Mark 4 engine gurus RE oil leak, opinions needed

[formula31]

I think I may have found the leak. Engine ran for 2 days (5 hrs)with no apparent leak or problem. Then a after a 1/2 hr rough ride, oil in the bilge. found drip, was where the starter meets the block and flywheel cover, pulled engine, pulled flywheel cover and flywheel, cam plug OK, oil galley plugs OK, BUT I found 2 threaded holes through the block on each side of the rear main that must be for a stamped pan but are not used on my cast pan. One of them is open to the inside of the pan. Could this be my oil leak problem? I dont want to change the oil pan gasket and rear main seal if this is where the oil came from. See picture attached. I feel like an idiot that I didnt see this when I put the pan on. First time I have ever used a cast pan and this engine was rebuilt before me and they must have used this pan and gooped up the unused hole.

[Mbam]

Sure looks like a good possibility. If you have an air compressor you can pressurize the crankcase and check it with some soap bubbles. Use a pressure regulator, start at zero and slowly increase it to 3-4 lbs. Normally I put the air in one of the valve cover breather holes and seal the other. Don't expect it to hold pressure as air will come back up through the rings and out the intake and exhaust.

[mopower]

If that hole is open to the inside of your pan thats got to be the leak! When you're running hard oil is flying all over inside the pan. The least bit if pressure inside and it will look for the first way out.(valve cover breathers to flame arrestor). Since this is down closer to the source of any pressure ,and there are no baffles like in the valve cover , guess what? Out it comes. After you pressure test it try cleaning the treads and screwing in a bolt the appropriate size from the top down with some tread sealer or silicon on it. That way you don't have to remove the pan.

[formula31]

Thanks Mark and mopower. Thats the way Ive got it figured. I didnt even need to pressure test this since when i squirted oil into this hole it ran right down into the pan. Im going to use some bolts and sealer and plugs these from the top. Damn, I was sure I did every thing right on the rear main and the pan gasket. This was a surprise for me. You learn something new everyday. This did give me a chance to check my cam break in and it looks good. However, the roller rockers I bought on ebay are junk. Driving to Summit in the morning to get a set of crane golds. Live and learn.
Tom

the hole you are showing must be plugged (use a set screw with sealer) with a cast pan it will leak 100 percent of the time, sometimes severe
dave

[formula31]

Thanks all, Just fired it back up and so far so good. Learned something new.