Have a rear main oil leak on a recently rebuilt 1930 engine. When the engine is cold (not run for a week) the leak begins after 12-15 minutes. My first thought is that the engine oil heats up, gets thinner, and comes out the rear main. This probably means that there is too much clearance, right? Seems like a plugged check valve (or stuck ball) would leak all of the time. Am I on the right track?
Thanks

I think you are on the right track. That is logical reasoning.

This info may help someone. When I originally posted regarding the leak from the rear main, I was using 10/30 motor oil. Many folks had recommended this viscosity in the past. Before doing major surgery on the rear main (shims), I went to straight SAE 30 non-detergent and the leak stopped. I'm not saying the 10/30 was bad advice, only that in this situation, straight 30 was the answer to my problem.

Happy for you the leak has stopped.

However, I think you could experience "unintended consequences"
by using the non-detergent oil. I suppose you don't put many miles
on the car, so maybe not....

Bill.

Wasn't all of the oil back in the thirties non-detergent with no filtering?

In ther 1930's oils had little or no additives.
Detergent will not cause oil to leak.
10W-30 is the viscosity of #10 when the oil is cold and the viscosity of #30 when the oil is hot.
This should not cause/stop a leak.
On some engines the leak will be worse when the oil is cold and can not drain back as fast.
The 1930 engine did not of oil prssure to the front or rear main bearing. They are fed by gravity from a pocket above the bearing.
There are several reaons for the rear main to leak including bearing clearence and oil slinger (on the carnkshaft) spacing.
Sometimes other things leak and the oil trails down the back of the engine.

The use of 30W instead of 10W-30 to reduce or eliminate the leak is a bit confusing to me. Let me try to explain. You said that the original leak did not occur until 12-15 minutes of running the engine. That should warm the oil and for straight weight oil (like 30W) reduce the viscosity (flowability) significantly. However, with multi-vis oil (like 10W-30) the oil viscosity is like a 10 straight weight oil at low temperatures but the same as a 30 weight oil at 100 C (212 F). So the viscosity at operating temperatures the 10W-30 and straight 30W should be close to the same if the oil temperature is in that range. Since we all know that normal operating temperatures typically boil off any accumulated water I would expect that oil sump temperatures should approach 100 C.