Can't seem to find the answer to rear main seal leak. Have a 1959 235 that has gotten the works. New cam, lifters, timing gears, pistons, rings, reconditioned rods, reconditioned (.020/.020) crank, all reconditioned 848 head, all new gaskets and seals. All machine work was preformed on the block by a reputable shop I have used for many previous builds. The only thing not done was to line bore the block, something I have never done on previous street motor builds. Running Fenton 2x1 with Rochesters and Fenton split headers, late model GM hei distributor and all backed up by a the stock cast iron powerglide in a 52 Chevy hard top.
OK so thats all the information, now to the problem.
Cannot seem to keep a rear main seal in this thing.
We are on the the 3rd seal and its leaking again. Rubber not rope.
In the car the pan can be removed and the seal changed but this is getting very frustrating. We have only been able to get between 300 and 700 miles on a seal before it leaks, and not just a drip, a major puddle and leaving a trail behind!
Have checked all the clearances every time it has been apart and found nothing out of spec. The last go round the rear main bearing did show some unexpected wear toward the rear edge of the bearing and was replaced with a new bearing shell along with the seal. Once again checked with plastigauge and buttoned it up only to have it leaking again after only 300 miles!
The engine starts right up, has great power, is smooth running and sounds great. Gets expected mileage and is in every other way a fantastic success.
This car belongs to a very good friend and as such I cannot vouch for the way it is being driven (he tends to have a bit of a lead foot) but I would think that this motor could handle cruising the highway at 70 which would put it at approximately 3500 rpm without this type of failure.
I am looking for any thoughts on how to fix this thing before it gets pulled and replaced with yet another belly button sbc. They are a great engine but not as cool as a stovebolt.
Sorry for the long post, I just don't know where else to turn for help at this point.

Chuck