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1951 Pontiac Chieftain - Gearbox

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I made Special Tool J-4670 out of some bolts and plywood.


Made an attachment for the air line out of a tire valve, trimmed it flat.


Tried with my little air compressor.


That just blew bubbles.



Employed bigger air compressor, win!


That represents the last piece to disassemble. It's all reassembly from here on in (barring taking old bearings out).



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Started today by cutting a groove in the bearings I wanted to remove.


Cold chisel to split it all the way.


Hammered out flat you can see just how bad this bearing actually was.


Removed all the seals and cleaned the grooves they sit in.


Rubbed all the sealing surfaces down smooth with 1000 grit because they were a bit fouled with alloy corrosion and dirt.


Refitted the piece with new seals


Fitting the piston was a pfaff, there is a step in the outer case where the outer static band locates, and something needed to be put in to remove the step. I didn't have the correct Special Tool so bits of cut up plastic of the correct thickness were added the piston then settled down into the bore happily.


Back together as much as I can put it for now. Typically in all this I missed one piece which needs to be replaced, a wavy spring washer that sits behind a retainer (that I forgot to pull off before this afternoon).

I think I will get some dry ice to put the bearings into before fitting them, that should help.



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1 hour ago, Tickman said:

Amazing seeing you just getting stuck in and making actual progress. Then some more progress, then more...


First automatic transmission I've ever taken to pieces, ever. Hopefully it all goes back together and actually works afterwards.

The gearbox has seen several fairly significant failures; I'm hoping with this overhaul that the weak points are refreshed and will be good for another 100,000 miles.




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"It does move if you put it in gear and it does select more than one gear. However, I have never driven a car with this type of gearbox before and as such do not have a baseline for its behavior".



Slightly later model with different valve block but the internals are the same and the shift points are the same.



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I decided to revisit the governor. Having sat a little while it was again sticking in the bottom of the bore.



The smaller weight on the right does the what-speed-the-gears-change-at pressure.

It must move smoothly throughout its entire range. Getting stuck is not an option.

I lapped the surface that it hits into at a rolling angle and that appears to have remedied the issue.

It now clik clak's happily from end to end, even if given a firm prod. That should help with making the gears actually change. I think it was getting stuck before, would certainly explain the behavior.


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New pieces fitted! New big return spring in the front servo ready to go in.


First, put the opposite end piston in place, then fight with the back end one to get the rings to seat correctly in the bore.


Main return spring goes in.


Locator hat sits on top and keeps the main spring in place.


Smaller booster piston spring sits in the hat. The booster piston gets pressure from the compensation valve, connected to the gas pedal; makes for a more positive gear change when you're accelerating hard and grips the band tighter the more you push down on the pedal.


The two halves them come together and bolt down.


One down, many to go!



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On 8/10/2019 at 3:38 PM, PhilA said:

I just stopped at AutoZone and they've changed up the transmission fluid they stock. Up until 2 weeks ago, they stocked Dexron-IIe. Now, they've rearranged everything and the oldest type oil they keep is Dexron-III compatible "universal" oil.


That made me think. Possibly the wrong oil had been put in, because the seals are rock hard and split like so much cheap plastic, and the only bearings that are bad are the phosphor bronze ones in the hubs. 


That would explain why it wore to such extremes.




 Hi, Putting Dexron fluid into older 'boxes can finish them off, unless they have Dexron compatible linings, because Dexron is a friction modifier.  So check with the supplier of the linings what fluid to use.


P.S. I'm sure you know this already so apologies if so.

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