transmission pan leak
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Joined: Aug 2017
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transmission pan leak
Hello everyone, my 69 Corvette steel transmission TH350 pan keeps leaking, I've tried the cork, rubber, and silicone type gaskets but still leaks, someone recommended to try the Moroso gasket #93102 since it is made out of rubber and steel that it might work. shall I just give up on the original steel pan with the raised edge or should I just switch to a flat aluminum pan? Any help would be appreciated. Thanks
Race Director
Joined: Sep 2001
Posts: 10,560
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From: Wellington, FL
St. Jude Donor '11 thru '25
First off, are you sure it's the pan that's leaking? To be sure, I would clean the entire tranny area and linkage and then run the car at idle to see if the leak can be spotted. If the pan is actually leaking, be sure its contact surface with the tranny is perfectly flat, including the holes. Use a flat edge to check. Be aware that there are seals at the linkage on the tranny which over time can leak.
Last edited by Jud Chapin; Jan 16, 2022 at 06:05 PM.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
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From: At my Bar drinking and wrenching in Lafayette Colorado
The stock steel pan, with a stock cork gasket with no silicone sealant, seals up just fine and is leak-free as long as the pan is not warped or damaged. It's a very simple and reliable seal - they don't leak. There are plenty of places for a tranny to leak from, "wicking" the fluid down around the pan gasket to make it appear that the pan is leaking. Your leak is probably coming from one of the other penetrations into the case - not from the pan gasket. To check it, wipe everything clean and then wrap clean, white paper towels around very penetration and seal: Twirl the paper towel into a "string" and wrap it around the perimeter of your pan & gasket and tape it in place. Wrap your speedo cable attach point with a towel. Do the same with the modulator penetration, governor cover, gear selector shaft, and any other penetration into the case. Whichever paper towel turns red is your actual leak point. If more than one turn red, the leak is the highest location.
Lars
Dr. Detroit
Joined: Mar 2012
Posts: 10,097
Likes: 4,027
From: New Braunfels Texas
The stock steel pan, with a stock cork gasket with no silicone sealant, seals up just fine and is leak-free as long as the pan is not warped or damaged. It's a very simple and reliable seal - they don't leak. There are plenty of places for a tranny to leak from, "wicking" the fluid down around the pan gasket to make it appear that the pan is leaking. Your leak is probably coming from one of the other penetrations into the case - not from the pan gasket. To check it, wipe everything clean and then wrap clean, white paper towels around very penetration and seal: Twirl the paper towel into a "string" and wrap it around the perimeter of your pan & gasket and tape it in place. Wrap your speedo cable attach point with a towel. Do the same with the modulator penetration, governor cover, gear selector shaft, and any other penetration into the case. Whichever paper towel turns red is your actual leak point. If more than one turn red, the leak is the highest location.
Lars
The kiss of death on any pan is overtightening.....if you pull the pan and the bolt holes have a "cone" shape to them.....tap them flat with a hammer and flat bar or get a new one. I always tell most customers here to get an aluminum pan......sealing an aluminum pan is to easy......two machined flat surfaces.....
A common leak point on any TH trans is the dipstick tube O-ring as the fill level goes above the pan when shut off......check this.
An old trick a trans guy showed me 30 years ago is to use a 1/4" air ratchet with the PSI set to 50 psi....tightens the bolt to the perfect tension. Try it!
Jebby
Safety Car
Joined: Aug 2001
Posts: 3,685
Likes: 202
From: Waterford Mi
I had a similar issue with the TH400 in my 73
the problem was the casting was full of porosity that nothing would seal
i ended up replacing it with the 200R4 but that was my decision
to fix the porosity I would have welded and recut the seal
the problem was the casting was full of porosity that nothing would seal
i ended up replacing it with the 200R4 but that was my decision
to fix the porosity I would have welded and recut the seal
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
The stock steel pan, with a stock cork gasket with no silicone sealant, seals up just fine and is leak-free as long as the pan is not warped or damaged. It's a very simple and reliable seal - they don't leak. There are plenty of places for a tranny to leak from, "wicking" the fluid down around the pan gasket to make it appear that the pan is leaking. Your leak is probably coming from one of the other penetrations into the case - not from the pan gasket. To check it, wipe everything clean and then wrap clean, white paper towels around very penetration and seal: Twirl the paper towel into a "string" and wrap it around the perimeter of your pan & gasket and tape it in place. Wrap your speedo cable attach point with a towel. Do the same with the modulator penetration, governor cover, gear selector shaft, and any other penetration into the case. Whichever paper towel turns red is your actual leak point. If more than one turn red, the leak is the highest location.
Lars
The white paper towels will tell you what you need to know, and with only a few possible leak points that are relatively easy to get at, it's probably the way to go. (Far be it for me to second guess Lars!) but in general, a great way to find a mystery leak is to clean the surface well, and spray the whole area down with a thick coat of aerosol spray foot powder. Any leak will show up extremely well against the bright white background of the foot powder spray, and will show you exactly where it's coming from, to boot.Scotty
