I have a recuring rear main leak and can not seem to get it fixed. i just pulled the engine (400 block with eddy heads ) took it apart put new bearings in it a new ultra seal, ( the one that looks like a rope seal) billet alum. rear main seal retainer. all new gaskets. shined up the rear main seal area (68 383 steel crank, with the serrations ) put it back together . stuffed it back in the car , run it in for 20 plus mins like a new engine, (no leaks ) took it for a drive, about 20 miles. on the way back, the new seal started leaking again, real bad. pulled it in shop and shut it off, 10 inch oil puddle. I am at a complete loss. I am not a professional , but i have built 25 big block mopars over the years, and this is the first one ever to have this problem. This engine holds about 70 pounds of oil pressure at speed after warm up. drops down to 40 at idle. any thoughts, i know i got to pull it again, and tear it down , would like for this to be last time, want to drive this car (70 superbee, 4 speed ). any help would be appreciated.

Not uncommon. We recently went through this on JCastle's RR with a 505. Think he had it apart 3 or 4 times finally the last time it was good.

Search for previous posts on this subject. Several good, long discussions.

His did not leak too bad right away either.

But a couple of points:

Make sure that you are not building pressure inside the motor - make sure you have adequate breathers. Don't overlook this.

In the back of the seal retainer (by the flywheel or TC) make sure you clean that area and fill it with RTV.

On the RR it was not hard to pull the pan and get to the seal with the motor in the car. We fortunately were able to work on it on a lift.

And good luck.........

I'm assuming it is the seal/retainer & not the pan flange. yes find those prior threads

I always put a good film of moly paste on the seal's lips and crank journal before I bolt the crank in for good so I don't "burn" the seal up on break in, rubber or rope, I've had good luck doing this over the decades on many builds

Mike

A puddle that large from a leaking rear main does sound excessive. Are you certain that oil pan not leaking at retainer bolts!

Pull flywheel cover, start engine and watch pan gasket action with a good light.looking for seepage.

If leaking at flange/gasket, incorrect too long bolts at retainer will cause the leak.

Further information in Motor's Repair Manual, 1974 edition. Also a Chrysler posted a bulletin on this and the cause.

Rapid Robert is correct on finding prior threads.

Awaiting usual commentary from peanut gallery.

You are certain that not leaking from back of either valve cover gasket?

thank you all for the things to look for . most of the stuff listed i have already checked and removed from possible problems. it is not the valve covers, or the oil pan, its coming from the rear main seal. i always lube the seal and silicone the bearing retainer, and with the ultra seal they give very specific instructions on how to install it. I did not know about the bulletin and will check that out. I will have to pull the engine there is no way to get to the ultra seal or remove it without pulling the crank. i will look fro the threads and get back to you all with what i find. again thank you for your thoughts.

Maybe I don't know what an ultra seal is, but you can pull the big block seals in and out all day long without pulling the crank........

Just ask J Castle!

Spent 2 days one time removing top half of a 'rope' seal from wife's 55 T Bird 292. This after pan and main cap removed. Was sweating that may have to pull crank, until coming up with a scheme to get it out. Hopefully will never see one of those critters again.

Replaced with neoprene,,,would take 45 seconds on that if there was a next time. Does not leak

Neoprene too on my Mopar takes just seconds, cannot speak to a rope seal on it.