Drivetrain C4 transmission leak question

Justin87

Active Member
Aug 7, 2017
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Hello all!

I don't have a classic mustang but I do have an old school classic mustang transmission in my Foxbody and I am hoping to get some help with a leaking pan.

My 1990 GT has a C4 transmission from a 1973 Mavrick with a 302. It was rebuilt a few years ago to go with my new engine. I got the engine and transmission in the car this summer and have been driving and enjoying the upgraded ride.

However the C4 has developed a leak from the pan. (or what I think is the pan) and I am having trouble tracking it down and figuring out how to get it to stop.

First I pulled the pan, check it for flatness and simply replaced the gasket. I used a rubber one piece. That helped but it was still leaking pretty good.

Next I replaced the gasket with a THICK blue silicone and steel gasket, replaced the bolts with studs, and used sealant RTV. It still seems to be leaking, although not nearly as bad. I also pulled the flexible transmission dipstick, checked the O-rings and applied RTV to it before re-installing it.

I am looking for any advice from guys who know this transmission better than I do and my questions are:

1. Is this a common problem with C4 transmissions? and If so, what has worked for ya'll to fix it?
2. Could it be a leak from another area? are their known trouble spots on these?
3. Every torque spec I could find said 12-16 ft/lbs for these but with a deep pan performance pan, and a thick gasket; should I try and tighten it more? I assume most of those specs are from a short steel pan but who knows.

Any help or ideas of places to check would be helpful. It doesn't seem to be leaking anywhere else but who knows.

Some photos showing that the trans looks dry everywhere else. (note the picture of the pan was from BEFORE I changed to the silicone gasket and replaced the bolts with studs)

thanks for any input

-Justin



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brianj5600

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Sep 19, 2003
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I tried several methods to seal a C4 pan. I tried aluminum pan, expensive gaskets and still had seepage. The only way I got it to seal was a perfectly clean pan and trans mating surface, cast pan and paper gasket with the lightest film of silicon on both sides. There is little to no silicon squish out and I don't put fluid in until the silicon is fully dry, usually overnight. As my car got more powerful I weeded out the weak links of my C4 and perfected leak free pan installs. My last 4 pan installs have been 100% successful.
 

Az4stringer

New Member
Sep 21, 2021
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I tried several methods to seal a C4 pan. I tried aluminum pan, expensive gaskets and still had seepage. The only way I got it to seal was a perfectly clean pan and trans mating surface, cast pan and paper gasket with the lightest film of silicon on both sides. There is little to no silicon squish out and I don't put fluid in until the silicon is fully dry, usually overnight. As my car got more powerful I weeded out the weak links of my C4 and perfected leak free pan installs. My last 4 pan installs have been 100% successful.

This ^^^ Same technique I have used for a leak free C4 pan. It is also possible the leask is coming from the dipstick tubve. It can leak at end of C4 dipstick tube where it connects to the transmission. The dipstick tube end at the transmission is sealed with an O-ring and when the O-ring goes bad it will leak and will look like the leak is coming from the pan.

The dipstick tube O-ring replacement is simple. Unbolt the dipstiuck tube from the head and pull it up, clean the end really good, put on a new O-ring and a very thin film of RTV on the end just below the O-ring, push it back into the transmission, bolt back onto the head.
 

manicmechanic007

Active Member
Sep 26, 2017
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Couple of things
If It isn't leaking it isn't running right
When you slam a bunch of HP thru a C4 the pan tries to pop off due to case flex
You have done what can be done and the new pan as well
Except the silicone
Trans fluid eats silicone
Use the Toyota or Mopar red rubber stuff if you want additional sealer
You can use what you want because I doubt your pan will be on there long enough for the silicone to deteriorate
It takes a year or two for the silicone to get eaten up
I always use some silicone on the dipstick tube o ring
I have seen pinholes in the dipstick tube where the flange for the bellhousing bolt is welded on
 

Justin87

Active Member
Aug 7, 2017
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the whole thing looks very clean
Like the H pipe
Thanks!

I appreciate the help. The leak is not bad by any stretch but man I got this nice freshly built 302 and transmission at the same time and it's super annoying that it's got this issue still.
 

Justin87

Active Member
Aug 7, 2017
129
36
48
34
This ^^^ Same technique I have used for a leak free C4 pan. It is also possible the leask is coming from the dipstick tubve. It can leak at end of C4 dipstick tube where it connects to the transmission. The dipstick tube end at the transmission is sealed with an O-ring and when the O-ring goes bad it will leak and will look like the leak is coming from the pan.

The dipstick tube O-ring replacement is simple. Unbolt the dipstiuck tube from the head and pull it up, clean the end really good, put on a new O-ring and a very thin film of RTV on the end just below the O-ring, push it back into the transmission, bolt back onto the head.
Sorry for the lateness of my response.

I am going to do this job again and change out the gasket that I used and check the O-rings on the dipstick.
 

Justin87

Active Member
Aug 7, 2017
129
36
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I tried several methods to seal a C4 pan. I tried aluminum pan, expensive gaskets and still had seepage. The only way I got it to seal was a perfectly clean pan and trans mating surface, cast pan and paper gasket with the lightest film of silicon on both sides. There is little to no silicon squish out and I don't put fluid in until the silicon is fully dry, usually overnight. As my car got more powerful I weeded out the weak links of my C4 and perfected leak free pan installs. My last 4 pan installs have been 100% successful.


I am going to do this job again with a different gasket and pan. Hopefully I can get it to stop if not slow down considerably. Thank you for the response!
 

Justin87

Active Member
Aug 7, 2017
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Hey All!

Just wanted to give an update and say that my leaking C4 problem has officially been solved. (for now anyways)

Thanks to all for the advice!

-Justin
 
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7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
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Sorry this is late. If the trans flexes, I’d look into Permatex #2, non hardening form a gasket.