Pontiac renovations, part 25.

I continued to work on the parts of the gearbox, stripping everything down. A large volume of swarf, metal chips and bits of burned friction material from the clutches had found its way into the hydraulics.

Front drum

I managed to clamp down and disassemble the clutch packs. All the friction material (an organic fibrous material and come cork) on the friction rings was gone, burned to a crisp. The six light spots are bright patches worn into the metal.

Rear drum

The rear drum was significantly more difficult to take apart. It is not internally spring loaded and is a tight interference fit. It was less badly worn but still unserviceable.


With the drums apart, I checked the thickness of the thrust washer shims. They were below limits.

I ordered a pair of new ones. The copper is completely worn through on one of the old ones.

Valve block

The cleaning continued. I disassembled the valve block, which was all partially jammed up with dirt and swarf. The bores and pistons were all in good shape, and upon reassembly all move well.

Servo spring

I started to disassemble the rest of the hydraulics. The front servo relinquished a fractured spring. This is going to be the main causal reason the gearbox failed, with this slipping there is extra stress on the clutches.

4-3 downshift valve

I invested in a large screwdriver and was able to remove all the plugs from the front servo. The 4-3 downshift valve (above) is a good example of how dirty the hydraulics in the system were.

I had a little difficulty disassembling the front oil pump, because the thing was held together with 4 Fillister head screws, all of which were stuck fast with sealant and all had been previously rounded off. One needed to be sawn down and another drilled. The pump itself was in surprisingly good shape inside, considering the amount of swarf in the rest of the pipes. The overpressure relief valve (the dark circle on the bottom right of the right hand assembly above) was also jammed, so that was freed up and cleaned out.

Rear servo, in pieces

The rear servo is fully spring loaded. It holds the rear band on when the gearbox is not pressurized, meaning that if reverse is engaged it has both reverse and 3rd gear selected simultaneously. This effectively locks the gearbox solid and acts as a parking brake.

Rear servo, assembled

I managed to reassemble the rear servo without losing any parts or breaking any of the oil rings. There’s enough tension in the spring to launch the thing into orbit.

Reverse gear

Finally, the reverse gear assembly.

The reverse gear cone is a little scored- turns out there’s been some swarf stuck in it and that’s caused hotspots. I picked all the steel out of the alloy. Thankfully this mechanism only comes into contact with the drum when reverse is engaged, and only slips of the vehicle is moving when that is done.

A good drum

In contrast, a new drum is fairly smooth. (Above).

Special tool

The annular piston that forms the central drum cone is held in by a disc and a snap ring. The disc needed to be pushed down, compressing the springs so the snap ring could be removed.

Modified valve core

I trimmed the base of a valve core flat so it would seal against the case.

Reverse cone removed

Application of compressed air saw the piston removed. Surprisingly, this was the only part of the system not full of swarf and dirt. The seals were fairly new (though in bad condition) and the roller bearing at the back is quite new also. The reason for this became apparent when I found half of the wavy washer under the retainer in the drum. It appears that fractured, came loose and jammed the reverse gears, causing a bit of damage. Someone had taken it apart, done half a job to pull the pieces out and thrown it back together.

Removing seal

I overhauled the reverse mechanism. Removed the seals, cleaned the grooves they sit in. Whoever was inside the gearbox previously neglected to do this.

Cleaning sealing surfaces

The sealing surfaces were not very clean so I rubbed then back with 1000 grit paper until they were smooth.

New seals were applied.

Reverse gear assembly

I reassembled it all, having to improvise another special tool to get the outer lip seal to be able to get past the step the static outer cone sits in. Now awaiting new bearings.

Governor and rear oil pump

The governor assembly was also in need of cleaning. The smaller governor weight would stick at the bottom of it’s bore because it was worn. I lapped it carefully with some metal polish and now it doesn’t jam- the piece it used to get hung up on doesn’t need to seal, it simply needs to prevent the piston bottoming out.

The governor is two parts- the left hand one has a large weight and opens up that valve at low road-speed. That engages the reverse gear lockout pin. The smaller of the two pins is thrown out at higher speed, and provides pressure to the valve block to engage different gears as the speed of the car increases. If the pin sticks then it will change gear too late each time.

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