Some possible reasons for the leak from your rear main seal

May 1, 2012 | By Richard Prince


I’ve had my 289 cid, 3-speed manual transmission Mustang for 35 years. The motor was replaced (just the short block) in 1977 but after that the car was never driven very much. I started driving it regularly on short jaunts this past year and it now has a total of 5000 miles on it over the last 28 years.

Since I started driving it last year, it has a very slow leak at the rear main seal area, which showed up on the garage floor after each jaunt. There is not much oil coming out and a piece of cardboard could handle the leak until it stops because the motor was turned off, but it bugged me. I took the car to a shop that knows Mustangs and had the rear main bearing cap seal replaced while the motor was in the car, which meant draining the oil, removing the pan and the rear crank bearing cap. The seal in it was the rope type, which is probably from the 1977 rebuild. The new gasket is a two-piece Fel-Pro BS 30136.

When I got the car back, the leak was worse and now needs a pan to catch the drips running down the thin clutch cover. I went back to the shop and complained. When asked what they should do, I said I thought the seal was put in backward because there is only one way to correctly put it in and the leak was worse. We agreed that they would redo it and if it was in right, I pay and if wrong, no charge. When the mechanic took off the rear main cap, he immediately said, “My bad, it’s backwards” so he replaced the seal (both pieces) with a new one.

However, the leak is just as bad. I let the shop know and they are telling me the oil must be coming from somewhere else in the same vicinity, possibly the oil galley plugs. I find this hard to believe because of the increase in leakage with a new seal. The mechanic also told me they had done one to another car that initially leaked and by driving the car, it stopped. That hasn’t happened for my car. The oil pan is new and the one-piece gasket is not leaking. There is no leak at the valve covers. I don’t have the guts to take it back to this shop and insist the seal is in wrong again, plus my confidence factor is zero now.

What would cause the leak to be more with the new seal than with the old rope type? Any ideas before my garage floor gets warm enough to lay on and do this operation the third time?


Rope-type seals were universally used to seal the rear main bearing area in antique cars and were still in common use into the 1960s before being replaced with the now ubiquitous neoprene rubber seals.

Rope seals are more prone to leaks but only because they frequently are not installed well. They come longer than they need to be and have to be trimmed by the installer and if cut too long or too short they usually will leak. While it is certainly possible that your engine is leaking oil from one of the galley plugs located at the rear of the block above the rear main seal it is unlikely for precisely the reason you cite, which is that the leak got considerably worse after the rope seal was changed to a neoprene seal. Changing the seal should have no impact whatsoever on the function of the galley plugs.

Rope-type seals generally work quite well when correctly installed in an engine that’s run on a regular basis. Rope seals in engines that are not run regularly tend to dry out and shrink, leading to leaks when the engine is put into regular service. Sometimes dry, shrunken seals will swell when soaked with oil from the running engine but often they don’t swell enough and leak.

A neoprene seal installed backward will normally leak. I can understand making a mistake and installing it backward once but doubt very much that your shop repeated the mistake and installed it backward a second time. A new neoprene seal that’s correctly installed may leak if it was damaged during installation. Any seal is only as good as the surface it’s in contact with and damage to the seal’s seat or, more likely, to the sealing surface of the crankshaft journal, can prevent it from sealing correctly.

A common cause of rear main seal leakage even when the seal is in good shape, was correctly installed, and has good sealing surfaces is excessive positive pressure in the engine’s crankcase. It is normal for most engines to develop positive pressure in the crankcase and some sort of a ventilation system handles this. In the old days crankcase pressure was normally just vented to the atmosphere but because the vented gases contained oil mist and combustion byproducts it was a significant contributor to air pollution, so beginning in the early 1960s car makers began installing systems to capture crankcase gases and put them back into the combustion chambers to reduce overall emissions.

If the venting system in your car is obstructed, disabled, or simply overwhelmed by excessive pressure, this extra pressure will tend to distort the rear main seal by forcing it outward, thus allowing oil to leak out.

The most common cause of excessive crankcase pressure is poor piston ring sealing. Worn or otherwise damaged cylinder walls, worn or broken compression rings, or anything else that causes poor cylinder sealing will cause blowby, which is a leakage of the pressure that’s supposed to be contained in the combustion chamber.