[mmm]

Does anyone have a TH400 that does not leak, and if so, how did you get it to be that way? I have tried 3 gasket changes, all carefully torqued down:
1) cork w/ silicone
2) dry composite
3) dry cork
On the last one, I removed, cleaned, and resituated the dipstick tube. I checked the modulator seal, speedo seal, resealed the place where the kickdown switch connects, used a ball peen and straight-edge to ensure pan flatness...
Still leaks. Not horribly, but just enough to annoy me. Is there anything I forgot, or should I just bust out a diaper and some baling wire?

Thanks, Mike

I'm right at home in a filthy garage

[mmm]

Does anyone have a TH400 that does not leak, and if so, how did you get it to be that way? I have tried 3 gasket changes, all carefully torqued down:
1) cork w/ silicone
2) dry composite
3) dry cork
On the last one, I removed, cleaned, and resituated the dipstick tube. I checked the modulator seal, speedo seal, resealed the place where the kickdown switch connects, used a ball peen and straight-edge to ensure pan flatness...
Still leaks. Not horribly, but just enough to annoy me. Is there anything I forgot, or should I just bust out a diaper and some baling wire?

Thanks, Mike

I'm right at home in a filthy garage

[Fox]

I have had a few of these things. None have leaked at all aside from the latest addition of my 69 Firebird which isn't being driven right now. I have a feeling that the seals would be fine if I were to drive it (hopefully soon). Gets down to 3 quarts and then stops leaking. You do have to run these trannys occasionally to keep them from leaking. My dad (who worked on motorcycles) used to omit the gasket alltogether and use high temp silicon sealer. Said it worked great so long as the surfaces were clean when the stuff was applied. Not sure if this is feasible on cars or not.

[Lemans1]

2 things you can do to help that have always worked for me. 1st you can try using the Permatex gasket adhesive. Its a red liquid that comes in a little bottle and is applied with a brush. It is super sticky and dries like glue. Let it set before you fill the trans and you will be fine. I wouldnt recommend it if you go into your tranny often as it is a pain to get off of the sealing surfaces. Another option is a better tranny pan. Something aluminum with a thicker flange. Good luck.

[gto1967]

You might also want to check the "o"-ring around the vacuum modulator and speedo. I have had these two areas lead me on a wild goose chase thinking the pan gasket was the culprit when all along,the tranny fluid was seeping out from these and leaking down onto the pan.

"Why that's impossible...and yet it happened!"

[critter]

Here's an odd one. I had an annoying drip. After a year of fighting it I found that the yoke to the drive shaft has a pressed in metal cap. You have to pull the drive shaft to see it. It's inside the carrier where the U joint fits. These can leak, especially after years of use. I filled it with black RTV, no more leak (at least not from that spot).

[mmm]

I didn't think about the non-use theory. It never leaked waaaayyy back when the car was my daily driver. For the last several years, the can has seen VERY limited street action. Maybe it is like a dog that pees on the carpet when it feels its not getting enough attention. Christine has nothing on this car!
Thanks, Mike

I'm right at home in a filthy garage

[Jack Peters]

The greatest things to keep the pan gasket sealed are 1) be sure the pan lip surface is FLAT, 2) be sure the transmission where the pan attaches is FLAT and 3) don't overtighten the pan bolts as it will over crush the gasket or gasket sealer that is used.

Use a straight edge to guage straightness of the pan and transmission surface where they mate. Gaskets do no good if surfaces are not flat. Get a motors or other manual to find out the torque specs and tightening order (if any) on it.

Double check all other places where leakage may occur to be sure you are working on the right areas before pulling the pan.

JLP

[stevep]

Before you replace another pan gasket,see if there is fluid above the pan area.If so,it could be the modulator or filler tube o ring,the linkage seal,or the o ring around the case connector for the kickdown switch.
Any of those leaking would look like a pan leak because the fluid would run around the top of the pan rail.

The difference between in-laws and outlaws-outlaws are wanted

[Skip Fix]

One cause I've heard for non daily drivers is the convertor slowly leaks back into the tranny and does get the fluid level above the pan so they are more prone to leaks. Fact or fiction I'm not sure. I'm not a trany expert but it makes sense to me.But EVERY auto tranny in my hotrods has done the same thing you describe.

[78 GHOST]

Take a look at the seal or O-Ring sealing the Manual Shift Shaft where it goes into the side of the case. Mine is leaking right there and I'm preparing to pull the pan to fix it.

Due to wind, etc. there is fluid all around the entire bottom of the tranny. If I carefully wipe all the fluid off and drive for a while at low speeds, I can stop and assess the leaks without too much wind effect.

I really twigged to this being the leak when the rest of the pan was dry after my test drive and the only fluid was on the front left side. After looking closely, I noticed the grease and dirt completely clean directly below and behind the manual shift seal.

On the upside, ATF is a nice preservative for the undercarriage.

Hopefully Cliff will chime in soon, but I think the converter drainback theory to be right on the mark.

____________________________________
It was a hot summer night. Slowly I spread her legs and my hand was trying to find it's way to her nipple... I was so excited! I never milked a cow before...

[stevep]

78 Ghost,you do not have to pull the pan to change the linkage seal.There is a tool available to pull the seal out.It also includes a seal driver.I think it's made by KD.It works on all RWD GM trans except Powerglide.It's not expensive.

The difference between in-laws and outlaws-outlaws are wanted

[78 GHOST]

Best news I've heard all week! Thanks a lot! I was really NOT looking forward to tearing into this thing.

____________________________________
It was a hot summer night. Slowly I spread her legs and my hand was trying to find it's way to her nipple... I was so excited! I never milked a cow before...

[Cliff R]

In most cases the leak is from above the pan as mentioned. Potential leak points are the "O" ring for the dipstick, manual shaft seal, modulator and front pump. If it's the front pump transmission removal is needed to make the repair. The best repair for the dipstick is to upgrade to the later style multiple lip seal, they WILL NOT leak under any circumstances. You can either replace the entire tube or buy the weld/glue on replacemnt end for the original tube. Manual shaft seals can be changed w/o removing the pan, a special tool is available to pull the old one drive in a new one. Often you can take an awl or small screwdriver and remove the old seal, but take care not to damage the case. A deepwell socket of the correct diameter will drive in the new seal. As for the pan, I highly recomend upgrading to the fiber pan gasket over cork or rubber, they never leak....Cliff

[78 GHOST]

As mentioned, K-D Tools makes a GM Transmission Seal Remover/Installer P/N 2393.

I spent 10 whole minutes changing this seal today and it worked like a CHARM! Thanks guys, turned out to be a totally easy fix.

-Matt

____________________________________
A little boy asked his father, "Daddy, how much do it cost to get married?"
And the father replied, "I don't know son, I ain't done payin' yet."

[mmm]

Way to go Matt, I'm just encouraged to hear that it is actually possible to make one of these beeatches quit leaking! I have my engine / trans out and seperated right now, so I will certainly replace everything while it is accessable. Thanks for all of the pointers.

I'm right at home in a filthy garage