TH400 Overhaul Kit 1966-1991, new and ready to install. Fits the TH400. Ships fast from the USA.
If your Turbo 400 lives behind a big-block in a drag car, a street machine or a hot rod truck, and it has started to slip on the 1-2, flare on the 2-3 or take forever to pick up reverse, this is the kit built for that job. It is the TCI 259000 Master Racing Overhaul Kit, catalogued by TCI for the 1966-1991 GM TH400. TCI describes it as an excellent choice for racers who maintain their own transmissions: everything usually required to freshen up a competition 400 in one box, without padding it with valve body parts or rarely replaced pieces you will never use.
TCI states that valve body modifications are not included, and neither are bands, a modulator or hard parts. If you want a calibrated shift package, pair the rebuild with a TH400 shift kit. If you only need a fresh seal and gasket set for a unit that is otherwise healthy, the lighter TCI 228600 racing overhaul seal and gasket kit is the cheaper route.
The TH400 makes three forward gears with three multiple-disc clutches (forward, direct and intermediate), a front band, a rear band and two one-way clutches. Every gear change is a clutch pack applying against hydraulic pressure, so a race 400 lives or dies on friction material and seal condition. Worn frictions glaze and slip when the engine makes more torque than stock, and hardened piston seals bleed off apply pressure so the pack never clamps fully. This kit replaces both halves of that problem at once: new performance frictions and steels for holding power, and new seals, gaskets and bushings so the oil the pump makes actually reaches each piston. The bushings matter more than people think: worn ones let the shafts and drums wobble, which leaks pressure past the sealing rings and lets the rotating parts wear each other.
Pull the pan first. Clutch material, a lot of fine metal or a burnt smell tells you the packs are done and a seal kit alone will not fix it. Before ordering, rule out the cheap causes: a leaking vacuum modulator, low fluid, or a sticking valve. Our TH400 common problems and fixes guide walks through each failure, and the TH400 delayed engagement guide covers the slow drive and reverse pickup that often points to worn seals and bushings. A line pressure gauge on the case port helps too: pressure that is low and will not rise usually means leakage inside the unit.
TCI lists this kit for the 1966-1991 GM TH400, the same box that GM renamed the 3L80 for 1987. That covers the 400s behind Chevrolet, Oldsmobile, Pontiac, Buick and Cadillac V8s as well as GM trucks and vans of that period. If your unit is a 1965-1967 switch-pitch (variable pitch stator) 400, recognizable by its two-prong case plug, or it has been built with non-stock clutch counts, send us your year, VIN and ID plate before you order so we can confirm. The full range of parts for this transmission is on our TH400 parts page.
Soak every new friction in clean ATF for at least 30 minutes before stacking the packs, and coat the lip seals and sealing rings with assembly lube. Measure clutch clearance on each pack as you build it, and check total endplay before the pump goes on; Sonnax recommends four separate endplay checks on a TH400 to prevent gear train damage. Install the new bushings with a proper driver, not a socket, so they go in square. Refill with a Dexron-family ATF, using TCI's capacity chart for your pan and converter. While the case is apart, check the pump gears and stator: a scored pump is a common reason a fresh rebuild still runs low pressure, and a replacement TH400 pump is far cheaper than doing the job twice.
| Fits | TH400 1966-1991 |
| Part type | Overhaul Kit |
| Condition | New |
| Brand | GM |
| Reference | 259000 |
| Ships | From the USA |
| Warranty | 1-year |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
TCI catalogs the 259000 for the 1966-1991 GM TH400, which includes the 3L80 name GM used from 1987. For a 1965-1967 switch-pitch unit or a modified race build, send us the year, VIN and ID plate first.
No. TCI lists performance frictions, steels, seals, gaskets, bushings, a filter and a drain plug kit. Bands, a vacuum modulator and valve body modifications are not in the box, so inspect your bands at teardown and order replacements if they are glazed or worn thin.
It is aimed at people who race or run more power than stock, but it works fine in a strong street build. The performance frictions hold better than standard material, which helps any 400 behind a modified engine.
The TH400 was designed for Dexron ATF, and a current Dexron-compatible ATF is the normal choice. Use TCI's fluid capacity chart to work out the fill for your pan depth plus an empty converter, then check the level hot.
Inspect the pump, bands, drums and planetary parts while the unit is apart. A worn pump or scored drum will undo a new clutch kit. Many builders also add a shift kit at the same time, since the valve body is already out.
