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!
It’s crazy that the seal even holds on so long. They look so gosh darn fragile! I really liked all the pics in this one. It can be really hard to get good ones under a car like that.
So glad I found your site. I just finished the rebuild of a 1965 Riviera, and I had the trans professionally built. I did everything else to this car, I just didnt want to deal with transmission issues….which I have dont nothing but deal with leaks. I just had the car back at their shop and they dropped the pan and replaced the seal on the kickdown connector but didnt identify the shaft seal leaking.
with your tips I’ll tackle it myself. Thanks saved me a huge hassle
My th400 does not look like the one in your picture s. There is no exposed seal to pull out. The casing of Trans is made where it covers over seal . It looks like the only way to change seal is fromm inside.
Good question Jim. I haven’t had to do that before, but I did find a good forum thread about it HERE
The post that talked about it was by ‘bruiser’ a little over halfway down. Here is what he said: “I just took my shaft out on my 77 th350. It slides out without having to lower the transmission. First you get the pan & filter off. Then you remove 1 bolt that holds the lever that touches the rooster comb. Remove the wide clip on the selector shaft. Then remove the 11/16 nut on the selector shaft inside the transmission. The shaft should slide out with a little prying and turning.”
It’s crazy that the seal even holds on so long. They look so gosh darn fragile! I really liked all the pics in this one. It can be really hard to get good ones under a car like that.
So glad I found your site. I just finished the rebuild of a 1965 Riviera, and I had the trans professionally built. I did everything else to this car, I just didnt want to deal with transmission issues….which I have dont nothing but deal with leaks. I just had the car back at their shop and they dropped the pan and replaced the seal on the kickdown connector but didnt identify the shaft seal leaking.
with your tips I’ll tackle it myself. Thanks saved me a huge hassle
My th400 does not look like the one in your picture s. There is no exposed seal to pull out. The casing of Trans is made where it covers over seal . It looks like the only way to change seal is fromm inside.
how do you replace shifer shaft on 350 chevy trans not the seal
Good question Jim. I haven’t had to do that before, but I did find a good forum thread about it HERE
The post that talked about it was by ‘bruiser’ a little over halfway down. Here is what he said: “I just took my shaft out on my 77 th350. It slides out without having to lower the transmission. First you get the pan & filter off. Then you remove 1 bolt that holds the lever that touches the rooster comb. Remove the wide clip on the selector shaft. Then remove the 11/16 nut on the selector shaft inside the transmission. The shaft should slide out with a little prying and turning.”