Sept. - Oct. 1996 Lane Racing And Rodding Article By Jim Kaekel, Jr.
Plentiful and durable would be an accurate description of the GM Turbo-Hydramatic 350 three-speed automatic. While it may not have the torque capacity of it's "big brother," the TH 400, it can certainly fulfill the needs of many small block bracket and class drag cars.
Three-speed automatics are usually preferred when vehicle weight exceeds 3200 lbs. with driver. The TH 350, price wise, is a cheaper transmission to build, due in part to the high core prices of Chevrolet TH 400's and two-speed Powerglides.
Locating a suitable core should be relatively easy, as literally tens of thousands were built from 1969 to 1986. However, there are a couple of different versions that should be avoided for serious performance usage. The TH250, which was introduced in 1973, is identified by the location of a band adjuster on the right (passenger) side of the case. The 350C began production in 1980 equipped with a lock-up torque converter and is identified by an electrical connector on the left (driver's) side of the case.
Be sure the bellhousing bolt pattern matches the engine that you are using. Three different styles were produced; the Chevrolet pattern, Buick-Olds-Pontiac, and a universal bellhousing that is relatively rare. Also, three different tail housing lengths were produced in 6", 9" and 12". The vast majority of applications will use the 6" version.
To ready a TH 350 for serious street action or drag racing, proper preparation and cleanliness are essential for long transmission life. When a suitable core is located it must be stripped, cleaned and inspected for cracks and stripped threads. A transmission repair manual would be invaluable at this point, especially to a novice transmission builder. HP Books offers a book (HP 511) on the TH 350 which has many detailed photos which can also aid during the rebuilding process.
Once the case has been inspected, begin by replacing the lip seals in the low-reverse clutch, which is located in the rear of the case. A master rebuilding kit (Coan #32107) will be necessary as it contains a complete paper and rubber set, metal clad seals, sealing rings, clutches and steels.
The case bushing should now be inspected and replaced if necessary. The TH 350 is particularly noted for needing a full set of bushings (Coan #32500) and thrust washers (Coan #32400) at time of rebuild. Proceed by installing the output shaft, rear planetary and low-reverse clutches. The planetaries should be thoroughly inspected to verify that the gears are in good condition and spin true on their individual needle bearings.
The low-reverse clutch support can be installed after the roller clutch has either been found satisfactory or replaced. Next, install the inner sprag race, sun gear shell and front planetary, after they have been carefully examined.
Proceed by replacing the lip seals in the forward and direct clutch drums. A press would be very helpful when rebuilding these drums to aid in piston removal. It is very important that all of these seals are replaced throughout the unit since they harden due to age and internal transmission heat. Five clutches and five steels should be installed in each respective drum for performance applications. The direct drum was produced with only four clutches and four steels from the factory. To increase the clutch capacity by one, the piston will have to be milled about .170" (thickness of one clutch and steel) by a machine shop. This will increase torque capacity when the transmission is shifted into high gear. Be sure to check the sealing ring bores of both drums as the direct drum is particularly noted for having the bore wear heavily. If this is the case , it should be replaced to correct any oil pressure leakage in this area. Recommended clutch clearance for the direct drum is .010"-.012" per clutch. For example, five clutches and five steels would require .050"-.060" clearance. The forward clutch can be run somewhat tighter since the only time it isn't applied is in reverse.
The intermediate sprag assembly, located on the direct drum, should be thoroughly inspected. The outer race should be replaced (TCI #328900) with a heavy duty replacement. This sprag assembly is the most fragile part of the transmission and the whole assembly should be replaced in ultra high horsepower or big block applications with a special drum that has a 36 element sprag with custom (Coan #32802) inner and outer races.
After the forward and direct drums are installed, the band and intermediate clutches can be installed. The band has no adjustment and is only applied in kickdown and manual low. This band is commonly discarded when a manual valve body is installed.
The front oil pump can now be cleaned, inspected and rebuilt. Inspect the pump gears and body for wear and replace as necessary. Be careful when replacing pump gears and bodies as four different thicknesses of gears were produced; .721", .723", .725" and .727". After the front bushing and seal are replaced, intermediate clutch re-sealed, and sealing rings replaced, the pump can be assembled and installed. End play can be checked with a dial indicator by prying up on the input shaft. It should be between .010"-.050" and can be adjusted by use of selective washers located on the back of the oil pump cover.
The valve body should then be cleaned and inspected to be sure that the valves can move in their bores without sticking. A shift kit (B&M #30262) can be installed to firm up the shifts in street applications. A full manual valve body (TCI #23253) would be advisable for a full time drag car. Manual valve bodies are engineered for full manual up and down shifting with no automatic features or manual low braking. If the transmission is going into a medium to lightweight car, a trans-brake valve body (Coan #32020) would be desirable particularly for serious, high RPM motor combinations. Keep in mind that transmission and converter life are reduced substantially when a trans-brake is installed due to increased load and fluid temperature. A deep oil pan (TCI #328000) , quality cooler and transmission temperature (Auto Meter #3451) gauge should be added when a trans-brake is used.
Taking care in rebuilding and selecting the right parts will ensure your TH 350 will make many a pass down the quarter mile.