Leak from Main Seal...whats acceptable?


Sep 14, 2019
So, I've had my 1973 Z-28 for about three months now, and part of that time has been devoted to dealing with what has been an annoying leak at the Main Seal. About a month after the car arriving, I noticed that what had been a an occasional one or two drops a week was getting steadily worse. Now, I've long been told that old cars will leak a drop or two, so I wasn't particularly worried at first, but if the issue was getting worse I wanted to address it sooner rather than later.

I took the vehicle to the same transmission shop in my area that GM Muscle of Hudson, a great local restoration shop, use to for their transmission jobs. Turns out, I had a leak of both the transmission and the main seal. Two weeks, and $1,500 later, I had a new clutch, a rebuilt M21 with new gaskets and seals, and a new main seal installed. Happy to get the car home, I notice that the leak directly underneath the main seal had actually gotten worse. I take the car back, and the mechanic agrees that something is amiss, replaces the main seal (again), keeps the car for a while to make sure the issue doesn't resurface, yet, after getting it back, I notice that one day later, I still have a leak (though not as bad as the previous time).

So, I guess my question is...even with a new main seal, is this something I should just get used to? What else should I be looking for? Do I just need a better local mechanic, or am I over-reacting to a what should be expected from a 47 year old car? Appreciate your thoughts.


Veteran Member
Nov 28, 2014
Knoxville, TN
What often happens to the seal area on the crank is a slight groove is worn in the crank from the seal. A new standard seal won't fix it for long. Victor and Felpro make an offset rear main seal that moves the seal away from the worn area on the crank. That being said, I have never had or seen a sbc 2 piece rear main seal stay completely dry over the long haul. Over time they will weep and maybe drip once in a while.


Veteran Member
Lifetime Gold Member
Jul 31, 2001
Ft. Lauderdale, FL
The joys of having someone else work on your car. Priceless.
You are positive it's from the rear main crank seal? Rear oil pan seal gasket?
If it's dripping from the bottom front edge of the bell housing it's coming from the front input of the trans. It's common on Muncies because they really don't have a front seal. Mine did it when I over filled it.
If it's dripping off the oil pan....then maybe....
Remember a rear main leak will not soak and waste a clutch, an input shaft leak will.
Did he know enough to offset the seal when he installed it in the two halves of the rear main bearing cap? So the seams are not matching up with the rear main cap seams? Did he put some sealer in between the two halves?
Try and make sure they put the right gear oil back in your Muncie, Usually they pour in anything and everything except the correct lube GL-4 80/90

And no, a leaking seal is not normal or OK, screw that...

..........We are looking for Moose and Squirrel............
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Veteran Member
Aug 1, 1999
Or the oil is coming from above? Hard to detect this sometimes, corner of intake, oil pressure sender, rear cam journal welch plug?


Veteran Member
Mar 25, 2010
marietta ohio
Have you checked to see if your leak is from the rear of the intake. It could even be something as simple as a valve cover leak. The oil will travel a long way when hot. Start at the top and work your way down when looking for the leak.


Veteran Member
Oct 1, 2016
Waupaca WI
^^^^ dye and uv light will make finding the leak a lot quicker....and cheaper IMO. On the extreme side the crank journals can get out of round when that happens there is nothing you can do to correct it but by having the journals align honed or bored to get the crank to sit correctly otherwise with the crank skewed a bit there will be a bit of a gap in the seal and will leak. Had this same issue with my engine leak started small and over time got pretty bad. the align hone took care of it.


Sep 14, 2019
Thanks guys. I'll look into the dye/UV light combo. I checked this morning, and there was no leak in the last 12 hours, but I took her out for a 20 minute drive, and will check again this evening. There is a small drop pooling right at the front of the transmission housing, so I have to assume its from right above. I'll let you guys know.


Jun 8, 2005
ARE YOU USING THAT ROPE SEAL? use a Tin Indian or BOP seal , also I ran across a situation using the rope seal on my 400 and using synthetic oil THE REAR MAIN SEAL LEAKED and after spinning a couple bearings the engine was rebuilt using the Tin Indian plastic seal and NO more leaks with synthetic oil