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tonys1969

1977 400 turbo transmission pan gasket replacement

Question

I have a 1969 Firebird with a 1977 400 engine and transmission. I recently replaced the transmission pan gasket. It was a rubber gasket and I was told not to use gasket sealer with it. I got about 4 quarts of fluid in before it started leaking. I read some comments on line that you should still use gasket sealer with the rubber gasket . Can anybody shed some light on what I need to do? Thanks Tony


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Tony - Gasket sealer is a good thing, just use a small bead around the perimeter. Also, are you certain the gasket is the correct one for your transmission and pan. This might sound like a silly question at first, but experience has taught me otherwise.



GM made a variant of the TH400 called the TH375. A true TH400 should have a "4" stamped into the tranmission pan. You can identify a TH375 because it will have 375-THM cast into the tailhousing. GM adds to the confusion by making some TH350s into the TH 375-B - but these are based on the TH350 case. Another variant is the 3L80HD which is also sometimes called the TH475. It's been my experience that if you don't have a true TH400, that variations in the gasket exist, so insuring you have the correct gasket is critical to proper sealing.



Finally, insure you tighten the transmission pan bolts in a criss-cross pattern, much like you would a cylinder head. Don't over tighten the bolts.


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Tony - Gasket sealer is a good thing, just use a small bead around the perimeter. Also, are you certain the gasket is the correct one for your transmission and pan. This might sound like a silly question at first, but experience has taught me otherwise.

GM made a variant of the TH400 called the TH375. A true TH400 should have a "4" stamped into the tranmission pan. You can identify a TH375 because it will have 375-THM cast into the tailhousing. GM adds to the confusion by making some TH350s into the TH 375-B - but these are based on the TH350 case. Another variant is the 3L80HD which is also sometimes called the TH475. It's been my experience that if you don't have a true TH400, that variations in the gasket exist, so insuring you have the correct gasket is critical to proper sealing.

Finally, insure you tighten the transmission pan bolts in a criss-cross pattern, much like you would a cylinder head. Don't over tighten the bolts.

Curious, is there a ft-lb torque that is specified for them?

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Yes there is but I will have to look up since I don't know it off the top of my head. The key is not to over tighten the bolts and distort the pan, you make the gasket's job that much tougher if you do.


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Is it the gasket leaking or the speedo gear seal. I chased that leak for about 3 years before a $0.25 part would have fixed everything.


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