I set two goals for this book: to provide the information I’ve learned over many years to others and to present that information in a way that puts this task well within reach of the average enthusiast. Basically, I wanted to make the information available and also keep it simple and easy to understand. In addition, I want to show you that you really can do the work at home, by yourself, and make a lot of what you need to accomplish the task from common hand tools, or other items typically found in most home garages.
At a glance, transmissions are somewhat complex and contain a lot of parts and components. The good news is that with the help of this book, a lot of the complexity will be taken out of the equation. Transmissions are actually quite easy to rebuild. These units in particular are older designs and relatively simple in construction and function. There is no need to be afraid of them, and armed with the correct knowledge, tools and information, they are certainly not beyond the scope of the average hobbyist. The user still needs to become familiar with the basic theory and design of the units, and which tools and equipment are going to be required to complete the rebuild. Once armed with knowledge and the required tools, you can proceed. If you are pulling your transmission out and putting it back into the vehicle yourself, you may want to reference Chapter 3 (removal) and Chapter 7 (installation) first, for additional information, helpful hints, tips, etc.
The TH350 is an automatic 3-speed transmission. It is considered by many to be one of the best automatic transmissions ever built. It was introduced in 1968 and developed to replace the Powerglide and Super Turbine 300 2-speed units used by General Motors through the late 1960s. The TH350 quickly became known for its strength, versatility, compactness, and availability. They were available in both Chevrolet and BOP (Buick, Oldsmobile, Pontiac) bolt housing patterns, and also produced with a universal bolt pattern to cover all Chevy/BOP applications. Short-tail–shaft versions were also used for four-wheel-drive applications with the transfer case bolted directly to the transmission. Special HD 4x4 cases were also produced in the later 1970s due to their use in Chevy and GMC trucks, Blazers, and Suburbans. These HD cases are easily identified because they have a solid cast aluminum bolt on torque converter cover, and the case is reinforced in the bellhousing area. This special cover had provisions for large strut rods that ran forward and bolted to the engine motor mounts. These were intended to help support the cast bellhousing area of the transmission, as case failures were occurring at a high enough rate to warrant this upgrade.
Very late production TH350 transmissions were also available with a lock-up torque converter. They are somewhat rare and General Motors quickly moved on to the 200-4R and 700-4R units; the TH350 lock-up units were discontinued. I do not cover the TH350 lock-up transmission here. Even so, the rebuilding procedures for them as essentially the same as the standard units, with the exception of the wiring, lock-up solenoid on the valve body, lock-up torque converter, and associated passages that provide fluid flow to the converter for lock up.
General Motors also produced a smaller cousin to the TH350, the TH250. It is similar to the TH350, except that it has a band adjustment screw on the passenger’s side of the transmission just above the pan rail.
TH350 transmissions were produced in great numbers, and many used units are still for sale. They have become even more popular in recent years thanks to their good reputation for performance applications, and even severe duty with the addition of a few aftermarket upgrades. Some controversy exists as to whether the TH350 actually consumes less power than its larger cousin the TH400, but TH350s are more readily available, especially with the Chevrolet bolt housing bolt pattern. They are also a great choice for small-block–powered vehicles and handle the power output of the ever-popular small-block Chevy engine without a lot of additional help during rebuilding.
The factory made numerous changes to the TH350 during its production run, but stuck with the same basic design; most of the internal parts interchange. As with anything else that relates to this hobby, it is always best to use all the original components during the rebuild. If you need to swap out drums, shafts, oil pumps, and other parts, try to obtain that piece from the same year, application, model, etc. As far as upgrades for the TH350 are concerned, the biggest differences were adding Torrington bearings between some of the parts instead of metal or plastic thrust washers. Some plastic and Teflon parts found their way into the later units.
Overall, it is a relatively strong transmission, and suitable for high-performance use. Gear ratios are 2.52, 1.52, and 1.00:1. One of the weakest points in these units is the intermediate sprag/race assembly. High-mileage units often show considerable wear at the center support case lugs as well. The aftermarket produces stronger parts and upgrades for both of these areas. When correctly prepared, the TH350 can handle a lot of power, and it is a good choice for high-performance racing applications. Folks are too quick to jump to the TH400, when a few minor modifications to their TH350 transmission produces a unit that is capable of holding up to great power as does the stronger TH400 transmission. The only difference at that point is the case.
The TH400 case is stronger where it flares up to the bellhousing area. I’ve seen a good number of TH350s break at this location, but all of those were being used in drag racing applications with big horsepower and good traction. However, with that said, pinion angle, engine mounts, and transmission mounts play a huge role in case breakage. I’ve had perfect success with TH350s but also make sure that steps are taken to minimize the twist and flex that can be imposed on them in racing applications.
The basic rebuilding information is the same for all years and models. When applicable, I point out factory upgrades, and places that require close attention when rebuilding these units. They are not overly complicated, or difficult to rebuild. As with any other transmission-rebuilding project, close attention to detail, good parts, and careful assembly is required.
You will also notice that I am replacing all of the bushings in every transmission rebuild. Most of the plastic or metal/brass thrust washers are also replaced. This is not a common practice among transmission builders. I have also found that the factory bushings (in units such as the TH350) may pass a visual inspection, but just about fall out of their locations with hand pressure during removal. This is especially true for the two bushings located in the sun gear, as they get “worked” pretty hard over the life of the unit. Typically, bushings are given a visual inspection and left in place, as some are somewhat difficult to remove and install.
Thrust washers are also reused in most cases, provided they pass a quick visual inspection. From a rebuilding standpoint, it is a good idea to spend the additional time and funds to replace all of the bushings and thrust washers. The results from doing this are all positive for the transmission. Having less fore/aft and up/down movement between the parts reduces gear noise and improves sealing of the rings, which produces a final product that runs quieter, shifts better, and lasts longer.
So, let’s get started . . . .
This chapter is one of the most important in this book.
Transmission rebuilding is not overly difficult, but it does involve attention to detail, and exact precision in every area. This does not mean that completely and correctly rebuilding an automatic transmission is beyond the scope and capabilities of the average automotive enthusiast. Automatic transmissions have always had a mystique about them that keeps away even the most skilled automotive technicians. There is no need to carry your TH350 off to a shop or to someone with more