Topic: Transmission Gasket

[priceless]

For years, I've had problems with transmissions leaking around the pan gasket. I've heard so many ways to fix the leak,and have tried them all....but, to no evail, continue to have a leak of tranny fluid around the transmission pan gasket.
 If anyone has a tried and trued fix, please share it with me. FYI, it's a FMX transmission in a 69 Mach 1 with a 351W engine.
 Thanks for any info....Rich

[carlite65]

well first make sure it is not leaking from above and blowing down around the pan seal fooling you. i have for years used a rubber gasket glued to the pan using 3M black weatherstrip adhesive. i do not have any leaks.

[bryancobb]

Just went down THIS road a couple of weeks ago and mine is bone dry and not leaking a drop.
My successful solution:
I actually had an advantage over you because I rebuilt the transmission myself and it was out of the car.
I had it disassembled and had cleaned the case parts and ran them through our kitchen dishwasher with 2 Cascade tablets.  There was not a tiny molecule of oil anywhere around the gasket surfaces. 
My pan was in good shape and the gasket flange on it was not distorted.  It too went through the dishwasher. 
The real thing that gave me success though, was 3M fast drying gasket and weatherstrip adhesive.  I put it on both sides of the gasket and snugged the bolts up barely with a 1/4" drive ratchet.   SUCCESS!

[priceless]

ok,,,,bryancobb.....did you use a cork gasket or a rubber gasket? you didn't say in your text.  and Charles, there are no leaks coming from above,, the leaks are coming from the gasket. I know some people have a tendancy to over tighten the nuts. but, how should one tighten them? most people don't have a inch/lbs. torque wrench. I see where bryancobb says he uses a 1/4" drive ratchet,,,,that would seem to work.....
  Thanks for any help you guys,,,,Rich

[priceless]

 Also Charles and bryan,,,,where can you get a rubber gasket?? local parts stores here don't have them. Should I try mustang parts vendors?,,,,Thanks for any help,,,Rich

[T Lea]

Most pan leaks are from distorted pan or bolt holes. Check the pan on a piece of glass, pien any defects (off glass) and recheck till its perfect.

[carlite65]

got my rubber gasket at NAPA. i believe it was ATP brand. sorry don't have a p/n handy. agree with the others about a flat pan mating surface. i usually run a wide flat file over the bolt holes. i also use a 1/4 in. t-handle with a calibrated wrist.

[bryancobb]

Mine was the el-cheapo cork one that came with the $69.95 C-4 Rebuild kit.  I spent 5 or 10 minutes making sure the pan flange was as straight as I could get it and the bolt holes were not "coned" inward.  I didn't waste my time looking up the torque # and trying to do it scientifically.  It's not crankshaft or head bolts!  I just ran them all in snug with a nut driver, and used the 1/4 drive ratchet to turn each one a quarter turn.

[priceless]

I know this is an old topic. But, once AGAIN, I've got a leaky trans pan gasket!!!  I've followed ALL the things you guys have said.

It don't leak right away though. It's been about a year since I replaced the gasket and up until now, hasn't leaked.

Do some of you think that because the Mustang is not driven too much right now it being winter, that it could actually spring a leak?

I went out to the garage yesterday and...There it was, a fine stream of tranny fluid right behind the LF tire.  I drive it quite a bit during spring-fall, to local car shows and cruise-ins, and just basically out driving it.  But during the winter months, just usually crank it up once a month. I would say, I drive it maybe 3-4 times from October-March.

Thanks, AGAIN, for any help.

[carlite65]

why would trans fluid be behind the l.f. tire? do you have power steering? it uses type F fluid just like the trans.

[priceless]

why would trans fluid be behind the l.f. tire? do you have power steering? it uses type F fluid just like the trans.

Now that I'm thinking, I should've worded this differently.

No, it's leaking from under the trans pan and the leak has found its way to the drivers side and that's how I noticed it.
Floor un-level I reckon.  I write the date and how many miles I drive the Mach every time I get it out. And I just drove it on Dec. 1st for 12 miles. And at that time, didn't notice a leak.

[Murf]

Make very certain that your leak is not from the transmission VENT TUBE!  After sitting the converter will bleed down causing an overfull condition and the excess fluid will be forced out the top of the tranny through the vent tube, run down the side of the transmission and along the seam between the case and the pan giving the appearance that the pan gasket is leaking  If you can, hook a rubber or plastic tube on the open end of the vent tube extending it past the pan gasket area. Clean up the previous leak and refill the transmission and run your car for a bit, you can then tell where the fluid is actually leaking. It may be some time before it leaks fluid again which is a very good indication it was coming out the vent.   I chased a pan leak for years with the same symptoms you are experiencing only to finally figure out it was not the pan.  This overflow looks exactly as if the transmission pan is leaking and it takes weeks or more for it to start when a car is not driven.  Just one persons long journey to a car that doesn't leak.  Good luck. If the overflow is a problem the only cure seems to run the car more often.

[ruppstang]

Make very certain that your leak is not from the transmission VENT TUBE!  After sitting the converter will bleed down causing an overfull condition and the excess fluid will be forced out the top of the tranny through the vent tube, run down the side of the transmission and along the seam between the case and the pan giving the appearance that the pan gasket is leaking  If you can, hook a rubber or plastic tube on the open end of the vent tube extending it past the pan gasket area. Clean up the previous leak and refill the transmission and run your car for a bit, you can then tell where the fluid is actually leaking. It may be some time before it leaks fluid again which is a very good indication it was coming out the vent.   I chased a pan leak for years with the same symptoms you are experiencing only to finally figure out it was not the pan.  This overflow looks exactly as if the transmission pan is leaking and it takes weeks or more for it to start when a car is not driven.  Just one persons long journey to a car that doesn't leak.  Good luck. If the overflow is a problem the only cure seems to run the car more often.

+1 on Johns experience. Changed the pan gaskets and even pulled the transmission just to find out it was just bleeding down due to the lack of driving.

[MattDoscher]

I betcha the fluid is leaking from your speedometer drive gear O ring and running down the speedo cable and dripping where the cable curves up and towards the firewall which would cause fluid to pool on the drivers side just behind the LF tire.  This is what's happening on my '65 Mercury Comet.  If I don't drive it for say 2 weeks it will start to leak but if I drive it regularly it keeps that O ring lubed up and warm preventing the leak.

Matt