I have a 390 with similar probs, after curing all the oilleaks from rocker covers, inlet manifold etc it still leaks out the rear main. (this motor is producing around 425 hp & has less than 2000 miles on it but was built by someone else before I bought the car)Apparently they all do it. It is luck if you have a 390 that doesn't have some degree of leak past the rear main. It seems to be an inherent design fault in the block.
Last time I checked Keith Crafts site he was not guaranteeing FE rear mains on his motors so if he can't get it right you see the problem you are dealing with.
My engine builder has worked for Ford Special Vehicles & Brabham Formula 1 and has been trying for 30 years to cure this problem with no real sucess yet.
He is going to try something new with the 428 we are building at the moment. We are going to machine the back of the block to accept a modern high tech single peice seal. He has done this on smaller motors & it works.
The other things that will help are using a girdle plate and/or cross bolting the crank to reduce flexing which wears the seal more rapidly.
Sorry I can't offer an easy solution, but if we are successfull on the 428 I will keep this forum posted.
Regards
David
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