Updated: Oct 29, 2021
Heres how I install a rubber rear main. First things first... Not all blocks can take a rubber seal! take note that the seal groove is centered on the crank journal. there should appear to be even seal pressure on both sides of the crank.. if not, then a rope seal should be used instead.. About half the seal lip should come in contact with your journal on both visible sides of the crank journal, which should be polished to 2k grit with no pitting or imperfections where the seal rides. the seal lip of any seal should point toward the inside of the engine. Theres 2 different seals sold for this application... A Brown one and a black one. I prefer the black one because it is stiffer and feels more durable to me, but i've had good luck with both so it doesn't really matter. the brown seals are slightly thicker.
I then pull the crank back out and trim the female end with no flag on a bench grinding stone. I typically want a fingernail thickness of seal sticking up on both ends to give the seal a good crimp on the back side upon cap installation, without distorting the actual seal. Since these engines didn't have a rubber seal, there's no actual formula for how much seal should be sticking up.. It would vary like ring gaps depending on thickness of seal and diameter of journal.. But i've had good luck doing it like that. Then I trim the female end of the seal at an angle slightly so that both seal halfs fit together like a puzzle piece when you put the two halves together like I pictured below... Lubricate the seal journal with light weight assy fluid or engine oil before installing the cap. I use Indian Head sealer on the bottom of the cap and in the corners, so bearing crush isn't impaired. that product and similiar ones have a very wattery viscosity, which is what you want. The side seal I make damp with black silicone. When installing the cap, I leave the side seals hanging down about 5/16 of an inch so the catch up flush as the cap is pulled down. you can also add the side seals after cap install, but it can be harder. different side seals are thicker than others, and there's variations in how tightly fitting the main cap is. Main cap bolts are torqued to 110 ft pounds. Steak rods go on the inside.
as a side note! get calls commonly regarding people with unrestored original engines that have a leaking rope seal and want a rubber seal to install without pulling the crank. i HIGHLY recommend the "rope seal trick" article as an alternative. on unrestored engines its highly doubtful the seal journal will be polished well enough for the rubber seal, and what happens if you remove the old rope and put in a rubber seal, and the seal makes very poor journal contact? what then? you'll be pulling the engine.
as a recommendation to shops performing the job: NEVER make promises to customers that replacing a rear main seal will solve the leak ESPECIALLY if the engine has other oil leaks already that the customer refuses to pay to fix. i only guarantee rear main seals on fresh rebuilds i perform, in which the crank is polished like a mirror and block has been tanked. if the engine has multiple leaks and you know the head gaskets are original, its almost always a better game plan to pull the engine and reseal the whole thing.