How to fix a leak around the shift shaft on your TH400 / TH350 Chevrolet automatic transmission. This is one of those projects that I have been putting off for a long time, but I should have done it a long time ago because it was really easy.
TH400 and TH350 GM Automatic transmissions are notorious for leaking. My transmission in Ol’ Blue leaked like the Exxon Valdez for 3 years before I finally ran across the information online on how to replace the shift shaft seal myself. I had even asked transmission shops about it to which I only received vague “Oh, that might take some time to fix, bring it in so we can look at it” answers. I feel like they were telling me that with dollar signs rolling around in the back of their head knowing it was an easy job.
I got my tool and seal from NAPA auto parts. At NAPA the Transmission Seal tool is Product Number: BK7769250 and the Man Control Lever Shaft Seal is Product Number: ATP13887.
For those with an Amazon account, here are two links to exactly what you need:
So there you have it. The actual TH400 or TH350 seal replacement only takes about 10 minutes once you get access to the side of the transmission (might have to remove exhaust or crossmembers depending on your setup). Don’t let the transmission shop laugh all the way to the bank, at least try to fix it yourself. Good luck!