TH400 Valve Body Shift Kit, Trans-Scat 400000 66-91

This is the Trans-Scat valve body recalibration kit for the GM Turbo 400 (TH400 / Turbo Hydra-Matic 400), part number 400000, covering 1966 through 1991 units. It is a shift-quality kit, not a rebuild kit: it retunes what the valve body already has so the transmission applies clutches faster and holds line pressure where a stock TH400 lets it fall off.

The stock TH400 valve body was calibrated for a soft, slow apply. That long apply time is what burns the direct clutches and the intermediate band in a truck that tows or a car making more torque than the transmission left the factory expecting. Slipping a clutch pack for an extra half second on every 1-2 and 2-3 is what puts the friction material in the pan. This kit shortens the apply, which is why a firmer shift actually makes the unit live longer instead of shorter.

It is a bench job with the pan and valve body off. Drop the pan, pull the filter, unbolt the valve body, and follow the kit instructions for the spring, check ball and separator plate changes before you bolt it back up. You do not have to pull the transmission out of the vehicle to do it. Plan on fresh fluid and a filter when you are done, because the pan is already off.

Full automatic operation is retained -- the transmission still shifts on its own in Drive, and downshift and kickdown control are improved rather than removed. It is not a transbrake, not a manual valve body, and not a reverse-pattern kit. If you want a street or tow TH400 that shifts crisp and stops cooking clutches, this is the change to make. Kit ships as parts and instructions, 1.16 lb, 11.8 x 10.1 x 2.1 in packaged.