TH400 Shift Improver Kit 20260 Stage 1 or Stage 2

This is the B&M 20260 Shift Improver Kit for the GM Turbo 400 -- TH400, and the same family TH375 and TH475 units -- covering 1965 through 1987. It is a valve body recalibration kit, not a rebuild kit. Nothing in the box replaces a worn clutch or band; what it does is change how fast and how hard the ones you have get applied.

A stock TH400 valve body is calibrated soft. The apply is drawn out on purpose so the shift feels smooth from the driver's seat, and that long apply is exactly what slips the direct clutches and the intermediate band in anything that tows, hauls or makes more torque than the factory planned on. Shift overlap -- one element still holding while the next one comes on -- is the other half of the problem. This kit shortens the apply and cleans up the overlap, which is why a firmer shift makes the unit last longer rather than tearing it up.

There are two calibrations in the one box and you choose which one you build. Stage 1 is the RV and heavy duty setting: noticeably firmer than stock but still civil for a truck or motorhome that gets driven every day. Stage 2 is the street and strip setting, a harder, quicker apply for a car that sees the track. You follow the instruction sheet for whichever stage you want; the spring, check ball and separator plate changes differ between them.

It installs with hand tools with the pan and valve body off -- the transmission does not have to come out of the vehicle. Drop the pan, pull the filter, unbolt the valve body, make the changes on the bench, and bolt it back up. Since the pan is already down, put a new filter and fresh fluid in it while you are there. This kit does not fit the 4L60E and is not a transbrake, manual valve body or reverse-pattern kit -- the transmission still shifts on its own in Drive. Backed by a one year limited manufacturer warranty.