I freshened up my '71 377 motor over the winter including a new rear main seal and 1 piece pan gasket. I always had a oil drip hanging from the front of the bell housing and on the floor, so I assumed it was the main seal and or pan gasket. I clocked the main seal and sealed it on the corners. I also sealed both sides of the pan gasket over the main and timing cover.

After the rebuild it started to leak from the initial 20 min start up. It really doesn't look like the rear main or pan. I have oil drips on the inner starter bolt, down the front of the flywheel cover and over by the oil filter near the edge of the oil pan. I know it's internal to the bell housing, there is no oil at the back of the motor outside the housing.

Could it be the cam plugs? I know when I had the motor out I took a look at them. The had screw in plugs with sealer. What about oil viscosity, I'm running 10w-30 synthetic. Is this too thin for older engines with bigger tolerances by design (compared to todays engines), and 2 piece rear seals? Should I go back to a conventional 10w-40 or 50? Would it make a difference?

Thanks, John

John the syn is still 30w when its warm, just like a petro oil is 30w when its warm. The viscosity when its 'cold' is thinner but when warm they are the same, so the oil wont make a difference. I dont know how to help with your leak but any oil will leak if there is a glitch in the engine seals somewhere.

Thanks Jim. The reason I mentioned conventional oil vs. synthetic was I read on here that synthetics have a smaller molecular structure and have a tendancey to creep or leak where conventionals may not. I'm going to put in some 20w-50 syn and see what happens, then probaby use a dye and light.

Put the dye in now before changing.

Well I found out where I screwed up, and thought it was important to report back so other amateur engine builders don't make the same mistakes. I installed the wrong rear main seal. The block was line honed and I didn't realize that the seal support lip gets honed as well on a 400, requiring a seal with a larger OD. Needless to say it was leaking like a shower head and the seal had spun with the crank. Strange thing is it didn't seem very loose when I installed it or I would have known something was up. Live and learn.