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.
These two parts and two wrenches are all you need to do the actual seal change.
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:
First you will have to remove your exhaust/crossmember, or anything else that might be within ~4″ of the shift shaft on your TH350 / TH400. In this picture I have already removed the nut and gear selector bracket. If there is nothing in the way, then the job just got easier.
Next, I took my air hose and blew the mud out of the cavity. Then I inserted the seal removal tool and twisted it clockwise with a 5/8 wrench until it got tight in the hole. With one wrench on the seal removal tool, get a 9/16 wrench and tighten the bolt until the seal comes out.
Looks like this when it comes out. No need to drain the fluid to do this, mine didn’t leak at all during the seal change.
Here you can see the tear in the seal that I took out. This leak has literally been plaguing me for 3 full years. If I knew then what I know now I would have done this a LONG time ago.
Clean off the shaft and the surrounding area. I pre-lubed the seal lip with automatic transmission fluid so it would go on easier. Be VERY careful while putting the seal over the flats on the shaft. If you rip the seal, you get to do it again.
Place the installation tool over the seal once it is on the shaft and lightly tap with a hammer until it stops.
Viola! Now you are done short of putting everything back together. No more listening to your significant other whine about your oil spots in the driveway/garage…
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!