Jump to content
Forums Gone... but not forgotten!
Pontiac of the Month

Debbie Harris's 1969 Grand Prix

2022 September
of the Month

  • Welcome!

    Welcome to Forever Pontiac, where we keep the memory of Pontiac alive with great discussion, maintenance tips, restoration/modification progression "blogs" and help from professional & DIY mechanics. Also, wonderful competitions that occur regularly. Please register for an absolutely free account to join in!

1977 400 turbo transmission pan gasket replacement


Recommended Posts

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


Link to comment
Share on other sites

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.


Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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.


Link to comment
Share on other sites

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.


Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

Tired of these Ads? Purchase Enhanced Membership today to remove them!
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.