on your own car or truck and get perfect results from your efforts. To this day, I have never understood why hobbyists farm out many of the components of their cars to others instead of rebuilding them in their own shops or garages. Transmissions, in particular, are at the top of the list for the items that get farmed out. There has always been a mystique about transmissions that makes even the most skilled automotive technicians avoid them like the plague! They also get hit with some pretty hefty bills from not doing them at home.
Similar to most other things associated with this hobby, you can get results that are just as good, or better, and save money by doing it yourself. The key component here is information, followed closely by motivation. This book was designed for the hobbyist.
It was a Saturday afternoon in early fall 1978 in the small garage detached from the house we lived in at that time. Without any outside assistance I had decided to tackle a TH350 transmission rebuild. Not that I really wanted to do it; I would rather have gone hunting or someplace with my friends. I still chuckle every time I lift the valve body off of a transmission, and think back to the six check balls that were rolling across the concrete floor in my garage. I had removed the valve body and turned the unit over to allow the fluid to drain out. At that moment I was really thinking that I should have carried it to a shop, but my income level at that time just didn’t have any room in it for a high-performance muscle car that wasn’t moving because I had fried its transmission.
I managed to find five of the six check balls, and after careful observation could see the “tracks” in the case where they were supposed to be. The story has a good ending; although that particular rebuild took me several weeks, and I wasn’t at all confident that the car was going to move under its own power when I lowered it to the ground for a test drive!
Over three decades and hundreds of units later, I’m armed with quite a bit more knowledge, and a toolbox full of homemade tools dedicated strictly to transmission work. I don’t give a second thought to stripping any automatic transmission down to a bare case, then putting it back together with my large assortment of hand-crafted custom tools and self-learned special procedures.
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
