Recalibrates the GM TH400 valve body for firm 1-2 and 2-3 shifts and better kickdown control, while keeping full automatic operation. Fits 1966-1991.
A TH400 that shifts fine but soft - a slow, mushy 1-2 or 2-3 that lets the clutches slip for an extra beat on every shift - is exactly what this kit is built to fix, and it is a different job than replacing a worn-out valve body. This is the Trans-Scat 400000 shift recalibration kit for the GM TH400, covering 1966 through 1991 units, and it works on top of your existing valve body rather than replacing it.
A worn TH400 valve body has damaged bores that leak pressure no matter what springs or check balls are in it, and the only real fix for that is a new casting. A healthy valve body that simply shifts soft is a different problem: the factory calibration was set up for a smooth, slow apply, and that long apply time is what lets the direct clutches and intermediate band slip for a fraction of a second on every shift. Over miles, that slip is what puts friction material in the pan. This kit swaps the springs, check balls and separator plate inside your existing valve body to shorten that apply time, so the clutches lock up faster and line pressure holds instead of sagging off during the shift. Full automatic operation is retained - the transmission still shifts on its own in Drive - the kit just changes how firmly and how quickly it gets there.
This is the right part for a tow vehicle or a performance-oriented TH400 that is still shifting under its own power but feels soft, or for anyone who wants to stop cooking clutches on a unit that has been slipping through its shifts for years without actually failing yet. It is not a fix for delayed or no engagement into gear - that symptom usually traces to a worn pump, low line pressure from a different cause, or clutch wear rather than shift timing, and our TH400 delayed engagement guide covers how to tell those apart before you order. Our TH400 common problems and fixes guide covers the drum, servo and clutch failures that a firmer shift calibration helps prevent going forward.
Part 400000 fits the GM TH400 (Turbo Hydra-Matic 400) from 1966 through 1991. Because this kit modifies an existing casting rather than replacing it, your current valve body needs to be in sound mechanical condition - if the bores themselves are worn or scored, this kit will not restore lost pressure the way a new casting does. See the full TH400 parts collection for the full OEM-pattern replacement valve body if yours needs to be swapped outright rather than recalibrated, or the full manual, reverse-pattern valve body if you are building a dedicated race unit instead of a firmer street or tow setup.
This is a bench job done with the pan and valve body off - the transmission does not need to come out of the vehicle. 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 bolting it back up. Since the pan is already off, this is the right time for fresh fluid and a filter; our TH400 overhaul kit covers the gaskets and seals for that part of the job. If you want an even firmer calibration than the base kit gives you, our TH400 shift improver kit offers a Stage 1 or Stage 2 option built on the same principle.
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Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
It is a kit - springs, check balls and a separator plate that go into your existing TH400 valve body. It does not replace the casting.
Yes. Full automatic operation is retained in Drive - the kit firms up and speeds up the shifts, it does not convert the unit to manual control.
No. It is a bench job with the pan and valve body removed and reinstalled in the vehicle - the transmission itself stays in place.
The GM TH400 (Turbo Hydra-Matic 400), 1966 through 1991.
This kit recalibrates spring pressure and timing - it will not restore pressure lost to worn or scored bores. If your valve body is worn out rather than just shifting soft, our full OEM-pattern replacement valve body is the right part instead.
Change the fluid and filter at the same time, since you are already dropping the pan for the kit - our TH400 overhaul kit covers the gaskets and seals for that.
