Oil pumps are designed as two halves. You have to take them apart to access the pump gears, bushing, and seal. When they are ready for assembly, you must align the two halves or the pump will not fit back into the case. Make a band from a large hose clamp or put several large hose clamps together. The pump’s bolts should remain finger tight while the band is tightened. Use a Phillips screwdriver or awl to align the bolt holes and then drop the attaching bolts through the holes while the pump’s bolts are tightened. This ensures that the halves do not slip out of alignment. Once the pump’s bolts are tightened, test fit the pump into the case, and then start all of the pump’s attaching bolts by hand. Do not install any seals for the test fitting procedure.
A couple of 5/16-inch bolts can be fabricated and used as alignment studs to install the oil pump.
General Motors threaded several bolt holes on the oil pump. Use two slide hammers to easily remove the pumps from the case without damage.
A large hose clamp can be used to align the pump halves when tightening the pump bolts. If not in alignment with each other, the pump is difficult to install in the case and the bolts may not line up enough to get them started.
Alignment studs guide the oil pump into place during final installation. Use at least two studs. Place the gasket on the case first, then hand thread the studs into the case. To make a couple of studs, simple cut the ends off a couple of long bolts and grind them to a point.
The TH350 and TH400 transmissions have two corresponding pump bolt holes tapped for a slide hammer. It is best to use two slide hammers to pull the pump evenly from the case. You can use only one hammer, but you may have to alternate back and forth between the holes in the pump to pull the pump from the case effectively. Thread a slide hammer into the pump to remove the pump from the case. Make sure to thread the slide hammer at least five full turns into the pump to avoid breaking the tool off or pulling out some of the threads.
Rebuilding the TH350 transmission effectively doesn’t require special tools. If you purchased all the special tools that you could use for a TH350 rebuild, you would quickly offset any savings from doing the rebuild yourself. For that reason, throughout this and other chapters, I provide alternatives: How you can use an alternative procedure, substitute a common tool for a special tool, or use a different procedure to accomplish a particular task for which a special tool would normally be used.
The TH350 1-2 accumulator cover can be very difficult to install. Insert a long 5/16-inch bolt through the pan rail as shown; then use a small pry bar to effectively compress the cover and spring to install the retaining ring without dislodging the seal.
Simple modifications to standard shop tools can make some operations easier and save money at the same time. I have modified a large C-clamp and turned it into a compressor to remove valve body accumulators. The springs are pretty strong under these accumulators, which makes them difficult to compress and remove the retaining clip at the same time.
Even so, there are a few procedures that can be difficult to complete without a special tool. Installing the TH350 1-2 accumulator cover is one of them. The accumulator uses a very strong spring under the cover, which is very difficult to push into position without dislodging the O-ring under it. If the O-ring is dislodged, you’ll have a huge oil leak when the transmission is placed into service. You can use a large pry bar to easily compress the accumulator cover to remove or install the snap ring. You need a long 5/16-inch bolt, which is inserted into one of the oil pan bolt holes. This makes a perfect location for levering the pry bar to compress the accumulator cover.
The accumulators in the valve body of the TH350 and TH400 can be difficult to remove. A modified C-clamp effectively compresses the accumulator piston so you can remove the retaining clip. Large channel lock pliers also work for this purpose. Gently clamping the valve body in a shop vise with soft jaws or wooden blocks makes the job even easier.
Large channel lock pliers also work for this purpose, although an assistant is probably necessary to compress the piston while the retaining clip is removed.
Once fully compressed, the retaining clip is easy to remove with a small screwdriver or needle-nose pliers.
Automatic transmissions have always fascinated me. I can still remember the very first unit that I took apart more than 30 years ago. It was a GM TH400 from an early 1970s Pontiac Bonneville. It seemed like the parts coming off the case never ended. Even the valve body, which is quite simple compared to most other models, contained dozens of parts. I was so afraid that a part or two was going to be left over after the rebuild was complete that I laid each part out very carefully in a long line across the workbench. I also paid very careful attention to the direction in which every part was installed. And also if any thrust washers, snap rings, or other parts were above, below, held a part in the case, or were inside another part in the case.
During the rebuild, each part was given special attention. This was not only to make sure that I didn’t get any piece upside down or out of place, but also to make sure that all parts were in good serviceable condition and could be reused without causing the unit to function incorrectly. My close attention to detail paid off, and after many hours of cleaning, installing new seals, gaskets, sealing rings, and clutch plates, the unit was placed back in service and worked flawlessly in all areas. I have since built hundreds of TH400s and, although I can’t remember the first, I have also built hundreds of TH350s. I always adhere to the same attention to detail and this plan has always served me well.
The GM TH400 transmission is a strong unit and came in several configurations, including two bolt patterns and several different overall lengths. Shown here is a BOP (Buick, Oldsmobile, Pontiac) unit with a short tail shaft. It came in a Chevrolet bolt-pattern version, long tail shaft version, and even a model with a smaller TH350-size output shaft. The unit shown is the most common junkyard or swap meet find, and the TH400 was the factory’s choice for full-sized cars with big-inch engines in the late 1960s through the late 1970s.