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Old 03-22-2019, 07:26 AM   #20
BRENT in 10-uh-C
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Join Date: May 2010
Location: Eastern Tennessee
Posts: 9,017
Default Re: Rear Main Bearing Oil Behavior

Originally Posted by dareheart View Post
When main bearing clearance begins to increase, the ability of the tube in the rear main to drain the oil back into the pan can become overwhelmed. First order of business should be to adjust the bearing clearance .At the same time the oil hole in the valve chamber above the rear main is in most cases too large and drains more oil onto the rear main than is needed. So with the excess clearance the oil ends up being slung into the bell housing. The simple fix is to adjust the clearances in the bearings and at the same time use a 1inch long piece of 1/4 in. tube with a flare on 1 end, and drop it in the oil drain hole over the rear main in the valve chamber. The rear main will still receive adequate oil and the tube will stay in place by gravity. I was told this by a professional engine builder who helped me with my oil leak into the bell housing. We did the above and I have been without leaks ever since. Happy Motoring!

I'm not sure I 'buy into' this theory. I can't help but wonder how that "professional" determined that??

I would think the restricting the oil flow to decrease volume is a band-aid for masking a problem hidden elsewhere When you consider the crankshaft journal has approximately 0.00075" (- approximately of a thousandths of an inch) clearance between it and the babbitt when the total clearance is set at 0.0015", ...and that effectively restricts the oil from flowing freely down the supply tube. From there, if the passages inside the rear main cap are clear from the slinger area to the bottom of the drain tube, then any excess oil from the journal area should be able to drain freely.

I will share with you that many engine rebuilders do not take the time to remove the welch plug in the rear main cap and clean the passages due to the extra time it takes. Often times there is all kinds of dirt, sludge, babbitt,& debris that is in there even after the cap has been cleaned. All it takes is just a small amount of 'krud' in the passage to restrict the flow which can cause a rear main to overflow and leak. It probably takes an extra -hour of time to thoroughly clean a rear main cap, so when the budget is not there, this step generally gets omitted.

BRENT in 10-uh-C
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