I'm on the edge of my seat with this one. I am actually dealing with this exact issue right now. I have assembled several Big Blocks over the years and have never had a leaking problem at the back of the block. The block and seal retainer I'm using was used in the past with a factory crank and no leak. The difference now is a Ohio Crankshaft 4.250 crank and BRC aluminum main caps. I am running it on a run stand. It doesn't start leaking until the oil get's hot and then it only leaks when you rev it. When I initially assembled it, I was a little concerned with the side seal on the one side of the cap because it didn't seem like it slid in all the way so I figured that was the problem. When I pulled the pan off to have a look I noticed witness marks on the windage tray and the gasket was chewed up there from the main cap studs. I then figured that this must be causing a leak at the back of the pan. I clearanced the windage tray so it wouldn't hit the main studs replaced the side seals put a light coat of silicone where the retainer meets the block, a small dab of the Right Stuff on the ends of the rear main seal (I stupidly didn't change the seal). I ran it again no leak when it's cold as soon as the oil gets hot and thins out when you rev it it starts dripping. I can't tell where it's originating from because everything is so clean and the Valvoline VR-1 is colorless. I bought a bottle of dye to put in the oil and will check with an UV light to try and pinpoint where it's leaking from. This is driving me crazy due to limited time to work on this stuff. I'm hoping to run it tonight. If I learn anything I will report back. Good Luck and thanks for starting this.