My two bits to this discussion...
I once ran into an extremely frustrating leak from the timing cover of a SBC. On and off, each time removing the entire cooling system and belt driven components. Couldn't ever see exactly where it was leaking from; but assumed it was the danged thick seal between the timing cover and the front of the pan. After about 3 attempts, I mentioned it to one of the local mechanic/machinists. He too, had had similar experiences before, when installing an aftermarket "cheap" chromed timing cover. After fighting the continuous drip, he mic'd out the dimensions of the hole in the cover, and found it to be several thousandths out of place...it was the seal around the crank leaking, not the gasket after all!!! When the cover was bolted into place, the seal was "over-seated" on one side of the crank, and not seating at all on the other. Taking his advise, I ditched the cheap chrome and went back to the original GM cover, and my leak went away. Seems as though the tolerences were far from accurate from the off-shore manufacturer who sold under several different name brands back here in the states. Guess you get what ya pay for. The money saved by not buying an original chromed GM cover in the first place was used up several times in multiple disassemblies to try to stop the danged leak. Education seldom comes cheap...
tjs