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Leaking rear main seal #1286733
Fri Nov 09 2018 04:10 PM
Fri Nov 09 2018 04:10 PM
J
JakeRyan  Offline OP
New Guy
Joined: Nov 2018
Posts: 3
Milaca, Minnesota
I am looking at buying my first classic pickup. A 1958 GMC 100. The current owner has stated that he believes the rear main seal is leaking only when he drives. Does this sound right or could the oil be leaking from somewhere else? As far as he knows it is the original motor and trany.

Re: Leaking rear main seal [Re: JakeRyan] #1286738
Fri Nov 09 2018 04:28 PM
Fri Nov 09 2018 04:28 PM
H
Hotrod Lincoln  Offline
Boltergeist
Joined: Feb 2004
Posts: 17,286
Dellrose, TN
Stovebolts are like Harleys- - - - -they mark their territory! If it stops leaking, put some more oil in it- - - -it's empty! The rear seal is in a groove machined into the rear main bearing cap, and it can either be a woven rope type, or a 2-piece neoprene seal. It's possible to change the seal with the engine in the truck, but it can be pretty challenging. There's a permanent fix that involves machining the back of the engine block for a full-circle lip seal, but that involves a complete teardown to a bare block.

https://www.zoro.com/skf-shaft-seal-3-78x4-34x38-crwa1-nbr-38647/i/G9397674/

Jerry


The murder victim was drowned in a bathtub full of Rice Krispies and milk.
The coroner blamed the crime on a cereal killer!

Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
Re: Leaking rear main seal [Re: Hotrod Lincoln] #1286742
Fri Nov 09 2018 04:56 PM
Fri Nov 09 2018 04:56 PM
J
JakeRyan  Offline OP
New Guy
Joined: Nov 2018
Posts: 3
Milaca, Minnesota
Don't you have to worry about oil getting on the clutch?

Jake

Re: Leaking rear main seal [Re: JakeRyan] #1286744
Fri Nov 09 2018 05:04 PM
Fri Nov 09 2018 05:04 PM
Hambone  Offline
Shop Shark
Joined: May 2007
Posts: 1,107
california, md
Leaking rms would not keep me from buying. They can be driven for years and changed when convenient. But it will leave a mark in driveway.


HB

1966 Chevrolet K-10
Flappy Fenders
In the Stovebolt Gallery
More pictures on Photobucket

1962 Chevrolet C10
Re: Leaking rear main seal [Re: JakeRyan] #1286745
Fri Nov 09 2018 05:07 PM
Fri Nov 09 2018 05:07 PM
J
Jon G  Offline
Shop Shark
Joined: Mar 2014
Posts: 639
Dallas, TX
It could be leaking from somewhere else, but it is likely the rear seal is also leaking. The best way to seal this is (as Jerry says) the full circle lip seal. Second best is the neoprene 2 piece seal...that is if one was available for the GMC. I can't remember if one was available or not and it might be hard to find today. Many have been discontinued. You didn't say if it was the 6 cyl or the 8 cylinder. Either way you will have to remove the crank and polish the rear area or install a speedi-sleeve there. If you don't, it will very likely look like this and nothing will seal it very effectively:

https://beta.photobucket.com/u/JonGoodman/p/cfe709da-fb70-45b9-a82b-819c082e24ce


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: Leaking rear main seal [Re: JakeRyan] #1286754
Fri Nov 09 2018 05:20 PM
Fri Nov 09 2018 05:20 PM
J
Jon G  Offline
Shop Shark
Joined: Mar 2014
Posts: 639
Dallas, TX
Originally Posted by JakeRyan
Don't you have to worry about oil getting on the clutch?

Jake


Yes, that can be a problem. The engine leans backward, so naturally oil is going to make its way to the rear surface of the flywheel. From there it can be slung off the flywheel and all over the inner surface of the bell housing. Works just like a cotton candy machine. Hot oil has a tendency to migrate and it doesn't take a lot of oil to mess up a clutch plate.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: Leaking rear main seal [Re: JakeRyan] #1286758
Fri Nov 09 2018 05:30 PM
Fri Nov 09 2018 05:30 PM
J
Jon G  Offline
Shop Shark
Joined: Mar 2014
Posts: 639
Dallas, TX
Suggestion...if the owner doesn't mind, slide under with a good flashlight and a screwdriver or crescent wrench. Remove the bottom clutch cover and look up in there. Look at the bottom clutch cover, too. See where and how much oil has been slinging in there and where it has gone. See if the pressure plate is oily. It will be if oil has been dripping on it from the top of the bell housing interior. Feel at the front where the bell housing connects to the transmission to see if oil has been running down there, also. Then you can talk to the owner with a bit more knowledge. Good luck. The 1958 was a nice truck.


Jon

1952 1/2 ton with 1959 235
T5 with 3.07 rear end
Re: Leaking rear main seal [Re: JakeRyan] #1286771
Fri Nov 09 2018 07:32 PM
Fri Nov 09 2018 07:32 PM
D
Dragsix  Offline
Shop Shark
Joined: Dec 2017
Posts: 299
Pennsylvania
If you can confirm that your motor has a two piece neoprene seal, you may be able to quell the leakage using Barr's Rear Main Leak Stop. I had the leaking issue raise its ugly head in one of my 261 motors two years ago. I used the leak stop (its clear and not like the radiator stuff of old) and it has done an admirable job of keeping the leakage to a very minimum. The Barr's will not work with a rope seal however.

Last edited by Dragsix; Fri Nov 09 2018 07:32 PM.

Mike
Re: Leaking rear main seal [Re: JakeRyan] #1286816
Sat Nov 10 2018 03:32 AM
Sat Nov 10 2018 03:32 AM
N
ndkid275  Offline
Shop Shark
Joined: Jan 2013
Posts: 1,150
Northern VA
When parked, my truck has an oil pan under it! As Hambone and Jerry stayed, they mark their territory! I just add oil when she is low and keep on driving my 216.

The oil may even preserve the underside of your truck! 🤣
Chris

Re: Leaking rear main seal [Re: JakeRyan] #1286880
Sat Nov 10 2018 05:02 PM
Sat Nov 10 2018 05:02 PM
C
cmayna  Offline
Shop Shark
Joined: Jan 2002
Posts: 1,723
Menlo Park, Ca
A few years ago, I attempted to fix a leak by replacing the rope seal. Lasted for awhile but eventually started marking it's territory again. Now, the green turd drips just enough that I don't allow it to sit on our driveway too long without having a drip pan underneath it. It typically resides under a canopy between a couple redwood trees on a gravel driveway. So much easier to top the tank versus chasing little leaks.

Last edited by cmayna; Sat Nov 10 2018 05:24 PM.

Craig's '50 Chevy 3100 5 window
My truck
If I'm not working on my truck, I'm fishing with the wife or smoking Salmon.
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