TransGo SK400 Shift Kit, GM TH400 Turbo 400 3L80

The TransGo SK400 is the classic valve body recalibration kit for the GM Turbo Hydra-Matic 400, the three-speed automatic most people just call the TH400, Turbo 400, or 3L80. It reworks the shift timing and raises apply pressure inside the valve body so the clutches and bands lock up firmly instead of dragging through the shift, which is what burns them up in a stock unit that is towing or making power.

It fits the TH400 and 3L80 from 1965 up, the versions found behind small blocks, big blocks, and diesels in Chevy, GMC, Buick, Oldsmobile, Pontiac, and Cadillac cars and trucks, as well as swap builds. The kit is installer-adjustable: set it up for a firm street shift you can live with every day, or for a heavier full-manual feel on a truck or strip car. It does not change the 3-speed ratios, it changes how hard and how cleanly the unit shifts.

Installation is done at the valve body. On most TH400s you can drop the pan, pull the valve body, install the supplied springs and check balls and make the plate changes the instructions call out, and reassemble with the transmission still in the vehicle. Read the instruction sheet fully before you start, because you choose your shift feel as you build it, and follow the drilling and spring steps exactly.

This is the cheapest insurance against a slipping, flaring TH400. Slow, mushy shifts let the clutches slip and cook the fluid, and once that starts the unit is on its way to a rebuild. Firming the shifts with the SK400 keeps the apply members from slipping, drops clutch and band heat, and is the first thing a lot of builders do to a stock core before it ever sees a hard load. Ships from the USA.