March - April 2002 Motor State Performance Report Article By Jim Kaekel, Jr.
Nearly forty years after its introduction, the General Motors Turbo Hydramatic 400 automatic transmission continues to fill the needs for street and strip enthusiasts alike because of its inherent durability. If built properly, the TH-400 can tolerate a significant amount of abuse, behind the stoutest of powerplants.
First produced in 1964 by GM's Hydramatic Division, the TH-400 could be found in passenger cars and trucks up until 1990 when production was halted. The unit proved so successful over the years that Hydramatic engineers used many of the same components when they designed the fully computer-controlled 4L80-E transmission as a replacement for the TH-400.
The TH-400's major drawbacks are its heavy static and rotating weights. An average unit tips the scales at around 135 lbs. with a whopping 55 lbs. of internal rotating weight. That's heavier than the TH-350 or the aluminum Powerglide, but the TH-400 offers much more durability than either of them, particularly in heavy, large cubic inch vehicles. Otherwise, there are very few weak spots and those that do exist can be easily remedied.
A word of caution. When selecting a TH-400 for street or strip duty, avoid single range '64 units which used different hydraulic circuitry and will not accept aftermarket manual or trans-brake valve bodies. Five different bellhousings bolt patterns were offered on the TH-400 over the years to mate the transmission to several engines. Some of which were not GM. The two most popular cases are the Chevrolet and the Buick/Olds/Pontiac units. Transmissions were offered with 4" or 12" tailhousing lengths. Be prepared to pay a pretty penny for the Chevrolet unit as the going rate for a rebuildable core is over $150.00. Less expensive Buick, Olds and Pontiac TH-400's can be readily adapted to Chevy engines with commercial adapters.
Let's look at some upgrades to this venerable transmission. Because of the TH-400's popularity, there is a vast selection of aftermarket parts including shift kits, heavy duty sprags, forward clutch hubs and low ratio gear sets. The accomplished builder can assemble a virtually indestructible transmission. Those who choose to rebuild a TH-400 themselves should acquire a quality transmission manual such as a "Motor Automatic Transmission Manual" or Motorbooks International's "How To Work With And Modify The TH-400 Transmission" by Ron Sessions before tackling the project. Extra life can be achieved through the installation of an extra capacity, deep oil pan. Deep, cast aluminum pans are available from TCI (#TCI228000) and B&M (BMM20280). There's also a factory version of the deep pan that might be located in a scrap yard, because in the 70's, TH-400's installed in 3/4 and one ton Chevy/GMC trucks were fitted with deep steel pans that held two extra quarts of transmission fluid. The OEM pan used an extended pick-up tube and an extra long filter bolt and spacer to help control fluid foaming in towing or snow plowing applications. Extra fluid capacity lowers fluid temperatures. Another common sense upgrade is replacement of the original filter with a high flow, screen type unit. Available from TCI (#TCI228500), this filter flows significantly better than OEM style filters. When installing the filter, use two "O"-rings where the pickup tube seals to the case to eliminate poor sealing at this point.
When overhauling this transmission, there are a couple of areas that need special attention. The intermediate sprag assembly, a one way clutch that must hold firmly when the unit is shifted into second, can occasionally fail. The stock sprag was offered in two versions, the early ('64-'71) sprag clutch and later ('72-up) roller clutch. Neither is well suited to extreme applications, however. An aftermarket sprag with 34 elements, manufactured by Borg-Warner, is available from TCI (#TCI227900) and B&M (#BMM20279). When installing this sprag, the builder must use a '71 or earlier direct clutch drum.
When rebuilding each clutch pack, replace the waved steel plates located at the bottom of the stack in the intermediate and direct clutch packs with flat steel plates. GM used waved plates to give a little extra cushion to the shift. The waved plate located in the forward clutch pack should be left alone since excessive harshness could be experienced when dropping the gear selector into the "drive" position when the vehicle is stopped.
Serious street and drag race-only applications call for the replacement of the bronze and selective thrust washer between the case and the output carrier with a needle thrust bearing (#TCI224400). This washer must survive a tremendous amount of load and its failure will often damage the rear of the case. Replace the nylon thrust washers, which can melt under extreme conditions, with ones made of bronze.