Fixes the usual Turbo 400 leaks in or out of the vehicle: cork pan gasket, front pump seal, rear seals, pump and rear housing gaskets and O-rings.
A Turbo 400 that shifts fine but leaves a red puddle under the truck does not need a rebuild. It needs the handful of external seals that dry out with age and heat. This kit gathers them in one bag for GM TH400 transmissions from 1965 through 1998, including units badged 3L80 in later GM trucks and the many 400s swapped into hot rods, tow rigs and drag cars. If you are tracing a drip from the pan rail, the tail housing or the front of the bell housing, this is the part to start with.
Every TH400 is sealed to the outside world at a short list of joints: the pan, the pump where the converter hub enters, the case extension where the driveshaft yoke rides, and the fittings and connectors threaded or pressed into the case. The kit covers those joints with a cork oil pan gasket, the front pump lip seal, rear seals, the pump-to-case and extension-to-case gaskets, the large pump body O-ring, and small fluoro-rubber O-rings for the external fittings and the detent solenoid connector. It does not touch anything inside the clutch drums, which keeps the job and the cost small.
Leaks travel. Air flowing under the vehicle carries fluid rearward, so a pump leak can show up at the tail. The GM service manual recommends cleaning the area, running the transmission up to temperature and inspecting with the engine running, then checking again with it off to catch drain-back. A red-dye or black-light check separates ATF from engine oil. The factory leak list includes pan bolts and gasket, extension gasket and rear seal, the connector and speedometer O-rings, the manual shaft lip seal, the front pump seal, pump bolt seals and the pump O-ring. Our TH400 filter and pan service guide shows the pan job step by step, and if fluid is coming from the lines rather than the case, see the TH400 cooler line replacement guide.
Listed for the GM Turbo 400 from 1965 to 1998: Chevrolet, GMC, Buick, Oldsmobile, Pontiac and Cadillac applications plus swapped units. The pan gasket follows the standard 13-bolt TH400 pan shape, and the pump and extension gaskets suit the standard case. Short tail housings, aftermarket deep pans and race cases can use different gaskets, so compare the pieces with your old parts before you start. Send us your year and VIN if you want us to check. All of our Turbo 400 parts are on the TH400 parts page.
If you are pulling the transmission anyway and the fluid smells burnt, step up to a full TH400 overhaul kit so the internal seals and rings are renewed at the same time.
| Fits | GM Turbo 400 (THM400 / 3L80) automatic transmission, 1965-1998 |
| Includes | Cork oil pan gasket, front pump seal, rear seals, pump and rear housing gaskets, pump body O-ring, fitting O-rings |
| O-ring material | Fluoro rubber |
| Weight | 1.35 lb |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
Partly. The pan gasket, rear seal and the external O-rings can be changed in the vehicle. The front pump seal, pump O-ring and pump-to-case gasket need the transmission out.
No. Install it clean and dry and torque the pan bolts evenly to 12 lb ft. Sealer and overtightening both make cork squeeze out and leak.
No. It only stops external leaks. Slipping comes from worn clutches, bands or internal seals and needs an overhaul kit and wear parts. Low fluid from a leak can cause slipping, so correct the level and recheck.
The kit is listed for the GM Turbo 400 through 1998, which covers the 3L80 name GM used later in the run. Compare the pan and extension gaskets with yours before installing.
Check the converter hub for nicks or grooves and the pump bushing for wear. A worn bushing lets the hub move off center and a damaged hub cuts the seal lip.
