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Transmission Pan Gasket on TH425

Started by Loveofclassics78, July 09, 2019, 02:45:39 PM

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Loveofclassics78

Good afternoon!
Changing the trans. pan gasket on my 69 Eldorado and my 72 Convertible.  What is brand is everyone using for a gasket?  Seems like those rubber gaskets leak.  Out of curiosity, is anyone using sealant and no gasket?

Thanks,
Don

bcroe

The biggest problem with the TH425 leaks is the chain cover.  The
cure seems to be gluing it back on, which requires removing the
transmission.  Other leaks are easier to fix.  Bruce Roe

hornetball

My pan doesn't leak.  Be fastidious about tapping the mounting flange nice and straight and then torque in a star pattern per FSM specs in several increments.  What makes the pans leak is bent flanges from over- or uneven torquing.

Big Fins

Cork gaskets, even torquing at hornetball suggests and before replacing the pan, make sure the bolt holes are on an even plane with the rest of the sealing surface.

I think that only if they doubled the trans pan mounting bolts would they really stop them from leaking. Today's 'O-ring' style oil pan and trans pan gaskets are great. Hard to find a modern car leaving a puddle in the driveway.
Current:
1976 Eldorado Convertible in Crystal Blue FireMist with white interior and top. (Misty Blue)
1969 Fleetwood Brougham in Chalice Gold FireMist with matching interior and top. (The Old Man) SOLD!

Past and much missed:
1977 Brougham de Elegance
1976 Eldorado Convertible
1972 Fleetwood Brougham
1971 Sedan de Ville
1970 de Ville Convertible
1969 Sedan de Ville
1959 Sedan deVille

Loveofclassics78

So, make sure the flange of the pan is straight?   I was looking at my pan and i did wonder if the pan flange may have become distorted from one of the previous owner(s) attempt to change the gasket? I used my torque wrench to prevent that.   If so, how can I correct this beyond buying a new pan?  Is this pan even made aftermarket?  The aftermarket ones that I have come in contact with haven't been the best quality.

I'm going to drop the pan again. Bought one a great fluid extractor.  Used it to remove the tranny fluid on my 69 Eldorado. Manual pump, but I don't mind the exercise.

hornetball

#5
Lightly tap with a body hammer and dolly to get the flange surface flat.  Same thing as you do for valve covers or any other sheet metal pan/cover.

The Tassie Devil(le)

While you have the pan off, replace the filter, and when you buy a replacement filter, it will come with a replacement gasket.

Bruce. >:D
'72 Eldorado Convertible (LHD)
'70 Ranchero Squire (RHD)
'74 Chris Craft Gull Wing (SH)
'02 VX Series II Holden Commodore SS Sedan
(Past President Modified Chapter)

Past Cars of significance - to me
1935 Ford 3 Window Coupe
1936 Ford 5 Window Coupe
1937 Chevrolet Sports Coupe
1955 Chevrolet Convertible
1959 Ford Fairlane Ranch Wagon
1960 Cadillac CDV
1972 Cadillac Eldorado Coupe

bcroe

There are after market pans, aluminum castings with drain plugs
and fins on the bottom.  That is a flat machined surface that will
mate well with the case.  Troubles are it cuts your ground
clearance, and an impact that would dent a metal pan would crack
the aluminum and loose the fluid.  good luck, Bruce Roe

TJ Hopland

I have an aftermarket aluminum pan.  I don't remember how long I have had it, at least 10 years.  It was from one of the GMC rv places.   I think it took about a dozen attempts to get it to be reasonably leak free.  It was a pretty crappy looking casting with a lot of pores or bubbles which I thought were part of the leak but apparently the are not.  One issues was the flange was a little to wide so it made slight contact with the back cover and the vacuum modulator.  Little grinding took care of that.  Still had leaks and I found with some test fits with no gasket that there was a clearance issue on the inside.  More grinding.  Still had leaks.  Sitting over the winter it would leak 2-3 quarts.  I finally found a shop that was pretty good with old stuff and told them to fix it what ever it takes.  New pan new old pan.  It took them 3 shots to get it and its still the same aluminum pan.  No idea what they finally did.   Only leak I have had since then is the shift shaft which has been worked on 3 times and the leak always comes back so it must be wear in the case.  If I had to have it out and apart again I would look at bushing it or at least finding a way to fit a lip seal like the 400's had.

I think the thing was almost $200 by the time it was shipped.  I didn't really complain to where I bought it who I think was having it manufactured for them but did ask a few times if they had had any issues or complaints and of course they said no they are awesome and no one ever has leaks so you must be doing something wrong.

At the time I think there was only 2 people making them and the other one cost a lot more.  No idea if the same people still do or if there is more or less options now.  No idea if the quality has changed.   I wound not do it again.   I would have rather taken the old one and paid a good old school body man to beat the original straight.   
StPaul/Mpls, MN USA

73 Eldo convert w/FiTech EFI
80 Eldo Diesel
90 CDV
And other assorted stuff I keep buying for some reason

bcroe

Quote from: TJ HoplandI think it took about a dozen attempts to get it to be reasonably leak free.  It was a pretty crappy looking casting with a lot of pores or bubbles which I thought were part of the leak but apparently the are not.

the shift shaft which has been worked on 3 times and the leak always comes back so it must be wear in the case.  If I had to have it out and apart again I would look at bushing it or at least finding a way to fit a lip seal like the 400's had.

At the time I think there was only 2 people making them and
the other one cost a lot more. 

Guess better buy the expensive one next time, I have never heard
of a problem from the GMC owners.  That shift shaft uses an O ring
seal.  The problem I have had with shift shaft leaks seems to be
caused by corrosion on the outside, which gets to the seal and the
roughness destroys it.  A new shaft is the easy fix if you have it,
reconditioning the original I have not figured out.  Bruce Roe

hornetball

I had a leak from my shift shaft and was preparing to go in.  While I was at it, I replaced the bushings in my shift linkage and made sure the shift linkage was nice and straight and not applying any side loading on the shift shaft.  Guess what?  Shift shaft leak disappeared.

Currently, the Eldo has no leaks whatsoever.  Knock on wood.