Rear Main Seal Leak

Tom, here's my first thought: the #5 main bearing cap has a very small pin in the groove for the rear main crank seal, originally intended to prevent the rope seal from spinning in the groove. If the pin is left in place when the neoprene seals are installed, it will distort the seal & cause a leak. If the pin is pulled & it's hole is not sealed with a dab of RTV, the hole will also leak. Finally, the neoprene seals will tend to leak where they join if they are installed flush with the ends of the cap/parting line of the block. The ends of the neoprene seals should be staggered, so that they hang out one end of the groove in the cap & block by about a 1/4", and are recessed below the parting line by an equal amout on the opposite side.

Your friend on the DTBB, George

quote:

Originally posted by Tom Todak:
I recently built a 381 cubic inch cleveland, Forged crankshaft (probe),the rear main seal leaks afrer 3000 rpm,drip drip approx.every 20 seconds.under 3000 rpm no leak,, I have tried 2 neoprene and 2 rope seals (1 new old stock asbestos)still a leak. the crank measures correct and true with a dial indicator.no cracks or porousity can be found in the block or cap. I'm tired of ( and my engine dyno guy is i'm sure and are mystified as to the cause, at this point we are concidering machining anouther block ! Also I am running a stock oil pump and the leak happens with or without the valve cover breathers installed,, Anyone have any ideas?

You have tried running the Engine "Without the Valvecover Breathers in Place"? I hope that you DID NOT plug the V. Covers All together, if so, in a few seconds of running; you would have blown out the crank seals. But I would expect this to cause more than just a drip every 20 seconds or so. You must have Breathers OR keep the Positive Crankcase Ventilation in place. If you have removed the PCV Valve and hose, replacing it may 'help' your leak problem; because of the Vacuum it creates in(where else)The Crankcase. Also check to make sure that it IS Engine oil leaking and Not gear oil/lube leaking out of the Transaxles' FRONT seal. Last I fully agree with GP, the rear seal pin or lack of RTV in the open hole, once the pin is removed; is suspect.

George & Marlin, its a mystery to myself and my engine builder,, the small pin in the cap was removed for the neoprene seals and the cap hole sealed with silicone, it was reinstalled for the rope seal instalation.none of the seals seal. at this point we can only think its a problem with the block some how or the crank shaft itself. we will figure it out at some point and I will post the cure, thanks for the imput. If you think of anything else let me know....

Tom,

I've two remaining ideas:

1. a groove worn ito the crank on the rear seal journal

2. Marlin's suggestion regarding crankcase pressure. Excessive ring blowby will create excessive crankcase pressure. If you are running too hot of a cam for the PCV system to function, I might suggest you experiment with a vacuum pump on the crankcase. If you have plenty of intake manifold vacuum at idle, you should be utilizing the pcv system.

George

The problem with the oil leaking turned out to be that the crank (new) was running out .0002 and was slightly undersize at the seal area. the crank was rewelded, ground .002 over size at seal area (tighter fit for seal) and walla no leaks. picked up over 30hp on dino ( it's amazing what a straight crankshaft will do) also found out one of 20 probe crankshafts are out of tolerance ....... Tom Todak Also thank you to everyone for your imput