TH400 Overhaul Kit 1965-1998, new and ready to install. Fits the TH400. Ships fast from the USA.
A tired Turbo 400 in a pickup, a motorhome or an older GM car usually shows up the same way: a slip into second, a flare when it shifts to third under load, and a drip under the pan that never stops. This kit is meant for that rebuild. Although it is listed as an overhaul kit, it goes well past a gasket set: it pairs premium friction plates with a complete Transtec gasket and seal set and adds the wear parts most stock 400s need at teardown, including the band, the modulator and the two main bushings. It covers TH400 units from 1965 through 1998.
Steel plates are not listed in this kit. Inspect your steels at teardown: flat, unscored steels without heat discoloration can be reused after cleaning, while blued or warped ones should be replaced.
The TH400 is a three-speed built around forward, direct and intermediate clutch packs, a front and a rear band, and two one-way clutches. Second gear depends on the intermediate clutch and the intermediate sprag, while the front band holds the sun gear for manual second and engine braking. The vacuum modulator is the part that tells the valve body how hard the engine is working: it reads manifold vacuum and raises line pressure and shift firmness under load. When its diaphragm splits, line pressure drops, shifts go soft and late, and the engine draws ATF through the vacuum hose and burns it. That is why a modulator is one of the most useful extras in a kit like this. The front pump bushing supports the converter hub and the extension housing bushing carries the driveshaft yoke; worn ones cause leaks at the front seal and tail seal and let the converter hub scuff the pump.
Pull the vacuum hose off the modulator and look for ATF inside; fluid there confirms the diaphragm has failed. Then drop the pan and check the debris. Heavy clutch material means a rebuild, not just a modulator. Our TH400 common problems and fixes guide lists the usual failure points, and the TH400 filter and pan service guide explains what to look for in the pan. If line pressure stays low with a good modulator, read TH400 pump failure symptoms before you buy, because a worn pump will not be fixed by this kit.
This kit is listed for the GM TH400 from 1965 to 1998, which includes the 3L80 version in GM trucks, vans and motorhome chassis. Clutch counts and some details changed by model code and application over those years, and 1965-1967 switch-pitch units use a different pump and stator setup. Match your ID plate, or send us the year and VIN so we can confirm before shipping. Browse every part for this unit on our TH400 parts page.
Soak the frictions and the new band in ATF before assembly, and lube the sealing rings and lip seals with assembly gel. Drive the new pump and extension housing bushings in square with a bushing driver. Check clutch clearances and total endplay as you assemble. After installing the new modulator, road test and confirm firm shifts under throttle. Refill with Dexron-type ATF. If your clutch drum shows a cracked hub or worn bushing bores, a replacement TH400 clutch drum is the time-saving fix while the unit is open. Many builders also add a TH400 shift kit at this point for firmer shifts.
| Fits | TH400 1965-1998 |
| Part type | Overhaul Kit |
| Condition | New |
| Brand | OEM |
| Ships | From the USA |
| Warranty | 1-year |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
Steels are not listed in this kit. It includes premium frictions, a Transtec gasket and seal set, sealing rings, pump and extension housing bushings, the intermediate band, a vacuum modulator and a filter. Reuse your steels only if they are flat and not discolored.
The intermediate band, which is the front band in a TH400. The rear (low and reverse) band is not included, so inspect it at teardown.
It is listed for 1965 to 1998 TH400 and 3L80 units, which covers most GM truck applications. Send us the year, VIN and transmission ID plate if you want us to double check.
The vacuum modulator sets line pressure and shift feel based on engine load, and a leaking diaphragm is one of the most common TH400 problems. Replacing it during a rebuild prevents soft shifts and fluid loss after the job is done.
Use a Dexron-type automatic transmission fluid. Check the level with the engine running and the fluid at operating temperature.
