Cracked piston puzzle

[Sam Glover]

I took the head off the ‘Amazon’ this week because I’d drunk too much Prosecco.

 

This led to an intriguing discovery: all four pistons are cracked.

 

All have cracks across the tops of their crowns at 90° to their gudgeon pins (pics 1 and 2). On one, the crack has spread down one side, leading to significant melty distress (pic 3).

 

These were new standard-size items, fitted by a previous owner c20,000 miles ago. The carbon buildup over the melty bit of the really bad one was consistent with that over the rest of the piston, suggesting that whatever caused the cracks did so fairly soon after they were fitted. 

 

The skirts are worn more than one might expect on the thrust side, considering the milage they’ve covered (pic 4). Otherwise, there are no further signs of distress. The bores are decent.

 

I can’t think of an entirely plausible cause. The consistency of damage across all four pistons is particularly vexing. Really advanced ignition timing could’ve done it, I suppose – or a prolonged period of overheating? A dodgy batch of pistons? A combination of these?

 

Theories invited…

 

Sam

 

 

 

 

[twosmoke300]

Silly question but are they definitely the correct pistons and installed the right way round ?

 

Otherwise it's timing and or overheating as you said .

 

Massive pinking maybe ?

[Mally]

We crack Pintos when racing, generally through overheating and over revving.

[Tamworthbay]

Manufacturing problem for the cracks seems most likely. I can’t see any issues that would cause that otherwise for every one to be so similar. The excessive wear on the skirt could be poor tolerance. Are there any makers marks on them?

[John F]

My very first thought was dodgy batch, but the skirt wear is intriguing. Do they have the correct dimensions for the vehicle?

[Sam Glover]

Silly question but are they definitely the correct pistons and installed the right way round ?

 

Otherwise it's timing and or overheating as you said .

 

Massive pinking maybe ?

 

Yes – they’re the correct pistons, the tolerances seem reasonable and they were fitted the correct way round.

 

It it were timing-related, I suspect it would’ve knocked and pinked like a bastard. Maybe it did – and it was simply assumed that this was what an engine was supposed to sound like when running in. You’d perhaps expect some corresponding big-end damage… but there isn’t any.

 

Overheating? You’d rather expect it to seize and/or the cooling system to shit itself before denaturing occurred.

[HMC]

Maybe the wayward timing on the 123 ignition you mentioned in your saga or the reason it was there to replace the original distributor caused undetected bother at the time? Maybe the head has been skimmed excessively in the past. What where the compression test values?

[Sigmund Fraud]

I'd blame the quality of the pistons, the bits you are showing us have far more casting defects than I would expect.

[MikeR]

to me , picture 2 seems to indicate that there is more deposits on one side of the crown underside then the other , could this indicate uneven heating ?

[John F]

 Yep. On balance they're probably just cheap, low quality pistons.

[Bren]

The fact that all four have damage as opposed to one, or possibly two, makes me think that they have not been good quality items.

 

There are lots of poor quality engine parts knocking around these days - I seem to recall Rover V8 specialists resorting to using genuine LR hydraulic lifters in their rebuilds because lots of pattern ones were failing very quickly.

[Sam Glover]

Maybe the wayward timing on the 123 ignition you mentioned in your saga or the reason it was there to replace the original distributor caused undetected bother at the time? Maybe the head has been skimmed excessively in the past. What where the compression test values?

 

Plausible. The 123 lump was set to provide bugger-all advance. Maybe in the past it was adjusted it to give reasonable performance, which would’ve made it massively advanced at lower rpm.

 

I should've done a compression test before taking it to bits – but I didn't. The head doesn’t look as though it’s been skimmed, or indeed touched in any way. It boasts excellent valve seat recession.

 

 

I'd blame the quality of the pistons, the bits you are showing us have far more casting defects than I would expect.

 

Worth researching. The only good thing about a Volvo ‘red block’ is the fact that everything’s over-engineered and over-specified. If replacement parts aren’t up to Volvo’s standards, I’d be better off fitting a Pinto.

[Asimo]

It doesn't look like pinking / detonation damage: that results in massive overheating/melting damage. The damage to the skirt is more than just wear, the metal has really picked up suggesting failiure of lubrication, possibly due to lack of water for a short time. Were any rings broken?

[somewhatfoolish]

The cracks in the crown of all 4 suggest poor alloy or crap casting, the skirt scuffing suggests failure of lubrication and/or cooling; were the oil control rings the right way up and are the oil supply holes(if indeed B18 pistons have such) present and correct?

[Des]

OE Quality Chinese monkey metal perhaps?

[Uncle Jimmy]

Are they cast or machined from billet?

If cast then the cracks in the middle of the pistons look like the results of tension in the castings. They were cooled too fast or cast at the wrong temperature or not allowed to weather before machining.

I'd be interested to know how dimensionally accurate they are across their width. I'm sure that the quality of these sort of aftermarket parts has always been variable, seems to me nowadays that it's very hard to know with certainty what you're getting and whether it's what it should be.

[Zelandeth]

Gut feeling from my side is pistons made of finest quality Chineseium.

Running issues severe enough to cause that I'd have expected to be massively apparent while it was happening. In an "it sounds like a stone cold XUD when idling" sort of way...

 

Edited to correct the usual idiocy caused by autocorrect on my phone.  Thing's a bloody menace.

[Joey spud]

I agree with the poor quality piston conspiracy too.

[willswitchengage]

Daftest question - is this a Volvo Amazon or Land Cruiser Amazon?

[Tamworthbay]

Volvo Amazon, has been in this parish a while.

[Junkman]

It looks like excessive piston slap caused it.

My suspicion is that someone installed standard sized pistons into worn/bored/honed cylinders.

It's time to dig your micrometre screws kit up. Without it it's pretty impossible to tell.

[Des]

Could it be worth measuring those pistons? Perhaps they're too mangled now but possibly they should be narrower along the pin axis and / or have inserts controlling the direction of expansion.

The obvious issue now is finding decent replacements, remember when Blue Peter used to collect milk bottle tops? Guess where they ended up, along with all the chewing gum scraped off pavements and old plasticine when it's picked up too much hair and biscuit crumbs for further use.

[forddeliveryboy]

Yes – they’re the correct pistons, the tolerances seem reasonable and they were fitted the correct way round.

It it were timing-related, I suspect it would’ve knocked and pinked like a bastard. Maybe it did – and it was simply assumed that this was what an engine was supposed to sound like when running in. You’d perhaps expect some corresponding big-end damage… but there isn’t any.

Overheating? You’d rather expect it to seize and/or the cooling system to shit itself before denaturing occurred.

 

Overheating can be very localised and it's surprising how many people don't hear a pinking engine, let alone recognise the sound. Were the rings fitted the right way up?

 

I'd say a bit of overheating combined with less than top quality pistons, but you need to measure the bores.

[alf892]

I'd say that has definitely been cooked and seized or a least semi seized. The picking up on one and the skirt damage evident are both symptoms of this.

[tommotech]

Take a look at page 12 and 13 of the following;

 

http://engineaction.com/FailureManual.pdf

 

It’s a really useful resource, I use it as guidance to clients when reporting on engine failures - especially when things become contentious.

 

Has the cooling system been modified in any way?

In my experience the root cause is cumulative here, cooling issues coupled with ignition or mixture related.

[Sam Glover]

Thanks for all this input.

 

My working conclusion is: defective pistons with possible exacerbation.

 

To rule out a few things: The bores don’t have a wear lip at the top. They’re of standard size and still show factory honing marks. The pistons are marked as standard and waggling them around in the bores suggests they’re the right size. I can’t find my micrometer, but sloppier measurement with a vernier suggests the tolerances are reasonable. The rings are reasonably-sized and correctly-fitted, and the oil supply slots are incorruptible. I don’t think it’s seized, as there’s no corresponding wear or pick-up on the non-thrust sides of the pistons. It certainly hasn’t seized badly.

 

Pertinent background information: I’ve only owned the ‘Amazon’ for 3000 miles. When I bought it, it was wrong in all manner of ways. It had incorrect needles in its twin SUs (too lean at mid-range; too rich at full load), a programmable distributor set to give practically no advance and a seized thermostat. The latter had stuck itself closed in the period of storage immediately before I bought it… it was previously owned by Ken, whose shitter-savvy enough to notice overheating. A redundant Kenlowe fan had been fitted before Ken and I owned it, though, suggesting that somebody might’ve been haplessly fighting an overheating issue. I think I subscribe to the theory that the angry skirt wear on the thrust side of the pistons is heat-related. 

 

So: in the c15,000 miles between the engine rebuild and Ken buying it, it may well have suffered timing- and heat-related torment. 

 

The thing is, I reckon 80% of shit old cars are driving around quite happily with significant fuelling, ignition and cooling system issues, or at least maladjustment. I’ve stripped more than 50 shit old engines over the years, some of which have sustained impressive abuse. I’ve never come across a complete set of pistons cracked in this manner.

[Sam Glover]

Take a look at page 12 and 13 of the following;

 

http://engineaction.com/FailureManual.pdf

 

It’s a really useful resource, I use it as guidance to clients when reporting on engine failures - especially when things become contentious.

 

This catalogue of failure is an absolute marvel. Thanks awfully.