rebuild, stories like this are never easy to tell, live or read but only add to the mystique of a machine.

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"Huston, we have a problem.."

Today, aside from other minor jobs, I moved over to working on the castings, starting off with the crankcase.  In motorcycle engine terms - the Sunbeam is unusual because the aluminium crankcase is cast together with the finned barrels ('cylinder block' in automotive parlé), which then is fitted (an interference fit) with austenitic iron sleeves / cylinder liners (..it's a twin-cylinder btw) onto a flange.

Regarding the finned aluminium casting, there are several things I wanted to correct ..in its slightly 'used' appearance, and a couple of others regarding slight 'design faults' (more of these things later).  However, in the course of seeing what was what - I noted that where the rear cylinder liner had been replaced (the original one had rust pitting in it) and its locating dowel had been re-drilled.  Red arrow below shows the liner's original dowel position, with the dowel fitted just to the right of it.


I had in fact been advised that this cylinder liner was second hand.  It's of standard bore (to match the forward cylinder and original pistons) but had next to no wear ..so that's fine.  And said professional friend* had taken the block to the engineering shop to have them fit it ..Also fine.  But judging by the position of its dowel cutout ..it must have been from a front cylinder position..

The dowel locates the liner in such a position that the cutout notches (blue arrow) clear the opening valves..  However, I now have an extra hole.  My thought was that this hole would leak compression directly into the crankcase.  So being technical an' all - I put my mouth over it and sucked. Well clearly the hole doesn't go all the way through, so no big gush of air.. But there was a notable seepage.  Here I might add that my suck is not in the league of combustion pressure  :shock:  so I was a little surprised ! 

I suddenly imagined, with bead of sweat 'glowing' from brow ..of my putting this all together, very carefully with new gaskets, then fitting the motor/gearbox assembly very very carefully into the frame, refitting all the electrics, petrol tank and fuel system, exhaust, et all, equally carefully  ..only to discover that I had no firk'n compression on one cylinder.!  Needless perhaps to say - I would have been both utterly confused and  TOTALLY PISSED OFF  :twisted:

Thankfully (?) ..as you might have gathered by now I'm pretty anal :blink: ..about doing thing and checking them..  several times over.   Perhaps even more thankfully.. I might think of having paid someone to have finished putting it all together, then I drive down to collect it, then not getting around for another year to reassemble it very very carefully into the bike., etc, etc.  

The air leak must be coming through the very fine crack between the liner and the aluminium block. 

Now as I say ..there are cutouts for valve clearance - which I know (but otherwise is common sense !) are not covered (sealed over) by the cylinder head gasket. In fact the cylinder head's design was so advanced for its day that it applies a tapered combustion chamber to achieve a squish effect.  And the in-line spacing of the valves means that the head-gasket doesn't cover much at all of this joint (between liner and case). 


^ an old gasket positioned to show just how much of that liner's rim gap is exposed.  Applying the aforementioned (and scientifically* recognised* ) suck test to where said clearance notch is and.., there's an even greater air leak !! :o

So my question to learned Autoshiters is : What if anything might I pour in the crack to seal it ..which will withstand prolonged combustion temperatures ?  Epoxy (such as Araldite) is too thick, but will something like Loctite or perhaps a cyanoacrylate adhesive like superglue work ?  :blink2:

Please.. ask anyone who might know for sure. Real experience or advice would be most welcome.   In the meantime (tomorrow) I'll search the internet for a unicorn tusk.

Thanks,  Bfg :mellow:

p.s.  ..and yes, I had noticed the top fin is cracked in two places.. and two thread inserts are needed too.

p.p.s  "Suspect that deep down you realised you were going to be doing all this yourself at some point!"  No, I'd really thought that I would be able to just get on and reassemble the engine from where he left off.  I only took the engine to him because I had confidence in both his abilities and his attitude (care & attention) to doing a good job at a fair price. He has seriously let me down.  

Perhaps I am however as disappointed in myself, because I really thought I was a better judge of character.  I had passed work on to him and on my own website I was proposing him to others who needed restoration work done, but now I can Not recommend he or his company.  


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Looks like Loctite 2422 (medium strength) thread-lock is specified to withstand continuous 650°F (340°C) temperatures. < Loctite pdf  >.  It also has a 30 minute cure rate - which is good for my application ..as I need the stuff to seep into the crack.  However it's also described as a 'High, thixotropic paste' - so that will not flow into such a tight crack.
I would prefer 650°C temperatures, so I'll give them a call and/or otherwise keep looking.
What is the temperature inside the combustion chamber of an engine?
Quote (from here ) :  "The temperature inside the combustion chamber is always transient and depends upon various parameters like cooling system, load on the engine, combustion chamber shape and the like. But the temperature is usually around 1500 degree Celsius. Here are some temperature range of other parts in an engine (in degree Celsius) 
  1. Intake manifold : 60
  2. Intake valve :     250
  3. Spark plug :      600
  4. Exhaust valve :  650
  5. Exhaust gases : 450
  6. Piston face :     300
  7. Cylinder wall :  185
  8. Piston ring :     220
  9. Piston skirt :    190
  10. Coolant :         105
  11. Engine oil :        70 "

NB. I'd guess these are for a water cooled engine.

p.s. I've just got through to Loctite's technical boys., and they say the best product they have is their Loctite 290, which has a 20 minute cure time & is water thin but only rated to 150°C.  I normally use hazard-free Loctite 2400 which has only a five minute cure time, is water thin, but only rated to 125°C.


I've also spoken to Stewart Engineering (the original post-war Sunbeam specialists), and they can't help other than to suggest taking it out again, buy a new one and try again. And I've also spoken to one of the old boys in the club who again couldn't advise because he'd never had the problem.  

p.p.s.  an internet search brought up Permatex high temp threadlock < 27200 > which is rated at 450°F (232°C) which is better. Their additional information indicates 20 minutes cure time and then also 'must be heated to 5-600°F (260-300°C) before parts will separate' (ie. to dismantle).

I'm still looking & making enquires..  

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I didn't expect to spend my day trying to sort this out. :?

I found Loctite 600 series produces which are rated up to 200°C and that's used to secure bearings in their seat.   So my investigation* led me from thread-locking to bearing-retaining compounds, and then onto valve guides and valve seat retaining compounds.  I reckon - if it can withstand the direct blast of gases that an exhaust valve seat is exposed to - then I'm sure it'll be fine tucked into the tiny gap between cylinder liner and the block ..which is constantly dissipating heat away. 

..searching led me to this stuff called < Seal-Lock Fluid-Weld > which is rated to 3000°F (1650°C) and is used for securing things like valve seats and guides.


..problem is that I can't find a UK stockist., or a UK equivalent, so I've dropped them an email to ask where can I buy it ?

Bfg ;)

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..in the meantime., not a lot of progress.  In fact further disappointment..

I'm not at all happy with the cylinder head fins being in such poor shape, so as the head's build number (stamped on the casting's dam inbetween valve gear and the cam-chain tunnel) neither shows externally nor in fact matches the crankcase engine number - I'm considering using one of the other cylinder heads I have (without damaged fins). 

Ok., I might have to replace other parts ..like the valve guides, &/or to re-cut or at least re-lap the valve seats, and perhaps even swap camshafts ..but at least then I'd have a neat and 'good' head to fit onto the Coronation bike.  And then I'll use this reconditioned and cleaned head on one of my more shite bikes. :-D   Oddly., this particular (cleaned / previously fitted) cylinder-head looks bog standard but has no build number stamped on it.

Btw., the fins get damaged because some brain-dead mechanic* has tried to get the cylinder head off by levering under the cast aluminium cooling fins - sometimes without remembering to undo the nut inside the cam-chain tunnel !  As is evident in the cyl. head's stud pattern - that inside stud/bolt is there so that the rear cylinder's combustion chamber is securely clamped down. Behind the rear cylinder is where the cam-chain runs top to bottom - so you have to reach in, past the cam-chain, to get to the nut. 


While on the subject - can you see what is obviously wrong with this engine below ?


^ Answer : Obviously, the cylinder head does not drop down freely onto the engine block.  It is in fact held up by the tightness of the cylinder head stud/bolts in their holes.  And that's because the bottom of those holes, being through aluminium, have been dented when the cylinder head nuts (which sit in the broader gap between the block's fins) have been done up so tightly. The highly compressed aluminium distorts & had to go somewhere - so the bottom edge of the hole closes up, which in turn pinches against the stud. 

I use a 12v hand held cordless to run a drill through them again - so its really not much of a task. I just don't understand why it hadn't been done by said professional ? 


Anyway.. I was talking about the cylinder head. And the first thing to do was to inspect for obvious damage ..and to test.  I wanted to know how well the valves are seated in the old cylinder heads I had - so I'd get an idea of any additional work I might be facing.   

A crude but simple check, when the cylinder head is off, is called a drain-down test, which looks something like this...


^ what I've got here is the already restored cylinder head (left) and another sitting inverted on a Black n' Decker workmate, in the bike shed so they're not in my way.  With each tappet adjuster released (so all valves are closed) - they've been approximately leveled horizontal and their combustion chambers filled to their brim with parts-cleaner (paraffin works just as well). 

If the valves are not seating properly then the parts-cleaner (or paraffin) will leak passed and run out of the inlet &/or exhaust port...


^ here you can see the professionally checked / restored head with fluid having leaked passed the closed valves.  You can see it's leaked into the exhaust (RH) port. Likewise, it leaked as much into the other exhaust port (LH / hidden behind the stud). 

The smaller port between these serves as an inlet duct for both cylinders. And again you can see it's dribbling (leaked passed valves) fluid.  I'd used slightly dirty parts-cleaning fluid in one combustion chamber, and if you look closely you can see in that inlet port both clean (blue) and dirty (brown) fluid flowing next to each other ..That tells me the inlet valves of both cylinders are leaking.  :-(     This was all within a few minutes. Normally a drain down test is conducted overnight so that even the tiniest amount of poor valve seating can be seen.    

Bottom line is that - despite this cylinder head's valve gear having been reassembled, it all has to come out again to re-seat the valves.   Even in modestly worded terms - It is a crap workmanship.

Of the two other cylinder heads I'm checking, one had not drained down at all (after 2 hours) and the other has one exhaust valve leaking. 

Disappointing progress, but I'm glad to find these things now rather than to have trusted said engineering professional, and gone ahead with the engine and then the bike's reassembly.  :-D 



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..and a sunny morning to you all 8)

Having slept on it last night, I realised this morning - it's really unusual for all of the valves to be leaking in a drain down test. Often one or two will seep a little but I don't recall ever having them all leak within minutes.. so I checked their progress now (18hours on)...


..and the third head I was testing..


Top photo LHS ..The clean one, supposedly checked and where necessary reconditioned, is half drained down in both cylinders from all valves.

RHS., The used and still dirty one has very slightly drained down on the rear cylinder, mainly from the exhaust valve, although a little damp is felt in the inlet duct (perhaps from both inlet valves). 

The second photo is of a cylinder head that's been sitting on a shelf in a detached Yorkshire garage for 10-15 years, and then on mine for another 3 - 4.  The forward valves are seating very well but again the rear cylinder's exhaust valve is leaking. Again there's dampness in the inlet duct - I feel it's also from the rear cylinder.  

What does this tell me (or you) ?  

1.  Non of the valves are bent.!  because in that circumstance the drain-down would be almost immediate and completely. So there would be no fluid in the combustion chamber.

2.  If the valve guide(s) had been replaced and the seats not been re-cut then the same rapid drain-down would (most likely) have occurred.

3.  Likewise the valves are not sticking open ..in their valve guide or because of broken valve springs.  NB. this test does not ascertain the valve springs effective tension, only that they have some !  These engines have two springs per valve, so one may be broken, but the other is holding the valve closed.

4.   Those which have drained-down a lesser amount, quickly may be because the valve is not sitting down completely. This is very likely with the old head that's been sitting on the slightly damp shelf for years because the open valve and valve seat would most likely have surface rust (or crud) preventing it closing tight. Taking this apart and cleaning those valves, and the valve seats, might be enough to resolve the issue. It's certainly worth a try before having the seats re-cut.

5.   Otherwise, the seat or valve is pitted or chipped ..and will need to be re-cut or replaced. 

6.   If a valve or its seat is cracked but not broken away then a slow leak might occur. This needs to be visually inspected for, once the valves are removed and these areas are spotlessly clean. 

7.   Likewise.. if a valve seat is not secure in the cylinder head

8.  ..regarding the clean cyl head : Possibly the chap who did the job had checked each cylinder's compression before taking the engine apart (which is good practice).  And then when he took the valve-gear out, to have the casting blasted clean - he carefully lined all the valves up in order. But during reassembly, he inadvertently got that order reversed (front -> back,  rather than  back -> front). That would account for non of these valves seating correctly ..on a supposedly checked engine. 

9.   Those that leak just a tiny amount need inspecting and just re-lapping (with grinding paste).

10.  The other valves that are sealing well - will still need to come apart, to check their valve guides are not badly worn, and thereafter might be 'cleaned' with a light lapping with fine grinding paste. 

NB. this test does not ascertain the condition of the valve guides. That has to be done by feel or very accurate measurement. However, the amount of oil in the soot of the combustion chamber also gives an indication. If it's saturated then that's likely to have either come past the inlet valves or past the piston rings. NB. These engines have two oil scraper rings., so oil coming up past the rings isn't so often a problem.  Very wet and oily deposits only in the exhaust port (and exhaust pipe) would indicate that exhaust valve guide is worn, damaged or cracked.

So there you go.. That's what I might gleen from this very simple and inexpensive drain down test.

Hope it's useful to others..

If I've missed any point - please tell me and I'll be glad to add to the list. Sometimes I might know something (sometimes something very obvious !) but just do not think of it as I write. 

Bfg ;)

- - - - -

Edit :   NB., regarding storage of an unused vehicle or engine.  Just so you know - practically Any stopped engine with more than one cylinder will always have at least one valve and it's cylinder's bore open to the atmosphere - wherever position the crankshaft stops.  So, vehicles &/or engines stored unused for a long period of time, or in anything but 100% dehumidified conditions will experience atmospheric moisture getting into at least one cylinder.  Blocking off the exhaust and carburettor intake, and the engine breather can help slow things down as long as the temperature remains reasonably constant (to avoid condensation of the air & moisture already inside the engine). 

Pouring a tablespoon of oil into the bores and slowly turning the engine over, before storage, so as to coat the iron bores & piston rings with oil, can also help but in time even that oil will drain down. And usually the valves & their seats do not get oil coated.  Doing this (on a dry warm day) and then releasing all the tappet adjusters will close all the valves / the cylinders. You might still want close off the carb inlet & exhaust (tightly wrapped plastic bags work !) and the engine's breather - to minimise moisture on the reverse-side of the valves and inside the engine cases. You ought then to label the vehicle / the engine with what's been done - so you or someone else knows ..when they come to restart it.!  

Turning the engine off and filling all the bores to their brim with oil has been tried, but be very clear about this in your warning notice - because forcing the engine to turn over when the cylinder is full of oil will almost certainly damage it (hydraulic it).  

In any case ; because dirty engine oil contains acids - the engine oil & filters ought to be changed for clean before the vehicle or engine is laid up. 

Petrol carburettors and motorcycle tanks needs to be drained down ..because over a month or two even a tiny seep (assisted by gravity on a motorcycle) passed a fuel petcock and into the engine will damage it.  Don't ask how do I know this :-(   IMO motorcycle petrol tanks are better stored for long periods - dry (or oil swilled) and then sealed closed.  Nowadays petrol goes off in about six months anyway.

Again I hope that helps.


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A very light grind of the valves with fine paste - just a few degrees once or twice may reveal something. I don't think i would reach any conclusions without doing that. It should also show wide the contact between the valve and it's seat is.


As far as filling up that dowel hole between barrel block and liner, a maintenance machinist I used to work with swore by some of the specialist filled epoxies from Devcon. Fairly easy to buy here from industrial supplies, RS components etc. I also wondered about an aluminium solder. The whole block would need oven heating to solder melt temp around 250 derees C.




I understand your frustration with so-called restorers. Too many of them are not engineers enough to care enough about engine and gearbox internals. As long as it goes and is shiny their customers will be happy. There is an expectation that old bikes will leak and break down and an assumption that they were like that from new and the "restorer" can hide behind that.


I once had some very expensive engine restoration done on a Vincent engine and I was very lucky to find someone to do it who was an engineer first and a Vincent specialist second. Everything he did was perfect but it required a lot of time and money. He told me that much of his work was correcting what others had done poorly, not just what had worn out but what had been bodged when the bikes were almost worthless. He showed me an engine which had been "repaired" by hammering masonry nails between a loose bearing and it's supporting aluminium casting!

The trouble with doing any kind of 5 star restoration is that every project car, house, bike, boat, whatever has passed through the

"run-it-into-the-ground-but-spend-nothing-on-it" stage of abuse and bodgery. The Autoshite stage if you like. 


Really enjoying this thread, thanks for sharing your project!

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Thanks Asimo, I'll check out Devcon and see what they have.  Agree with what you've otherwise said about restoration and engineers.  Said engineer who worked on this motor started off with Hawker Siddeley and went on from there. To speak to, he seems to know his stuff, but from what I've seen here is that he lacks the care and attention necessary for quality restoration work.


In the meantime.. I've removed each of the valves (in the cleaned head) and reversed their order. I'm now repeating the drain down test .. Fifteen minutes later I see weeping from only the rear exhaust valve. This is much better than the previous results (after five minutes) so can conclude - he got the reassembly order back to front. 

I note that there were no assembly markings on the valves. After cleaning I like to lightly scratch or use a felt pen to mark on both sides of the valve's head.  I also note that none of these valves or their seats had a shiny ring of being even lightly re-lapped in after the blast cleaning.


note the cut-away fin is not standard. It had again been broken off by someone levering the cylinder head.  But, as this is below the carburettor, behind an air filter and between the two exhaust downpipes, I was sort of OK with using it.  However, without the other more obvious broken fins being (neatly) repaired it's just too shabby for a 'restored' bike.  It'll be fine to re-use this head on one of my autoshiters :)

^^ After 30 minutes the rear cylinder's inlet valve is seeping.  The front cylinder valves are at this time still fluid tight.  This is generally good news, insomuch as its disappointing that an expert with these bikes should make such an elementary mistake, but pleasing to think that just a little lapping / lightly regrinding should sort this head's valve seating out..  :-)

Update after 6 hours : Same as after 30 minutes. I was working in there early today (using the parts cleaner) and with floorboards rocking - a little fluid spilled onto the top (gasket face). That now seems to have run away or evaporated. But there's no noticeable dribbling out of the ports, and the fluid level doesn't seem to be going down any further.. . .


Regarding other matters : I'm disappointed that he should have re-fitted pitted valve caps, which would make setting the tappet clearance accurately - impossible.  It would have taken just a couple of minutes to regrind their top surface. 

The cam lift face of one rocker-arm is too badly damaged to re-use. 

And I'm not pleased that he should use star-washers to secure the rocker shaft.

Before dismantling, I thought he had used plain washers (where non were originally used) but they turned out to be shiny new star-washers, which are not intended to take such tightening nor the constant hammering of valve gear lifting against camshaft and spring.  In a short time they would flatten and then the rocker shaft assembly would be loose & twisting.

End float in the camshaft is (imo) more than is acceptable. Again easily sorted - but not done.

And I'm not pleased that I had to resort to using a hammer and wedges under the rocker assembly to lift it off three 1/4" studs, when again those holes would be freed up in seconds, simply by running a drill through them.  How can anyone know an assembly is down tight if its bolts are binding that much in their holes ?  And they'd be no point at all in trying to torque things down evenly in such circumstance. 

No, I'm sorry but these and hundred other details I've not reported here all lead me to conclude this chap doesn't really care &/or doesn't bother to think.  A mistake or two might be overlooked, but not this many, nor of this sort. 

Bfg :(

p.s. My apologies..  So far this thread seems to have turned into little more than an ongoing moan. Tbh., I thought this thread would be a fast moving reassembly, because pretty much everything was ready to go.  I never expected what's been found..  

Hereafter.,  I'll just get on with the job and put aside that I've overpaid and feel cheated.  It's springtime, which is a time to look forward, not to what has now passed.  So its time for a more positive stance.

Hopefully there'll be something useful to learn as we move on   B)      

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Moving on ..and using the other cylinder head - whose fins and gasket faces appear to be really good.


^ after the first clean in parts wash. :) The in-line overhead cam & valve arrangement can clearly be seen, with its central rocker shaft and the valves down one side (not a cross-flow design). The sprocket seen on the LHS is attached to the end of the camshaft (hidden from view under a sheet steel splash plate). The sprocket is of course for the cam-chain.


^ if you look closely you'll see the valves had previously been finely punch marked with one to four dots.

The combustion chambers are distinctive in their wedge / sloped shape. This is an early example of the squish principle whereby as the piston reaches the top of the stroke - the air-fuel mixture is further squished into a more confined space - where the long-reach spark-plug is centrally situated. With less distance to cover the explosion is more thorough (= more powerful with less emissions). Likewise on the exhaust stroke the spent air is forced towards the valve. Clever but simple isn't it ! 

Later developments of this use many more curved surfaces, computer-aided designed to swirl (mix) and more smoothly flow the air-fuel mix in, and then the exhaust gases out.  Of course most later engines are of cross-flow valve configuration (either side of the combustion chamber) with two or more valves.  Especially if the valves are set into angled faces ; a cross-flow layout allows for greater valve diameters than the in-line valve configuration (which extend beyond the bore diameter) these Sunbeams had..  Needless to say it was very advanced for it's day but things have moved on., albeit rather slowly  ;)


^ clearly showing the camshaft laying in its trough, with its sprocket in the cam-chain chimney - which also serves as an oil-return gallery, and to allow movement of crankcase air when the pistons descend together (1/2 litre). 

Drilled internally - very unusual for a motorcycle (of any era !).. the oil feed to the cylinder head is pumped up to where the casting is wide (just forward of the sprocket). This is where the camshaft's rear journal is.  A controlled amount of oil goes through a drilling in that journal, and then internally along the length of the camshaft to lubricate the front journal. More oil rise up the hollow / stubby post you see to the LHS. of the photo. 

The rocker-shaft's front mounting clamp sits over this post ..and is likewise drilled to take the oil to the hollow rocker-shaft.  From there, the oil still under pressure goes to each rocker arm which has a small drill hole in its underside. Whence the oil squirts directly onto its cam lobe.  Excess oil is flung off as the camshaft spins - creating an oil mist under a steel plate cover. The camshaft operates in this oil mist. 

In the aluminium casting - under the first (rear) journal is a return hole for the oil heading back to the sump. On it's way down it splashes over the cam-chain. Then the oil splashes / mist is thrown upwards (with the surge of crankcase air)  all over the rocker arms and valve springs. All this happens within the closed (cast aluminium) rocker cover.  Spent oil, which has absorbed cylinder-head heat (dissipated through the casting) then returns, under gravity via the camshaft trough, down the chimney (collecting more heat, from the rear cylinder's block) to the wet sump which houses the filter. 


^ is where we leave it tonight.  Gasket faces and parts superficially cleaned for further work and inspection.  However there's no point in my spending too much time or trouble cleaning the casting ..as I plan to have this cylinder head bead-blasted.

Have a good weekend,

Bfg ;)

p.s. clocks change tonight at 1.30am .. welcome back BST   8) 

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Nothing new for today, as I was painting the underside of the Yugoslavian Shitreon's inner front wings. Stinky stuff that epoxy paint. :wacko:

However..,  I thought y'all might like to see a engineering drawing of the motor we're discussing..

NB. the drawing is actually of the early (1948) S7 engine and there's a thousand detail differences but hey., the design layout is pretty much  identical. :rolleyes:


Four stroke, in-line, twin cylinder, air cooled Sunbeam s7 motor 

..from top to bottom :

/\   Cast aluminium rocker cover, retained by just three studs (originally sitting on a cork gasket). Crankcase breather disk-valves fitted into the cover's front face. The motor's head-steady bracket is seen to rear. Between this and the frame tube are rubber snubbers & a friction damper.

\/    Rocker assembly and chain driven (@ 1/2 speed) overhead camshaft. A twin-lead distributor with centrifugal auto-advance is driven off rear of camshaft.

\/    Cast aluminium cylinder head with air-cooling fins and inclined in-line valves (2 valves / cyl.) Squish-design to combustion chamber. To the LH side of the motor is an oil pressure sensor. Spark plugs with HT leads are neatly protected under an alloy cover.  To the RH side of the motor are the inlet & exhaust ports. The two exhaust manifolds (not show here) were in cast iron (chrome plated) for early engines, but this was changed to being neatly cast in aluminium, which do not suffer the blueing of chrome tubes.

\/   11-off cylinder head studs. The cast aluminium block with front engine mount (uses a Metalastik type rubber) positioned high - so the engine is suspended between this and those under the back-end of the gearbox. The diagonal-line geometry of engine mounts minimises torque reaction (noticeable on other in-line and flat-twin engined motorcycles.. and some powerful cars).

\/    Air cooling fins around the cylinder / barrels. Lightweight alloy pistons with floating gudgeon pins. NB. this early drawing shows three piston rings but on all later engines there was an additional oil scraper ring in their piston skirt.  Early pistons were flat topped @ 6:1 compression (to avoid pre-ignition / pinking of the poor grade post-war fuel). Subsequently stepped pistons were introduced for export and then home market at 6.5:1 and 7.2:1 compression. The piston's step shape corresponded with the slope in the combustion chamber.

To the rear of the motor is the cam-chain. The 'chimney' this runs in is washed by oil returning from the camshaft & valve gear. That 'oil wash' helps dissipate heat from the rear cylinder. Also evident (in the frontal end view) is the outline of the engine-case casting which opens up to the size of the clutch bell housing. Again this is to aid rear cylinder cooling ..with the castings designed to conduct and dissipate that heat. 

Together the chimney and large rocker-cover almost double the volume of air within the engine - and this helps absorb the pulsation of air pressure caused by reciprocating pistons. Along with having so few case parts (lesser number of gaskets or seals joins) - which in turn helps minimise oil leaks.  Indeed many of those experienced by owners today are due to damaged gasket faces &/or the wrong sort of gasket material.

\/    Con rods are forged light alloy. The engine is short stroke, both to keep the crank throw compact (which also effects the height of this OHC motor) and to minimise piston velocity (..therefore wear) and vibration. These short con-rods are very stiff. Big end bearings are shell (at a time when split crankshaft & white metal bearings was usual). The size of these bearings is the same diameter & thickness, but 1/8" wider than the 1970's 1275cc Mini.  At the rear of the engine (in with the cam-chain) is the half time gear & pinion.

\/    The forged steel crank is one piece single throw, drilled internally to 7/8" diameter as oil way to the big end shells and as a centrifugal oil sludge trap.  The cranks with their huge counterweights were balanced on a machine. Balance ratio is 50%.  The front main bearing is a large ball bearing (later changed to roller) with deep groove to take the end thrust of clutch operations.  Forward of this is an oil seal and then the dynamo, mounted directly onto the front of the crankshaft (so having no bearings of its own).  

The rear main bearing is large diameter white metal (for quietness).  The crankshaft drilling to the big end's oil gallery can be seen illustrated. The oil is pumped into the white metal bearing and a drilling into the crank's journal transfers the oil forward.  The rear main bearing is interference fitted into a cast-steel rear engine plate. It is removable for purpose of fitting the crankshaft into its case. That bearing-carrier (drilled to transfer oil to necessary parts) is also the body of the geared oil pump (seen under the rear main bearing) and a pressure relief valve, as well as securing a steel post for the half-time timing gear.  

The rear of the engine is closed off with a pressed steel plate with rear crank seal.  Thereafter is then the flywheel (an earlier type is show here. Later bikes used a simpler design) and 6-spring dry-plate clutch.  The operation of the clutch is via a pushrod which extends through the gearbox. The gearbox main shaft is in line with the engine's crankshaft. The gearbox lay shaft sits to the right of the main shaft gears.     Those familiar with older British motorcycles will undoubtedly be more used to seeing a transversely mounted crankshaft, and then a primary drive chain (within a usually leaky case) which goes back to a transversely aligned gearbox. This in turn has a sprocket for the rear drive chain.  On the post-war Sunbeam the engine's crank, the clutch and gearbox at all in line (S-type : may refer to in 'series'). There is no primary nor final drive chain to adjust. Its final drive is via a shaft-drive offset to the right besides the rear wheel. 

On the LHS of the motor is the crankcase oil filler with dipstick.  NB. This drawing shows a mesh basket around the dip-stick, which was not used on later engines. 

\/    At the bottom of the motor is a cast aluminium wet-sump pan, housing a gauze (fixed/washable) filter. The sump pan has to be dropped to clean it.  Later engines have a much deeper and squarer sump pan with a different design of filter. Again changing in detail rather more than design layout.  NB. the deeper sump pan lost a little in ground clearance but gave a greater oil capacity. It also lowered the oil level in the case - so uses a longer dipstick.

Hope that's been of interest.

Bfg.    ;)


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Yesterday, to borrow a retired tennis professional's idiom..  Pain Stopped Play.!

On Monday I'd used the big grinder / polisher (it has a slow speed control) to trim the new fibreglass flange on the Ami's inner wing ..necessitating I hold the lightweight panel with one hand and the grinder in the other.  ..Just a few minutes work but the twisting weight of this big boy grinder ripped the tendons in my elbow again.  I had hoped it was getting stronger but clearly it'll take a little longer yet.   :?

explained in non-medical jargon.. Tennis elbow is the tearing of the filaments that hold a forearm muscle to the outside of the elbow bone. Of course once it starts to rip apart ..then it's easy to tear a whole lot more.  So stopping doing that sort of work is not so much to avoid pain - as to avoid more extensive damage.  

My own tennis elbow reoccurs every so often, having been weakened many years ago when long-boarding a boat's hull.  I can work through the aches & pains - but then I know it'll be weak for months rather than weeks ..And that's not good as I have a lot of project work scheduled for this year.  

An icy cold-press is often recommended, as this helps reduce inflammation, ..but imo that's only of use when first injured. Conversely, keeping it warm improves blood circulation - which aids / speeds up the repair / healing process.  Turmeric, St. John's wort, and ginger are all said to lessen inflammation, so I'll combine eating ginger biscuits with keeping it warm (wrapping the elbow in a woolen v-neck sweater) and resting it (..not use single-handed power tools !).  I also use an elasticated tubular bandage to support and otherwise pad it from knocks.  NB. my arm's too big for usual arm bandages (too tight restricts blood circulation), so I use one for a knee. And then, a little hand stretching / clenching exercise also helps blood movement & generally stops things seizing up. But then I'm an engineer and not a doctor !  :P

- - - - -

Anyway's up.., I had wanted to get on with the cylinder head.  Unfortunately I needed to use the cordless drill with burr, and again its side-pressure aggravated the condition. 

As a start, this was the job list ..

1.  visually check the cylinder-head casting for damage, cracks, or manufacturing fault.

2.  visually check the valve guides and valve seats for damage or cracks.  

NB. I know  from the (not oily) soot within the combustion chamber & exhaust ports that the valve guides are not too badly worn.

3.  check the line-bored camshaft journals (in the casting) for damage.

4.  check the oil feed route to the camshaft journals.

I've seen one cylinder head where this drilling was in the wrong place (from new) and its oil flow was restricted.  And then, because of the sharp turns and hard edges along the route - it's also not un-common for gasket-goo, bits of paper-wipes, or even metal burrs to partially block this oil-way (either at the front or rear journal).

5.  check any studs and threads for looseness, and correct as necessary. 

On these engines, the rear cylinder's centre stud (very..) often has partially stripped threads, which is not evident until tightened down. Then the thread pulls out of the aluminium.  This once happened to me after I'd rebuilt an engine and was running it in.  I then re-torqued the cylinder head bolts down again ..when the damn thing stripped out.!  The cylinder head had to come off again to fit a thread insert. Thankfully that was just about possible with the engine still in the bike's frame. Now I use a spacer tube and torque the stud down against that ..to check that it'll take the load before rebuilding the top end.     

..then to

6.  clean up 'used' appearance & surface blemishes (..some of which are original but unsightly).

7.  redress the sides of the cam-chain chimney

..which is usually damaged by the cam-chain having been too loose.

8.  cut away the casting flash, inside the rear access hole..

9.  'flow' the inlet port a bit

10.  flow the combustion chamber a little


11.  check for flatness & damage of all gasket faces, and to redress as necessary. 

NB. sometimes the gasket face can be in good shape but the aluminium immediately around the stud has pulled up a little.

12.  Re-lap the valves.

..Before I take the cylinder head for bead blasting.


I started off by cleaning up the casting of its 'used appearance' and a few sand-casting shortcomings, but generally its in really good shape (not least considering its geriatric age !).  This particular cylinder head, judging by its build number, is probably from 1950 or '51 and the cast finish is noticeably better than that of later ones from '54 to '56 (when production ended).


^ aside from the electric drill & plastic handles this photo might have been taken in the 1950's ? 8)

...and some folk worry about blast cleaning grit getting inside the engine !


Otherwise here are photos of work in progress...

7.  to redress the sides of the cam-chain chimney, which is usually damaged by the cam-chain having been too loose.



^  despite all that reshaping to be a smooth corner (right) the upstanding burr is still there ..so I've only taken the face back to where it was already worn.  After taking the photo I filed the burr off ;)

I did the same sort of work last week ..on the engine block..


8.  cut away the casting flash inside the rear access slot, in the back face of the cylinder head


  This slot goes into the cam-chain tunnel, and is used to access the inside rear-cylinder-head nuts (for tightening them down).  It's a small enough (!) ..and with the engine in the bike it has restricted access. Reaching passed the cam-chain makes getting to the nuts rather awkward.  The larger blue arrow indicates where the all-important rear cylinder head stud comes through. Onto this a thick washer and large nut has to be fitted and then tightened down. The small blue arrow is where a smaller stud with washer and nut go.. right in the firk'n way.  :roll:

This particular cylinder head has had rough casting flash all around the inside of this slot (red arrows) which when removed makes future life just a little easier.

- - - - -

As an engineer I'm wary of hard / sharp internal corners being the instigator of stress cracks. It's rather like the sides of cellophane packaging being incredibly tough to rip open (until it has just the tiniest nick/cut in it, then it rips open as easy as anything).  Most consumer cellophane packets have their top & bottom edge cut with serrated edge - so that you can open the packet easily. The sharp inside corners of the serration focuses the force of your pulling or tearing (the stresses) right into a sharp corner, which then cracks/tears open. 

Metal engine parts are similar when stressed.. a tear / crack usually starts from a hard /sharp inside edge. This 'fault line' can be there because it was machined in, or in some cases due to the sand casting or a forging's flash that has been coarsely ground away.  There are many places in an old engine where that might happen ..and so I generally run around it with the rotary burr on the cordless drill. 

Below are just two examples where I'm addressing the potential problem, before it cracks in use.

The first is particular to these Sunbeam engines and is renown to be problematic. In the engineering drawing previously posted you'll see I make mention of the front engine mount. This is at the top of the cylinder block and is a forward extension of the casting. The whole weight of the engine is hanging on it.  You'll also see that the underside of this has a machined-in 'hard' corner. That is a design fault which seems never to have been corrected in production. 

..Instead the manufacturer specified a steel support plate to go under it (..interestingly not shown on the 1948 drawing).  You can see one on the previously posted photo of the cylinder head I'm now using on this bike (drain down test posted on 25th March) which has it hanging on the front cylinder head nut. It's a very important albeit little plate with two holes in it. That steel helps carry the stresses from the engine mount to the front cylinder head stud, alleviating the aluminium from having to carry all the load.  Unfortunately that plate has to be fitted at the same time as the cylinder head is fitted/dropped onto the block so is often forgotten. (..and the reason I hang it on the front stud !).

Anyway, I always fit that steel reinforcement plate ..but I also like to get rid of that hard corner.  This is my rework of that issue, on this particular engine block..


^  As was, and then (right) a first coarse cut to round that inside corner 


^  ...the red arrow shows where I still have some filing to do ..That sharp edge has to go.  And then (right) with it smoothed off with emery paper I can see that the hard edge has now gone. I'll still use the steel plate (making sure that has no hard/burr edge that might dig into the aluminium) but I'm happier now :) 

And then there's the same thing happening here..


^ short stubby but lightweight alloy con-rods, now with flash edge smoothed off.

So there go for today ..a little more progress despite this ol' giffer feeling his age. And now it's time for cuppa tea and another ginger biscuit dear :mrgreen:


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Interesting stuff. Unusual to have gear reduction then the cam chain, suppose it allows a smaller cam sprocket so reduces overall height. Are the gears 2:1 with 1:1 chain or is there a pair of more complex ratios that give the overall 2:1?


I don't see why supporting the engine at the front of the head and the rear of the gearbox would have any mitigating effect upon torque reaction. Such mounting may reduce the coupling of the vertical shaking of the engine into the frame however.

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Interesting questions :)

Along with the crankshaft, this (below) is the timing gear, positioned on the rear main bearing-carrier...


The long end of the crankshaft of course sits in the rear-main white-metal bearing, mounted into the cast carrier. The driven pinion (shown bottom right) then goes onto the crankshaft. In geared ratios it drives the geared pump (37% faster than the crank) and the large diameter timing wheel (at half speed). 

Mounted directly/securely onto the back of the timing wheel is the cam-chain's drive sprocket. The cam-chain's top sprocket is the same size as the bottom one (..in the photo, it can be seen tucked under the aforementioned pinion). So as you say ..the reasoning was most likely to keep the top sprocket's size compact. B)

It's a very simple assembly with a low c.of g.  When installed the timing wheel / cam chain sprocket is just above the crankshaft bearing, but the cam-chain running at half speed is short (so needn't be so tight), which would lessen wear on its tensioner & bearings. 


Regarding "why supporting the engine at the front of the head and the rear of the gearbox would have any mitigating effect upon torque reaction?"    ..I'll have to think through ., not least in how I might explain it.  :shock:   ..having written what seemed to make sense last night, it doesn't seem so clear this morning.!  so I'll come back it later in the day (in blue type) to try and explain.  In the meantime the following might help...



^ Unlike most car and in-line or flat-twin motorcycle engines, the Sunbeam's crankshaft's axis (blue line) is vastly out of plane with its diagonally positioned engine mounts (signified by the red line). 

Engine torque has to act / react around this diagonal, and that again is out of line with the third axis of rotation ..that being the motorcycle's longitudinal axis (a horizontal line through each tyre's contact point)  ..so reaction to engine torque is not only laterally but longitudinally (ie., being countered by the length of the bike's wheelbase). 

Let me try and explain why ..

.  Torque reaction is not apparent when the engine is running steadily.

.  The felt 'reaction' is the force that acceleration pushes off against. 

.  When the 'the mass of rotating engine parts' is accelerating (caused by more air/fuel being introduced.. and so more powerful an explosion in the combustion chamber) - it pushes off against the cylinder head. 

.  But this push is not just upwards ..there is side element to the force - because the con-rod is not vertical but at an angle (to its crank-pin).  It's like sitting on a push bike and pushing your foot down on just one side, well then you and the bike start to topple sideways. 

.  It's similar with an in-line motorcycle engine,  but instead of your foot pushing down on one side of the bike - it's the engine mounts that are pushed down on one side. And that's rather like pushing down on just one side's footrest - the bike starts to topple sideways (rotates around a horizontal axis - that axis being a line between front & rear tyre's contact with the ground). .

.  That's all very easy to understand when the front and rear engine mounts are closely aligned to the rotating axis of the bike   ...But what happens when the engine mounts are positioned at an angle of 45 degrees (for example) to the bike's axis. ?

.  Well continuing with the analogy of pushing down on just one foot peg,  if that moves (rotates) not downwards but at 45 degrees (down and forward) then your foot's force is pushing not only to topple the bike but, in equal part, to spin it around its vertical axis.  That of course can't happen very much at all - because the bike's tyres grip on the ground.  But that's where some of the torque reaction is absorbed.    

Likewise the angle between engine axis and its mounts, with its own longitudinal element of reaction, deters self-excited rocking. And then, the engine's low centre of gravity (being below the red line) further helps to stabilise things ..more so than an engine bouncing around on top of in-line engine mounts.

.  Going back to "It's similar with an in-line motorcycle engine,  but instead of your foot pushing down on one side of the bike - it's the engine mounts that are pushed down on one side."  In practice the 'reaction' tries to rotate the engine around its mounts.  And where these mounts are low down and in axis with the crank ('the mass of rotating engine parts') - then it's the relatively lightweight cylinder-head which moves most.  And when it moves it picks up inertia, which in turn has to be controlled

.  So what happens when an engine mount is where the cylinder head is ?  Well, again the 'reaction' tries to rotate around the engine mounts.  But with this geometry - it is trying to move all the weight of the engine's bottom end.  Of course accelerating to move a greater mass absorbs more energy than moving a small (lighter weight) cylinder head. And that energy is absorbed even before it reaches / is felt in the frame.


Perhaps also significant is that there are just two Metalastik type rubber 'mounts' in this configuration, with the engine/gearbox hanging and happily rocking in suspension between them. :)  Much of the engine's rocking is not directly transferred to the bike frame, but there are two sets of soft rubber (read : absorbing) snubbers  (signified by the yellow lines) to counter the engine's torque induced motion. The top one is mounted to a friction damper plate - which seems surprisingly effective in preventing things from bouncing around, either up n down or rotational.   

NB. Car engine-mount designs ..higher up the suspension towers, eventually came close to this geometry :mrgreen:

I hope this at least starts to / better explains ?  :wacko:



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Some early transverse engined cars had a rubber bushed engine steady bar connected somewhere around rocker box height, connected to the bulkhead or other bodywork in a convenient position.  This was introduced to limit the engine's movement in reaction to torque because engine mounting initially followed front engine rear drive engine mounting practice i.e. mountings typically at sump interface height.  Transverse location of the engine tended to leave less room for the top of the engine, together with carburettor and air cleaner assemblies, to rock and, more importantly, the motion quickly fatigued the exhaust downpipe.  Eventually, engineers were allowed/asked to attend to these problems, resulting, as you mentioned, with the high set mounting at one end of a transverse engine which makes it much stiffer in reaction to torque.  It also made changing a cambelt more time consuming when overhead cams became typical.  Even with this improvement, further work was required to enable the exhaust downpipe to have a reasonable life, hence the flexisection featured in most modern exhausts.  Interestingly (or not!)  my 1961 Reliant has a steady bar to tame the engine's movements within its tightly cowled position between the front footwells.  The Sunbeam's mounting arrangement nicely addresses the resistance to torque reaction and reduces the transmission of engine induced vibration to the frame, something that many British singles and twins were notorious for.

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Yesterday evening and this afternoon I had a little fun..  (which might be saying more about my social life than I'd care to admit !)

When I used to work on customers Sunbeams ..I was obliged to do what I knew worked.  That usually meant putting things back to standard specification or., with the owner's OK.,  I might apply a proven modification already being used on one of my own bikes.  

Of course, when I worked on customer's bikes I rarely had time to work on my own, let alone play around with developing ideas.  So yesterday / today I played around with one such idea I've been carrying around in my ol grey matter for some time now. 

..I experimented a little bit of porting within the combustion chamber.  I don't know if it will help, or indeed if it does - whether it was worth four hours work ?  but hey.. it's my party ..

You've seen before what these motor's combustion chambers look like, and if you've read what I posted then you may remember that it was a pioneering (lovely word that !) application of the squish combustion chamber used in conjunction with in-line valves. Here it is again to refresh your memory..



^ imo., one of the major drawbacks of this in-line valve design is that there's just not enough room for the valves.., or rather the for the air flow around them.  In fact as you can see - around the back corner of either valve - over one-quarter of its perimeter is so close to the casting as to (imo) seriously restrict the gas flowing in or out of the cylinder  ..So that's what I sought to address...

I'd never tried cutting this before, so I did it first on an old cylinder head that I'm not planning to use on any of my bikes. 


^  Firstly I protected the valve seat. For this I used a longish bolt, which fitted into the valve guide, and a dished washer off an electric drill's sanding disc (which happened to be the right size).  Then my first cut was with a ball-shaped cutting burr (about 16mm dia.).  I think this is one from a cheap set intended to butcher wood, which I'd bought to butcher fibreglass.!  Crude as heck, but it worked for me here on the aluminium.  That cut a very rough grove around the outside of the combustion chamber's wall.  It was too big to get tight into the corner but., that's OK because I also had in my toolkit a 10mm metal working burr (somewhat finer).. 


^ resting this on the steel washer (protecting the valve seat) I got closer into the corner and broadened the grove being cut.  And then I got out the Dremel..


^ although these stone disks wore down as quickly as the aluminium they did a good job of smoothing the lumps out ..and particularly down the sides which taper to nothing - they work tight into the corner.


So what did I end up with ? ...


^ the blade is just a conveniently small straight edge to show the (getting smoother) trough. The sides were straight down (in the same plane as the valve guides) and I estimate to have removed to about 3mm depth. The valve is shown open 5mm and the pencil line marks its level on the side wall.  The valve's full travel is 7mm.

Below shows a 1/8" drill bit (,,convenient for scale) in the tightest segment alongside the inlet valve .first photo : where the side wall is as original...  versus second photo : the one I cut.


^ original


^ modified

That difference may not seem a lot ..but in terms of air getting in, around that segment of the valve, it's probably about 75% more than it was  ..And that's at the tightest spot where I was undercutting the gasket face and the side wall was only about 8mm high ...and tapering. 

Below : The pencil lines on each side/back wall (level with the top of the valve) illustrate how, around the corner, this cut groove doubles the clear air gap between the valve head and its side wall...


^ after cutting   . . . . . . . . . . . . . . . . . . . . . . and as it was  before ^

I did of course check the remaining depth of side wall (to the outside, inbetween cooling fins). And with the groove I still have roundabouts 8mm thickness left. NB. no gasket face has been cut away, these are grooves in the combustion chamber's back and side faces only.

I'm happy with the improved gas-flow clearance, so I did the same to the cylinder head I'll be using on this bike...


That was an interesting afternoon's tinkering ..I enjoyed that  :-D


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Continuing on today with the inlet port. It was like this ..


So firstly, I re-profiled the arch between the two cylinders to be very much slimmer ..


^ The close-up photo makes it look really rough inside there but in fact it's not too bad  ..perhaps just a little we n dry needed yet. ;)

The carburettor on this bike is a diminutive 24mm.  I had on my other beam's cyl. head - taken its port out to 26mm, with the intent of up-sizing the carburettor, but I never got around to doing that.  I note that the 1960's Triumph 500cc Tiger 100 likewise uses a 24mm carb - so that is probably sized OK., but enlarging the inlet port, when used with the small carb added nothing to the bike's performance.

However, as you might have noted (in the first photo above) - the carb's insulating (phenolic) spacer's hole is just a bit bigger than the mouth of the inlet port.  So the final task I wanted to do on this - was to enlarge the port's diameter, just enough to loose the lip / the step down in size.  

To ensure it didn't then neck tighter, just inside the mouth of the port, I used a penny washer of the right diameter as a sizing gauge...


So now the inlet port looks like this..


You may ask "Really, what's the big deal here - it was only a small lip ..what 1/2 - 1 mm step in size ?"   . . . Well, such a lip (or any other significant surface flaw) in an induction port might be likened to overtaking a fast moving lorry in a lightweight car.. you certainly know it when you hit the lorry's bow wave / the displaced air from around its front.  I recall such a circumstance when driving an old Citroen 2CV on a dual-carriageway, it just didn't have power to get passed :shock:  ..and I couldn't go wider (into clean air) because of a central reservation.  I had to drop back and take a run at it ..and then the momentum helped us push  through ..but still quite embarrassing.  :oops:

Can you imagine the rate of air-flow in that little induction port.. when the fuel-air mix is being sucked in at the rate of 500cc (1/2 ltr) x 5000 RPM.?  With those air-speeds., a square-edged lip in the sidewall might have a pressure-wave three or four times its own height.  Even though only 1/2 mm high - its pressure wave may be an additional 2mm.  And the lip is all around this port's perimeter ..so together with that on the opposite side-wall - it's like constricting 24mm diameter down to 20mm.  In terms of cross-sectional area, that's a huge difference (reminder: cross sectional Area = pi R2 ).  Btw.. carburettor gaskets with too small or an inaccurately positioned hole (which part intrude into the air-flow) can have the same detrimental effect.   

The exhaust ports on these bikes are fine as they are. Not only are they a very much larger in cross sectional area (34mm dia) but there are two of them.!  Hence the reason I put my effort into getting the air in / out passed the valves, and then only needed to work on the induction port.


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Guest Hooli

Impressive. I'd like to flow my Bonnie a bit when I rebuild her, it's nice to start to understand what people aim for when doing it.

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On 4/1/2017 at 9:23 AM, Hooli said:

Impressive. I'd like to flow my Bonnie a bit when I rebuild her, it's nice to start to understand what people aim for when doing it.

Very glad to offer some sort of guidance, from the garage workshop rather than from a bar stool.  ;) 

There's a good book called 'Triumph Tuning' by Stan Shenton (one < here  > on amazon ..but can be found cheaper) which may be an interesting read to you.  btw., I am assuming you are referring to the original Bonneville T120 or it's immediate relatives rather than the much later bike ?

Triumph were of course the biggest in the world of Brit performance bikes for many post-war years, and I'd suggest that is something many a boy-racer forgot when they tuned* their own bikes.  Youthful exuberance versus Triumph engineering and work's resources  :roll:

What I'm saying is that you might not want to try and improve on what was designed ..but rather to 'realise the design and development' ...which is often compromised in crude production. 

The post-war Sunbeam is (imo) somewhat different.  BSA-Sunbeam (I believe) originally had that expertise in Earling Poope - the bike's Designer (..I understand he even had a cross-flow ohc version of this engine) ..but he was dismissed* very soon after the bike was into production.  Thereafter, no-one else (of influence) was interested in performance, nor developed it's potential. And the later S8 / S7-deluxe engines were detuned in their redesign, presumably for longevity &/or 'refinement' purposes. BSA's Gold Star was their trophy winner.

'Realising the design' - is illustrated in what I did in the reshaping in the induction port (ie., more clearly defining 'the arch', correcting the port's size, and smoothing the air flow)  ..whereas my earlier intent of up-sizing the carb (and so needing to enlarge the port's diameter), and the reshaping I've just done within the combustion chamber are both examples of my arrogance., in-so-much as I believe I can improve on what was produced. :P

Btw., a good S7-deluxe, despite it's girth and sit-up n beg riding position (frontal area !!) will better 80mph in standard spec. (with 7.2:1 pistons). And that's enough for those brakes and low tyre presures :shock:

Perhaps the most useful advice I might offer is in how to think about air flow through the port and into, or out of, the combustion chamber.   Leonardo Da Vinci was a brilliant inventive mind, who tried to develop a flying machine (both a helicopter and an aeroplane).  In this regard, the single most error he made was to think of flapping or rotating wings as compressing (pushing down on) the air under them.  If he had realised that the low pressure air above a wing is where the lift is... then we might have had aircraft before AD1500 !  

Likewise with air-flow through an induction port. Think not of air being pushed through the induction port (which is how most people look at it when they peek in), but rather air being extracted from that port and the carburettor !  The air is being sucked in, in the vortex of the descending piston.

Why does that make any difference.?  Well., just as an example think of the valve head and the air being sucked out of the gap ..around its rim.  If you think about that then you'll be less concerned about the fairing or otherwise minimising the valve guide.!  

Another example is the arch between cylinders I more sharply defined..  If the fuel-air mix was being pushed in, then it would aim directly at what was the very rounded arch, The fuel would strike the metal and smear across it, in the meantime the back-pressure off that face would be tremendous. 

Now, think of it with air being 'extracted' first through one port and then the other.  When (for example) the rear cylinder is drawing fuel-air in - we don't want to draw air from under the valve of the front port ..and the nicely rounded edge of the arch encouraged that.  No., I sought to sharply cut off one port from the next, so air wouldn't be drawn backwards out of one port into the other.  I wanted a smooth, clearly defined route of extraction of air through the carburettor (where the fuel-air mix is) into the rear port. 

...And then, when the rear valve closed and the front cylinder's valve opened - then the air flow should switch instantly that way.  The sharp edge of the arch helps that switch insomuch as it's either one way or the other ..without confusing turbulence inbetween or diverting around the other port.

And as the objective is the extraction of air from the carburettor's venturi, then you'll begin to realise that opening up the ports to the biggest possible size is actually detrimental. The same size bore (cross sectional area being more critical than shape) ..all the way is best.

So it very much comes down to ; the cleanest, unrestricted route (from cool outside air to cylinder)., with least superfluous volume (as excess volume reduces pressure and therefore speed)., with the minimal boundary layer (caused by rough surfaces) or turbulence (steps in size, hard ridges, or intrusions)., and the smoothest flow around corners (so the heavier fuel particles don't fall out against walls). 

Btw., keeping the air flowing steadily through the carburettor, and neatly switching its direction from one port to the other is (imo) why single-carb twin-cylinder bikes often run smoother than twin carb bikes.  My own Norton 850cc has a single carb conversion. I simply wouldn't want twin carbs (which are only an attribute at the highest speeds).   

tis only my opinion of course.. and we all have one of those :D

..so anyone - please feel free to butt in ..to add to ..or otherwise contradict what I've said.


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Guest Hooli

That all makes a lot of sense, thanks.


It's a T140v, so yeah one of the old 'proper' ones. I only intend to do similar to you've done, smoothing the gas flow & making sure all the joints line up perfectly. I know it's got a 1/16" step from the manifold stubs to the head for example. I've already got E3134 cams in it (650 spec) and an uprated timing side bearing so it just needs everything smoothing out to take advantage of what it's got.


TBH the biggest difference will be a quick action throttle when I get one, having to take two grabs to get to fully open means I rarely get past about 60-70%.


All in all, it's fast enough already but I'd like to be as good as it can etc

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The complexity of flow through ducts, pipework and sundry components, whether compressible (gases) or more or less incompressible (liquids), is well known and difficult to model despite huge advances in computational fluid dynamics in the last couple of decades.  A reciprocating engine's inlet and exhaust manifolding probably absorbs the efforts of dozens of specialists who, with undoubted knowledge, skill and expertise, use powerful computers to crunch all the theories to tease out extra performance with economy for our benefit.  In the end, it still comes down to a series of tests to see how closely their digital predictions of flow behaviour and its effect on combustion and hence performance correspond to the real world.


In other words, the empirical approach has a lot going for it  :-D .  Sometimes, smoothing the passage for a gas (e.g. fuel/air mixture) can lead to a reduction in performance/economy because of the removal of steps and changes in cross sectional area that induce benificial turbulence and pressure changes (from velocity changes) i.e. the inlet charge remains nicely mixed rather than stratifying.


Keeping the CSA of a duct constant and as straight as possible is a good, safe approach (although rarely possible between a carb and engine on a multi-cylinder unit).  Wherever the duct (inlet manifold) narrows in CSA, the velocity of the gas will increase and pressure will reduce.  This is of course how venturis work, with Mr Bernoulli doing most of the ground work back in the 1700s.  When it comes to flight and the development of lift, Mr Newton's theorems provide the primary explanations for the resulting forces after using Bernoulli principles to resolve the pressure changes caused by velocity over parts of the wing.  As long as you remember that the aerofoil shape of a wing is mainly to improve the lift/drag characteristics for a particular application and is not the sole cause of lift, you'll be fine.  A flat board, or indeed an upside down aerofoil, will generate lift, thanks to the angle of attack.  This is largely ignored when TV presenters try to explain lift.


I am enjoying your Sunbeam thread.  Keep up the good work and may your remarkable motivation continue  :-D.

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My old man had an S8 when he was an apprentice; he reckoned the rear cylinder needed a few thou extra off the liner to avoid it nipping up under load/hillclimbing/etc. because the cooling for it was that much poorer than the front cylinder.

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I had a T140v at one time, with a full Avon fairing on it.. It was a very quick bike, but as had an annoying habit of blowing the head gasket. I think it happened three or four times so I got quite quick at changing it.  Toured across Europe together with a friend on a T150v. I rebuilt the engine in the evenings and the bike would always start so easily..  Converse his T150 was really reliable but a bitch to start :)  

On one occasion I gave a hand signal as the indicators weren't working again, and caught the front indicator just as it fell off !   ..Another time it led me off the road and into the woods, just outside Luxembourg.  The rear brake was a stainless steel disc which I'd just discovered didn't work in the rain.!   Can't deny that bike had character as it tried to kill me :mrgreen:

"Keeping the CSA of a duct constant & as straight as possible is a good, safe approach"  Yep.. it's just as easy to make things worse than to improve them when 'flowing' a cylinder head. So, as we're talking about 25bhp to shift 430lb + 200lb of rider I'm not going to spend more time in trying to squeeze every ounce of power out of it..  :P

I did learn to fly a fixed-wing at one time.. mainly in a Piper.  I was glad to learn all sorts of useful tidbits about lift and drag ..But the instructor wouldn't let me fly upside down. :roll: 


"My old man had an S8 when he was an apprentice; he reckoned the rear cylinder needed a few thou extra off the liner to avoid it nipping up under load/hillclimbing/etc. because the cooling for it was that much poorer than the front cylinder."  Have to say I do the same, but I do it for both cylinders.  But that's because (imo) there's no point in having such tight tolerances as 0.004" piston skirt clearance, when the piston is fitted with a bottom oil scraper ring.  The early bikes had three piston rings rather than four, and if my supposition regarding Sir Bernard's aspirations for this bike are close ..then near-silent mechanicals was really important.  After the bike's designer (Earling Poope) was fired, I don't think anyone in the works really knew or cared very much about the bike (West Midlands BSA boys working on a Gentleman's Sunbeam !) and so that early skirt clearance specification never got changed when the pistons were.

As I've said in a previous post I have blue-printed one of these engines to minimum clearances throughout. And it did nip up a number of times whilst running in (the first 1000 miles !) but seems to be OK now.  8)   Btw it was no quieter than other engines I've rebuilt with more realistic tolerances. 

Bfg ;)


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11th April 2017 :  Unfortunately on the 11th April, this server got hacked before I backed up the post I'd written ..regarding the Sunbeam's rocker cover and its breather.   Dave tried to recover that for me 8)   ..but it was lost,  so I've now rewritten it. 

Following on from reworking the inlet port of this bike's cylinder head and its inlet port..  I next needed to check and where necessary to make good / redress the gasket faces of the rocker cover and sump pan, and any other part where they sit to the cylinder head and engine block. 

Again the similarities between the post-war Sunbeam motor and car engine design-layout is quite apparent, insomuch as they each feature a one piece rocker cover and a wet sump.  This was at a time when almost every other production motorcycle had external push-rod tubes, bolt-on rocker boxes with external oil-feed pipes attached via banjo connections,  and then a number of small covers for access to tappet adjustment.  And their engines were dry sump - with an oil pump bolted (often externally) onto the crankcase, having pipes leading back and forth to a remote oil tank (usually situated under the rider's seat).

If it had been a car engine of its day - the Sunbeam's rocker cover and sump pan were not so unusual - insomuch as they are of cast aluminium (..because steel was in very short supply after the war).  Ironically, despite cars having primarily water-cooled engines, quite a few had fins on their rocker cover and sump pan.  In fact these were simply external stiffening / strengthen ribs ..but the effect was as if air-cooling fins.  Of course most later car engines used deep pressed-steel rocker covers and oil pans.

Our air-cooled Sunbeam motorcycle engines had no cooling fin nor external ribs on their rocker cover or sump pan. This was in part because the motorcycle design was to look especially clean - in the post-war modernist style (and also be easy to clean).  And then because of the single overhead camshaft arrangement, the engine was already exceptionally tall.  The  rocker cover cannot go any higher ..as it needed vertical air clearance above it (beneath the motorcycle frame) - to be removable over the valve gear.  

The early Sunbeam s7 engine did have stiffening corrugations cast into their shallow sump pan - which increased its surface area for cooling. But the engine's 1948/9  re-design called for a greater oil capacity,  so the sump pan was made boxy and without corrugations, and extended downwards. Its depth was limited by the need for a decent ground clearance, and its cooling was in part restored by the larger size (aerodynamics around this brick-like sump isn't favourable for cooling though).

N.B. Because of its shallow sump ; the early Sunbeam engine calls for a higher oil-level in their crankcase, so the dipstick was shorter. The dipstick is a flat blade rather than the later round rod type.  

OK.., from the top - The Rocker Cover :
As established, this is in cast aluminium.  It was fastened by just three shouldered nuts,  and very likely was originally designed to sit on a low pressure (thick) cork gasket.  The three studs are in its top - not around its perimeter,  and the top of these castings are really not very thick.. so there's a raised boss cast around each fastening hole, to beef them up a bit.   At the front of the rocker cover is an open-bottomed cap.  This hides from view the crankcase-breather mechanism.  An engine-breather venting down the front of an air-cooled engine was an unusually dumb idea, but otherwise it was all very neat and tidy .. although not usually understood by the owner nor their mechanic. 
It does however seem (from the illustration in the early-S7 parts book) that thick-paper gaskets were used, rather than cork ones, even before the engine's redesign in 1948/49 (when the engine's S7 prefix changed to S8).  This may have been part n' parcel of cost savings introduced for production.  The difference though - is that cork, if it hasn't been overly compressed but even after being subjected to the engine's heat and oil,  still bounces back.  It is, with care, quite reusable.  Conversely, once used and subjected to the heat and pressure - the fibres within a paper gasket flatten and do not bounce back.  :wacko: 
Of course for reasons of economy or availability - gaskets (particularly those under adjustment covers) are frequently reused.  But as used paper gaskets have no sponginess left in them ..they tend to leak,  so the owner / mechanic tightens the cover a little more ..and then a little more again.  As a direct consequence (..of the wrong material being used for the gasket) the three narrow-shouldered nuts holding this cast shell of this cover are over tightened - very commonly causing the alumunium to bend and crack. . .
(..castings do not like to bend at all !)
             ^ This one has been weld-repaired from the underside.  On the top face (seen in subsequent photos) the raised / reinforcing boss around each hole has been ground away - making it weaker still.   NB.  the inside stiffening webs were not exactly designed to help.  This it appears is an early rocker cover, but not off an early-S7 (..which has different ribs / baffles inside it).

Conversely, my engine's rocker cover had been part / weakly polished.  It wasn't nicely smooth and beautiful, but still lost much of its definition of being a casting.  It could be bead blasted to restore much of its former looks but.. during polishing they also rounded off the raised boss around each fastening hole.  


So often it seems, persons simply don't stop to think that the originally machined face of any such raised boss around a fastening - is in fact a gasket face.  And the nut or bolt with a fibre washer under it (..its gasket) needs to sit flat  on these raised rims ..if one aspires to build an oil tight engine. ;)



             ^ a combination of the nut having a very small shoulder,  the drilled hole (always) being too big and drilled to one side,  it having been dented by over tightening,  and then subsequently being rounded off & polished ..are not the ingredients for a decent gasket face - which the top of this raised boss should be.  If I were to take this down (re-dress it) to being flat & level then there would be very little left of the additional strengthening boss.  NB. It is the same cover as seen below (with the closed breather cap screwed on)

As it happens.. I had yet another rocker cover available to use - which retained it's cast-aluminium definition, was not cracked, nor had it been repaired, and the raised boss around each hole was in salvageable condition.   So I set to redressing its gasket faces. 
I also wanted to fit a closed crankcase breather cap - which has a take-off for a pipe that may be led clear of the engine, to a catch tank &/or into the air filter.   This is an approved modification  ..with the aluminium cap (below) being supplied new by Stewart Engineering (the Sunbeam Specialists).  It's not cheap but I think it's worth having . . .
 ^ Even aside from said professional's choice of inappropriate size of washers & cheese-head screw ..this cap is ugly for use on a 1950's cast aluminium motorcycle engine. Yes I know it's pretty well tucked under the fuel tank when fitted into the bike ..but I still know it's there and ugly.!  So I sought to modify the modification.   :-D

But before I did that I wanted to check the guts of the crankcase breather on the 'nice' rocker cover (the one I'm going to use on this particular bike).   I expected to find this . . .

              ^  This photo is of the weld-repaired rocker cover  ..you can see how the raised-boss around the fastening hole has been rather crudely ground away.   And recessed into the front of this casting are three round holes with one-way valves in them (more of those later).  To the right are six fine gauzes ..used to separate the oil from the breather / crankcase air. They are shaped to fit within the oval recess in the front of the rocker cover, and are retained by the steel baffle plate with its pillarbox slot and gasket. A small raised-cheese-head screw to secure / help oil-seal the plate's bottom edge. And then the open-bottomed cast-alumnium cap with its three raised-countersunk set-screws.  

In the cover I plan to use - which I thought was unmolested - there was none of these parts.  Just three holes in the front of the cover.!   :?
And in the polished one, onto which said professional had fitted the closed breather cap, were no oil-separator gauzes, nor steel baffle plate. But crud in with the breather valve disks  ..Thanks fella ! :cry:
So, I resolved to reuse the parts out of the welded cover, and will fit those in the unmolested one. . .  
             ^ here you can see in closer detail the working parts of the breather's three disk valves, just removed from the welded rocker cover.

The disks loosely sit over the holes through the casting, and are each retained in their recessed round hole by the almost rectangular soft steel plates.  These are an interference fit in those recesses ..and were originally slightly convex in shape, rather like the principle of a core plug.  Once placed in the hole they are tapped flat ..which lengthens them to fit tightly into the casting. The casting is then punched, over the four corners of each plate, to further secure them.  The pieces in this photo may look grim but they just need a good cleaning and are otherwise in good shape.     OK,  now have the parts I need.. 

Btw. The disks themselves appear to be sprung / high carbon steel (they are are very hard, do not bend, and barely wear).  In the past I have cut new ones from the thin steel plate found around some speaker cones.   NB. They do have to be very flat though.  I have also seen some which have been home-drilled with small holes in them ..exactly wrong again Bud !  :roll:

The crankcase air blowing (as a result of pistons descending and so pushing air out of the way, and otherwise by piston blow-by) through the holes in the casting ..lifts these disks away from their seats ..and so allows the air to pass.  Then, as the piston rise in their cylinders again - the crankcase pressure drops ..and air is sucked back in.  As this happens the disks are drawn to close off the holes and not allow any more air in.  That lower air pressure / partial vacuum then helps negate the high pressure build up inside the crankcase the next time the pistons come down ..which in turn helps keep the engine oil tight.   If these disks are drilled, or are not to free open and close properly ..then the whole thing stops working.  A disk valve is very simple with just one moving part.. and works rather well,  although it will tend to make a very light clicking noise. 

I mentioned (re. previous photo) the oil-separator fine gauzes.  Let me explain how they work ..and in doing so help differentiate between an 'oil-separator' and a 'filter' (as these might be thought to be ).. 

Crankcase air with mechanical parts being splashed in oil (more on that below) has tiny oil droplets (like a fine spray) flying back and forth.  These droplets are heavier than air and so don't go around tight corners as well.  So.., the oil droplets being carried (in suspension) in the crankcase air..  goes through the tight gap between these disk valves and their seats.  This route is not only tight but also involves sharp corners ..so some oil splats against the disk or against the side of the recess ..it then runs away through a small drain hole (through the casting) back into the engine.  NB. that hole should not be enlarged otherwise the air just goes through that and by-passes the one way valves. 

Any air + oil droplet that gets passed the disk valves then has to go through the maze of fine wires ..the aforementioned oil-separator gauzes.  The oil droplets hit the wire and drain down and back into the engine. This is why there is then a steel baffle with gasket to seal the bottom half of the oval recess.  The air with oil droplets must negotiate its way up and through the pill-box slot to get out ..before the pressure drops and tries to suck it back into the engine again.  Even Harrison Ford would not get through that in time! :D

So you see those oil separators are a critical part of the crankcase breather's design.  Yes 'design'.  It's not all happening by chance.!  :P

As an aside.. the gauze does also serve a seconday purpose ..of being a filter to help keep out larger bits of field dust,  small insects,  and airborne seeds.

What about that ugly closed breather cap ?   well . . .  
 ^ made at 17mm deep  ..it won't be soon !   2.5mm off front and back faces .. by hand as I have no milling machine ..and then flat enough for refacing ..to be oil tight !
             ^ And there we have it..  the closed breather cap reduced in depth by 5mm overall  ..and fitted with the original cast alumnium cap for 1950's style.  When bead blasted to match the casting's finish then I hope this 'modification' will blend in & be visually lost.  NB. I couldn't just leave it long and then fit the original cast cap in front of it, as it sits too close to the fuel tank bracket when in the frame.  
From the underside it looks like this . .
^ the cast breather cap (Orange arrow) is now just a dummy. The green arrow simply shows the inside of the breather valve holes.
NB. Two addition inner stiffening ribs are standard on later rocker covers (if you compare with first photo above). 

Unlike said professional.. I've redressed the gasket face.  Just one small section (half the width of the flange)  is still  showing dull (Blue arrow) but it's really so close to being flat that it will not effect the seal.  To get this bright and shiny would mean further cutting back (on emery paper) of the whole of the rest of the gasket face.   Redressing a gasket face is about getting it back to being flat, neither damaged nor distorted ..not to make it look pretty. 

The Red arrow shows where yet another amateur thinks he knows better than the Design Engineers who created this engine.!  Inevitably, because his engine was in poor condition and leaked oil ..he blamed it on poor design - so he drilled three holes through the end baffle plate, between the cam-chain chimney and the rocker gear ..to give the crankcase air a clearer route to the breather vent.  

In doing this - he doesn't realise that the air was meant to blow back / up through the camshaft chamber's oil drain hole.  In doing so it blasts / splashes oil back over the camshaft lobes and up through the gaps (seen above) in the cover's longitudinal web ..to keep the valve guides lubricated and to cool them.   I note that this baffle in the 'nice' rocker cover also has a small drill hole through it ..which I'll close up again.
That's it for today (..OK., for the 11th April then ! )

Bfg ;)


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Those fastener bosses might benefit from steel top hats bonding in, or even just with mastic. Your description of the design subtleties of the Sunbeam is very interesting; when working on a piece of equipment(particularly if it's gone through a lot of development or a long production life) it can be very absorbing trying to follow the design changes and understand artifacts that perhaps don't have a function any more but they remain on castings/forgings, like tattoos on middle aged people that were acquired in the impetuousness of youth.

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