Rear Main Seal Leaks (Part 1)

 

Rear Main Seal Leaks (Part 1)

 

 

So you've got that embarrassing oil spot under your Chevelle and it's dripping off the flywheel dust cover. You're thinking it's the rear main seal. That's one of the hardest leaks to fix and we'll deal with how to do it, but many leaks appear to be from the rear main seal and actually aren't. Valve cover, oil pan, oil filter, intake manifold rear seal, oil pressure sending unit, and transmission can all cause leaks that end up on the dust cover before dripping onto your pristine garage floor or driveway. You can put a piece of white paper under the leak to see if it's red (auto trans fluid) or brown (motor oil). Muncie 4 speeds have a bad habit of leaking gear lube out the front of the case and into the bellhousing too. You can feel for wet oil at the rear bottom corner of the valve covers and just below the intake on the rear of the block. Also jack the car up and use a light to look for the source of wet oil. Sometimes oil pan bolts will loosen over time and a SLIGHT tightening will stop a leak. Be careful not to overtighten as that can damage or deform the gasket and make the leak worse. If you still think it's the rear seal, remove the dust cover and use the light. If it is the rear main seal you'll see evidence of oil traveling from the crankshaft down across the rear main cap above the oil pan rear seal.


If it actually is the rear main seal leaking, you must of course remove the oil pan to get to it. That's a problem in Chevelles because of the crossmember. In smallblock automatic Chevelles, you can get the pan off by jacking up the front of the engine. In manual trans and/or big blocks, the trans must be removed and the rear of the engine raised as well as the front, so you may as well pull the engine on those. The seal is fairly easily replaced without pulling the crankshaft. If the leak is due to a groove worn in the crank from the old seal, Fel Pro makes a seal with an offset lip for '59-'85 smallblocks, part #BS11829-1, that puts the sealing lip on a virgin area of the crank. That worked for me on a 350.


Next issue: replacing the rear main seal and removing the oil pan with engine in the car (smallblock automatic only).