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An external reseal kit replaces only the seals and gaskets on the outside of the case: the ones that keep fluid off the driveway. It is the right buy when a TH350 or TH400 shifts fine but marks the floor, and the cheap insurance to install any time one of these transmissions is out of the vehicle.
It is not a rebuild. If the transmission slips, flares or engages late, the problem is inside and you need an overhaul kit.
Which Kit Fits Your Transmission
| Kit | Fits | What Is In It |
|---|---|---|
| TH350 external reseal kit | THM-250, 250C, 350, 350C and 375B, all years | About 30 pieces: metal-clad front, rear and shift linkage seals, speedometer housing seal, filler tube O-ring and boot seal, detent cable O-ring and boot |
| TH400 external reseal kit | THM400 / 3L80, 1965-1998 | Cork pan gasket, front pump seal, rear seals, pump-to-case and rear housing gaskets, large pump body O-ring, fitting and detent solenoid connector O-rings |
The two are not interchangeable. The TH350 uses a detent (kickdown) cable; the TH400 uses an electric detent solenoid with a case connector, and the pump, case and rear housing are different castings. Check the pan shape if you are not sure which you have, or see TH350 vs TH400 differences.
Find the Leak First
| Where the Fluid Is | Likely Source | Trans Out? |
|---|---|---|
| Bottom of the bell housing | Front pump seal or pump body O-ring | Yes |
| Tail shaft and slip yoke | Rear seal | No, pull the driveshaft |
| Along the pan rail | Pan gasket, often a split or crushed cork gasket | No |
| Around the speedometer gear | Speedometer housing seal | No |
| At the dipstick tube | Filler tube O-ring or boot seal | No |
| Side of the case at the linkage or kickdown | Shift linkage seal, detent cable O-ring (TH350) or connector O-ring (TH400) | No |
Clean the case with degreaser, drive it, and look again before you buy. Fluid runs back and down, so the lowest wet spot is rarely the source. For more detail, see the TH350 leak spots and TH400 leak spots write-ups.
One leak a reseal kit will not fix: a ruptured vacuum modulator diaphragm. It pulls ATF into the engine through the vacuum line, so the fluid level drops with no drip and the exhaust may smoke. Pull the vacuum hose off the modulator; if there is ATF in it, replace the modulator.
Reseal Tips From the Bench
- Install the cork pan gasket dry on clean flanges and torque the pan bolts evenly in a crisscross pattern so the cork does not split.
- Drive the metal-clad seals in square with a seal driver and lube the lips with ATF.
- Check the yoke surface. A groove worn by the old rear seal will cut the new one. Replace the yoke or use a repair sleeve; see extension housing bushing and seal replacement.
- Check the converter hub before installing a new pump seal. A scored hub leaks again quickly.
- Do the pump seal when the trans is out anyway. It is a few dollars of parts, and the only way to reach it is with the converter removed.
Shop the Part
We stock external reseal kits for both the TH350 family and the TH400, plus overhaul kits and hard parts. Free shipping on every order.
TH350 Reseal Kit → TH400 Reseal Kit →All TH350 Parts All TH400 Parts
Need a complete rebuild kit or a hard-to-find hard part? Our sister store Core Transmission Parts carries the full lineup.
FAQ
Does an external reseal kit fix a slipping transmission?
No. It replaces only the outside seals and gaskets. Slipping, flaring or late engagement come from internal seals, clutches or bands and need an overhaul kit.
Can I reseal a TH350 or TH400 without removing it?
Mostly. The pan gasket, rear seal, speedometer seal, filler tube seal and linkage or detent seals can be done in the vehicle. The front pump seal and pump O-ring need the transmission out.
Is a TH350 reseal kit the same as a TH400 kit?
No. The cases, pumps and detent systems are different. The TH350 kit also fits the THM-250, 250C, 350C and 375B.
Should I use sealer on a cork pan gasket?
No. Install it dry on clean, flat flanges and torque the bolts evenly. Over-tightening splits cork gaskets.
My TH400 loses fluid but nothing drips. Why?
Check the vacuum modulator. A ruptured diaphragm pulls ATF into the engine through the vacuum line.