Large persistent rear main oil leak
Thread Starter
|
1st Gear
Joined: Jun 2026
Posts: 1
Likes: 0
Large persistent rear main oil leak
Hello
I have a 302 small block I believe to be out of a '82 F100 that I have swapped into my '87 ranger. The motor runs pretty good, and was fairly clean when I bought it, but immediately after I started driving it, it developed a very aggressive oil leak from the rear main seal. It lost about a quart of oil in just a few miles of driving. I pulled the rear main out and found that it had been previously replaced with a rubber type seal, and the hole for the locating pin was plugged but the parting lines were not offset from the face of the rear main cap. The old seal was severely damaged, with the rubber sealing lip partially detached from the rest of the seal, so I figured it must have been the issue.
I replaced the seal with a new Fel-Pro one, and followed the recommended installation instructions, offsetting the parting line, applying a bead of rtv to the edges of the main cap, and a film to the ends of the seal. The seal did provide some resistance when putting the cap back on, but seemed to seat well when I torqued down the main cap. After putting the engine back, it quickly started leaking oil from the same spot, but maybe just a bit less. Still way too much to drive around with.
The oil is coming from the center of the back of the motor, and I cant imagine anywhere besides the rear main as most of the oil is coming down the backside of the block plate and dripping from the bottom of the bellhousing. The outside of the pan is fairly dry, as well as the valve cover gaskets and the intake manifold rear seal. I applied loctite to the flywheel bolts and the clutch assembly is dry, so I cant imagine its coming through there, and it seems like way too much oil to make its way thorough threads anyways. The crank sealing surface seemed fairly smooth and i cleaned and oiled it prior to installing the new seal. I pulled both of the breathers to check for blow-by but didn't notice anything major that could cause the seal to blow out.
Any help or ideas would be appreciated, as I am at a loss for what could be causing it. I have heard of crankshaft repair sleeves that increase the outer diameter of the sealing surface, is that something worth trying? Are there any other brands of seal that are recommended if this is a common issue with felpro seals? Any other places that It could be leaking from that would cause oil to end up on the backside of the block plate that I could have missed?
Thanks
I have a 302 small block I believe to be out of a '82 F100 that I have swapped into my '87 ranger. The motor runs pretty good, and was fairly clean when I bought it, but immediately after I started driving it, it developed a very aggressive oil leak from the rear main seal. It lost about a quart of oil in just a few miles of driving. I pulled the rear main out and found that it had been previously replaced with a rubber type seal, and the hole for the locating pin was plugged but the parting lines were not offset from the face of the rear main cap. The old seal was severely damaged, with the rubber sealing lip partially detached from the rest of the seal, so I figured it must have been the issue.
I replaced the seal with a new Fel-Pro one, and followed the recommended installation instructions, offsetting the parting line, applying a bead of rtv to the edges of the main cap, and a film to the ends of the seal. The seal did provide some resistance when putting the cap back on, but seemed to seat well when I torqued down the main cap. After putting the engine back, it quickly started leaking oil from the same spot, but maybe just a bit less. Still way too much to drive around with.
The oil is coming from the center of the back of the motor, and I cant imagine anywhere besides the rear main as most of the oil is coming down the backside of the block plate and dripping from the bottom of the bellhousing. The outside of the pan is fairly dry, as well as the valve cover gaskets and the intake manifold rear seal. I applied loctite to the flywheel bolts and the clutch assembly is dry, so I cant imagine its coming through there, and it seems like way too much oil to make its way thorough threads anyways. The crank sealing surface seemed fairly smooth and i cleaned and oiled it prior to installing the new seal. I pulled both of the breathers to check for blow-by but didn't notice anything major that could cause the seal to blow out.
Any help or ideas would be appreciated, as I am at a loss for what could be causing it. I have heard of crankshaft repair sleeves that increase the outer diameter of the sealing surface, is that something worth trying? Are there any other brands of seal that are recommended if this is a common issue with felpro seals? Any other places that It could be leaking from that would cause oil to end up on the backside of the block plate that I could have missed?
Thanks
Cargo Master
Joined: Oct 2020
Posts: 2,705
Likes: 800
A friend of mine ran into this one time on a 289 that he built for a customer. He tried several different seals and it always leaked so he ended up taking the crank back out and putting in a rope seal which solved the problem. His theory is that on the old blocks that were intended to be used with a rope seal that the groove which holds the seal is not necessarily all that concentric with the crank centerline. Maybe so. T Meyer and others have a one piece seal that you can put into the 2 piece main seal blocks which might or might not be better than the 2 piece.
The main problems with the rope seal are that it takes more time to install and getting good quality rope today can be a problem.
The main problems with the rope seal are that it takes more time to install and getting good quality rope today can be a problem.
