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.
Automatic transmissions are often avoided like the plague, even by highly skilled mechanics and automotive technicians. It seems that these days, because of the complexity of modern electronically controlled 4-, 5-, and 6-speed units, overhaul and repair has been limited to the dealership level. Few if any automotive repair shops will touch one, and in almost all cases a complete unit is ordered from a transmission rebuild shop or commercial remanufacturing facility. In the grand scheme of things, it saves time and money, but it doesn’t increase or improve the skills of the automotive mechanic. As with many other items, including smaller things, such as starters, alternators, master cylinders, brake cylinders, or calipers, the trend is simply to order a complete unit rather than take the time to completely and correctly rebuild the original factory components.
Because many hobbyists are not always concerned with time and efficiency when making their automotive projects operational, they have the option to perform many of the repairs themselves. As far as the automatic transmission is concerned, these efforts save great expense. It is simply one of the areas of this hobby where you can get a better overall end result, learn a great deal about a somewhat complex automotive part, and have the satisfaction of doing it yourself. Knowing and understanding the design and function of the internal components, as well as the basic fundamentals of how an automatic transmission works helps a great deal.
The automatic transmissions referenced in this book are all GM units that spanned more than 40 years of use in a wide variety of applications. Many similarities exist between some models, and some are simply based on the originals, or are highly improved versions. The TH350 doesn’t actually share any parts with the TH400, but the fundamentals for rebuilding it are the same, as is the basic procedure. The TH350 is slightly smaller and has lighter internal components, but uses the same basic principles to provide three forward speeds and one reverse.
All of the units covered in these pages use a torque converter that drives an oil pump located in the front housing, which is bolted to the case. The torque converter (which I cover in greater detail later in this chapter) is bolted directly to the engine’s crankshaft, and transfers all of the energy to the rear wheels. It routes these forces through the transmission’s internal components, and allows for enough slippage so that the vehicle can remain stopped at idle speed, and is efficient enough to provide very close to complete energy transfer at various vehicle speeds. Torque converters equipped with an internal clutch can actually be employed once the vehicle is in motion. They provide a complete or solid lock between the engine’s crankshaft and transmission’s input shaft, just like the clutch does in a manual transmission.
The torque converter serves double duty. At the same time that it turns the transmission’s input shaft,
