Roy- I've done "extensive scientific research" (well, I've putzed in the garage on it) on the leaky 216/235/261 rear main seals. I've tried just about everything. I know that leaky rear main seals are not caused by too much oil pressure or excessive crankcase pressure. Both of these are easy to check. Pull the trans./bellhousing/flywheel off the back of your engine and use a dummy distributor shaft and a drill to spin the oil pump and build oil pressure. You can bury the needle on the oil pressure gauge this way with a good fast drill motor. Watch the rear of the motor for leaks. Watch it for hours if you want while the drill spins, it still won't leak. Not oil pressure related. Build a brass slinger that bolts to the rear main cap and wraps around the rear crank diameter and protects the rear main seal from oil under pressure coming through the rear main bearing. No change.
To test for the buildup of crankcase pressure, I first cut apart my roadpipe and cleaned all the baffles and welded the assembly back together. No change. Then I built a simple PCV system to draw any pressure and fumes out of the crankcase. No change.
I believe the leaky rear main seals are caused by not enough press or squeeze on the rear diameter of the crankshaft by the seal. I think the current seals (made by Fel-Pro, even if you buy another brand such as Victor) don't fit tight enough. I also think that a rope type seal will work better at first (as it has plenty of press), but will eventually wear in and need replacing. The old asbestos rope type rear seals will last longer, but will also wear in and need replacing, especially if the vehicle is not run regularly and sits.
My current theory (which is most likely wrong again, as usual) is that the best way to seal an early Chev. 6 is going to be a large 1-piece seal as used on the newer Chev. V-6's and V-8's. Someone did an article on machining a block for this style of seal in a past issue of the Inliner's magazine. (The 12-Port News) I think if we could cut a window in the side of an oil pan and take a peek while the engine was running, you would be amazed at all the oil flying around. The crank is thrashing the hell out of the oil in the pan. I think it takes a real positive seal to keep oil from slipping by and getting on the back of the flywheel where it gets slung around the bellhousing, and eventually ends up dripping out the drain hole in the inspection cover after you have parked the truck.
