Every external seal and O-ring to stop a leaking Turbo 350 in the truck: front pump seal, rear extension seal, shift shaft, filler tube and detent cable.
A Turbo 350 that drips on the garage floor usually does not need a rebuild. Most leaks on these old GM three-speeds come from the outside of the case: the front pump seal, the tailshaft seal, the shift shaft, the filler tube, the modulator and the kickdown cable. This 30-piece external reseal kit covers those joints for the THM-250, 250C, 350, 350C and 375B, so you can stop the leak with the transmission in the car and without opening the clutch packs.
An external reseal replaces the seals that hold fluid inside the case at every spot where a shaft, tube or cable passes through it. The kit includes metal-clad front, rear and linkage seals that press into their bores and hold their shape, a speedometer housing seal, the filler tube O-ring and boot seal, and the detent cable O-ring and boot. The product photo also shows a pan gasket, a pump gasket and an assortment of O-rings. The filler tube and detent cable pieces matter most, because those joints share a drip point on the right side of the case with the modulator and are often mistaken for a bad pan gasket.
Air flowing under the car pushes fluid rearward, so the wettest spot is not always the leaking joint. GM's procedure is to degrease the case, drive to full temperature, look with the engine running and again with it off, and use dye to tell ATF from engine oil. Our TH350 fluid leak guide lists each leak point with its factory torque. If the fluid is low, also read TH350 fluid type and capacity so you top off correctly.
Listed for all THM-250, THM-250C, THM-350, THM-350C and THM-375B transmissions, any year. That covers the 1969-1986 TH350 in Chevrolet, GMC, Pontiac, Oldsmobile and Buick cars and trucks. Aftermarket tail housings and yokes can use different seal sizes, so compare against your old seal if the car has a short tailshaft conversion. See every TH350 part we carry, or send us your year and VIN if you are unsure.
If the pan rail is dished from years of overtightening, a new finned aluminum TH350 pan with drain plug gives the gasket a flat surface. A fluid loss with no visible drip often means a ruptured modulator; the adjustable vacuum modulator fits the TH350 and TH350C. Refill with a licensed Dexron VI.
| Fits | THM-250, 250C, 350, 350C, 375B (all years) |
| Piece count | 30 |
| Seal type | Metal clad front, rear, and linkage seals; rubber O-rings and boots |
| Includes | Front and rear seals, linkage seal, speedometer housing seal, filler tube O-ring and boot seal, detent cable O-ring and boot |
| Weight | 1.2 lb |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
Most of it. The rear seal, speedometer seal, shift shaft seal, filler tube O-ring and detent cable seal can all be done in the car. The front pump seal requires pulling the transmission and converter.
Yes. It is listed for THM-250, 250C, 350, 350C and 375B units, all years.
The filler tube O-ring, detent cable seal and modulator O-ring all sit on the right side of the case and drip down to the pan rail. Check those, and check the pan rail is flat.
No. It is an external reseal kit. For piston seals and sealing rings, use a full TH350 overhaul kit.
A licensed Dexron VI ATF, which is back-serviceable to the Dexron fluid the TH350 originally used.
