I recently had the 400 big block in my 72 Charger rebuilt and have had a constant slight leak at the main crank seal. The shop tried replacing it twice but with no better success. Can the seal be effectively replaced without pulling the motor and if so, what type of seal would work best, rope or rubber?

 

Are you sure its the main seal? I had my 440 leak at the oil pan/main cover/block juntion. A little bit of sealant fixed it.

You should be able to replace it my pulling the trannt and dropping the oil pan. I think its a 2 peice seal, if so I've been told to stagger the 2 peice seal so the crease doesn't line up where the crease is at the main block, hard to explain but try looking up an install writeup on google.

 

A lot of people have issues with the rear main seal leaking when the engine has been rebuilt. There was a thread about this a while ago. The best way to seal this was with an asbestos rope, and as we all now they can't use that material anymore. But the best way to seal it now is with a rope seal and the proper sealant. There can also be clearance issues now with less material in there forming a slight gap that now must be sealed and depending on the oil pan gasket may not be getting it.