My Procedure for Rebuilding the TH-425
At the shop I have three, tranny-holding fixtures mounted on the teardown table. I also have a roll-around cart with a tranny holding fixture. I can teardown four trannies side by side. I have several different fixtures for a variety of transmissions, but these days all I work on is the Turbo-Hydramatic 425.
Mostly I teardown three trannies at a time. At the beginning this was important because there are differences between these trannies. I did alot of research on the reason for these differences. Most are factory, some are after market. If it was for reliability and performance, I implement those changes on the earlier trannies so that they perform the same as the newer trannies.
I start by removing the 18 bolts holding the chain cover. Sometimes the cover needs to be pried off gently, most times it just comes off.
Next, with a measuring tape, I squeeze the chain together in the middle of the sprockets and measure the distance. For me, at 7-1/4” or more, the chain is still good. Less than 7-1/4”, I replace the chain.
With a large snap-ring pliers, I disengage each snap-ring in the sprockets.
With a wide pry bar, I lift one sprocket, then the other until I can pull the driven sprocket out. Once the driven sprocket is out, the drive sprocket can be pulled out.
I check the driven sprocket bearing, whether or not the race rotates, the shaft is pressed all the way down, and for groves on the area the bushing mates.
I check the drive sprocket bearing, often the bearing is loose and the race rotates around the shaft. The bearing is removed and a new bearing installed at this time. If it has a Teflon oil ring, it is removed. I take the sprocket assembly to the Lathe and machine the oil ring groove. A new metal oil ring is installed.
With the sprocket at the lathe, I also check sprocket balance and runoff. I also do the sprocket balance and runoff for the driven sprocket.
Both assemblies are put aside for installation later.
Unbolt the 23 pump plate bolts to the case, five bolts from the driven sprocket support, and six bolts from the stator support to the pump.
Using the wide prybar to leverage the pump plate from the case and pull up. Once out, remove the two oil rings and washer from under the driven sprocket support. I discard the oil rings and measure the thickness of the thrust washer for later reference. The three parts go in the washer for a good cleaning.
I use the hydraulic press to remove both sprocket supports.
The drive sprocket support stator tube is checked for oil ring grooves. Stator tube bushing is removed and a new bushing installed.
The driven sprocket bushing is also replaced.
The pump plate is checked for grooving and wear on the area where the pump rides. The oil return check valve is checked and placed back.
With new gaskets, the sprocket supports are pressed back into the pump plate. New thrust washer and oil rings are installed. This assembly is put aside, ready for install.