Rear main oil seal leak - could it be something else?

[fairlaniac]

I've ben running down a "rear main oil leak" for some time now. I'm comfortable now as I think I've identified the source of the leak and it's most likely not he rear main. After numerous tests with dye and black light I've identified my leak to be at the right rear of the intake manifold. However I was still concerned that I may have a leaking rear main and I wanted to know for sure or debunk that thought.

We often read about rear main leaks possibly being something else. Many times it is difficult to prove out the "something else". When I see oil coming from both sides of the crankshaft it makes me think rear main.

What I did was purchase a syringe and and dispensing needle from McMaster-Carr. During my tests I put oil dye in the engine. So I drained a few ounces to put in the syringe.



What I noticed was a small pool of oil was forming above the block boss for the middle bell housing bolt on the passenger side. Here is a picture of that area from another block.



So I cleaned everything up and proceeded to put 20 drops of oil in that recess and let is sit over night. With the engine tilted about 3 degrees, the oil would find it's way between the block and block plate and go where?

Here I am applying the oil in the recess. Hard to make out, it's tight in there.



So today I went under my Fairlane with my black light and the next three pictures are what 20 drops did. Certainly looks like a rear main leak. However I know that is not the case, based upon my test. It is my leak at the intake, pooling oil in the boss recess and draining down the block to both sides of the crank.







Now this may not be 100% conclusive but I have more confidence that is it not a rear main leak.

The engine was never started during this test.

I hope that you find this entertaining or informative.

[chilly460]

Good idea, have to keep that in mind next time I’m chasing a leak

[machoneman]

A very good idea, for sure.

[Henrysnephew]

Nicely done!  Far too many FE oil leaks are blamed on rear main seal/cap leakage.  John V has been saying this forever and this is a perfect demonstration.  Randy M

[John67427]

Good detective work to find real cause...

[fe468stroker]

Yes sir - good detective work at minimal expense.  Put one drop of oil in a pool of water and see how much it spreads on the surface or in your case follows gravity.  Our massive and heavy intakes (even aluminum) are extremely difficult to line up with both china walls and heads at the same time.  A little bit of misalignment setting the thing and subsequent movement to make it perfectly in place can create gaps in the sealer.  No cure for this other than taking some sealer and pushing some into the mating surfaces of the intake and china walls with the old trusty tool - your finger then torque to specs.  Not a cure all but will make you feel better that at least you tried.

[6667fan]

Gonna have to start calling you Mad Doctor Doug or Professor Doug or something. Nice work!

[Stangman]

Nice work Doug. Get that manifold sealed up.