1951 Pontiac Chieftain

Went to town to try buy Christmas lights for the house. Failed because apparently they've been a popular item this year.


Braved busy town traffic, the car behaved quite well. It did do something interesting as I pulled up to a red light- it downshifted, hot, through all 4 gears smoothly, then when I pulled away upshifted as cleanly as you could ever hope for.


The car settled down and drove superbly on the way home which gave me opportunity to think about what could probably be happening inside the gearbox.
The hotter the oil, the worse the behavior normally. 1-3 rough upshift at about 12-15 and then it would hold 3rd, acceleration would sometimes cause it to shift but it would slush into 4th, slipping badly until fully engaged. Occasionally taking my foot off the throttle would cause it to shift. Other times it would drop out drive and become like it was in neutral (normally backing off the throttle results in a significant drag, like with a manual gearbox).
I went look in the manual again and did a bit of analytical reading.


Apart came my spare (the original) valve block. Of particular interest, the throttle valve pistons, of which the 1-2 (center) valve has a spring which connects it directly to the 1-2 shift valve shuttle. I measured carefully and there's 0.16" gap between flush and the top of the shuttle when it's in the 1st gear position.
Press on the throttle to get the car to move and the pressure behind the throttle valve piston increases and it will shoot out against the spring and contact the 1-2 shuttle. Under light throttle the pressure from the governor should shift the shuttle against the throttle valve with ease, shifting smoothly to second gear.
Note, and this is important, that the spring is only held in place by a small peg in each end and has no other support.
Cold, the spring appears to compress nice and evenly. Hot, it buckles to one side in an arc. Technically this shouldn't cause any issues but the bores are worn so that applies a small radial force on both the shuttle and throttle valve piston.
I measured the maximum lift position of the throttle valve piston then applied a slight sideways force as I pushed it down the bore. Pushing downward, it would snag and bind up solid halfway down.


The engineers knew that a really accurate, sharp 90° edge on a piston will cause any debris to be scraped out of the way and not cause the piston to bind, however that's only applicable really for edges that remain in a bore all the time. The larger section is just an exhaust valve to allow any blow-by on the 1-2 shuttle to escape while the car idles in Drive, at a standstill. It passes through a hole in the valve body, and relies upon the smaller section to be an accurate fit to hold it concentric.
Well, it's not a tight fit any more because it's worn out- that's one of the most operated pieces of the valve body. As such, when the spring heats up and curves into an arc, if the spring is sitting downward (generally it is because gravity) it'll cause the sharp edge of the piston to sit neatly on the lip of the exhaust hole. That'll cause the 1-2 shift shuttle to become lodged in a halfway state, bleeding off pressure from the governor and generally causing havoc. I think this is why if I can get the gears to cycle through their shift a few times the new oil from the pan is cool enough to get the spring to stop compressing in an arc and allow the throttle valve piston to drop fully home, and allow the gears to shift correctly.

So, what I did was engineering taboo. I carefully machined a chamfer on the larger portion of the throttle valve piston. No matter what radial force is applied now the thing centers itself in the hole as it's pushed through.
If time allows tomorrow I want to swap this valve block out and see if that's made the slightest bit of difference. I hope it does, because it should make this a very well operating gearbox.

Phil