All- the original pan on my th400 has been leaking- a lot- and my mechanic brother recommends replacement. While an original style would be preferred, the main thing is to stop the leak.
I understand there are also several different styles of gasket now. The th400 is not original to my '74 Salon, though i've tried to keep it looking factory the best i can.
I don't drive it a lot, but I have driven it to Nats a few times, and hope to again soon (after I find the correct replacement tires for my 235 70R15s with correct 3/4 whitewall)
Appreciate input- my brother's a great wrench but not up on the better options for our old cars.
Thanks, and stay safe out there!
BG

 

Make sure the pan flange is straight, flat, with all surfaces clean, and use a cork gasket. It may not be just the pan leaking.

 

I agree, ensuring the pan rails are flat and clean should be all that is needed to get it to seal.

 

After removing and cleaning the pan, look for bolt holes dimpled toward the transmission from overtightened pan bolts. They can be flattened with a ballpein hammer. Overtightened bolts is a frequent cause of leaks, use an inch pound torque wrench or a low range foot pound torque wrench to tighten the bolts. It is uncommon to have to replace a pan.

Good luck!!!

 

Thanks for the replies so far..hope to address all those areas soon.

 

Following this..
My Vista Cruiser ran fine all last summer and didn't leak a drop.
Did some work on it over the winter, and went to back out of the drive, and it wouldn't move
Topped off the ATF and it was fine...
Didn't drive it for another week, and it leaked out again. Topped it off again, and it drove fine again...
Drove it to a cruise in on a brand new parking lot.
Only a couple tiny drips when I left after a few hours.
Next morning, there was a huge puddle under it and had to top it off again.
Got it in the garage, and will be cleaning and inspecting the transmission and all the gaskets.
Someone told me the pan bolts were prone to coming loose.
My car has headers into flowmasters, and nothing else at this point, so it does vibrate a lot.
Hopefully it's that simple.

 

Eric is correct, the pan may be warpped and causing a leak. However, don't cheap out on the gasket. Get the best one you can. I just did the filter and pan gasket on the C6 in my 86 F250 and I used a cork gasket. Big mistake. It leaks worse than before the filter change.

Here's the one I plan on using in my 71. It's due for a filter change.

https://www.summitracing.com/parts/mor-93103

 

After discovering “Duraprene” pan gaskets, I’ve never used anything else. Tried them all at one time or another over the ages and these, in my experience, are superior.

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When I worked at GM dealerships during the 80's and early 90's, the main reason for transmission leaks was over-tightened trans pan bolts.

 

And it still is.

 

would like to think overtightening was not the problem, but, since it leaked a lot initially, haste and frustration may have contributed.
My mechanic is not a fan of cork gaskets (my dad was firm believer whether pan, valve cover or whatever). I did get a new gasket from my GM dealer- supposed to be their replacement for cork.
hoping to get time to put her down, pull the pan, and confirm whether a new one is needed. Haven't noticed any performance issues, other than she won't chirp second like she used to..i'm also going easier on her.
Thanks for all the input.

 

I've had really good luck with these Lube Locker gaskets. They're supposed to be reusable also, but I haven't reused one yet.

 

i'm sure hoping that once this is squared away, I won't be dropping the pan again..
Thx, Joe- seeya in TN ..

 

those work well. The pan gaskets that come in the premium TransStar overhaul kits are kind of charcoal gray colored, some are tan. I have no idea what they are made of, but sometimes you have to beat the pan with a rubber mallet to get them to break loose from the case.

Replacing a gasket and having no leaks is 99% preparation. The best gasket will leak if the installation is sloppy. Make sure the sealing surfaces are absolutely clean and dry, including the holes the bolts thread into. Make sure the pan rail is flat, make sure the bolts are clean and dry. Install the pan, evenly tighten the bolts, then torque to spec. The spec is 120-144 INCH pounds (not fl lbs!!) or basically snug plus a 1/4 turn. Overtightening will distort the pan, and crush the gasket.

if the car leaks after sitting a few days, most likely it isn’t the pan gasket. It’s more likely the fluid in the torque converter is slowly draining into the pan, and raising the fluid level to the point the fluid is leaking around the o-ring for the dipstick tube, the modulator, or the kickdown solenoid case connector, or possibly all the above.

Before doing anything, go to your nearest auto parts store and buy a small bottle of A/C oil that has the fluorescent dye. Throughly clean the transmission, too off the fluid, and add a little of the A/C oil to the transmission. Drive the car, then look at the transmission with a black light. The dye will highlight exactly where the fluid is leaking.

Most likely your going to find the leak is coming from the dipstick tube. They tend to rust and crack right where they neck down for the o-ring. If the leak is coming from the tube, get a replacement that uses the sleeve type seal. They work much better.

 

Those Lube Lockers are really nice gaskets. I have a friend that used one on rear end of his 72 442, may have used on transmission pan as well. Too bad there isn’t enough market for LL to manufacture the reincarnation of the extinct Perma Dry Plus valve cover gasket that these are reminiscent of.

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