I just replaced a 1970 250 block with 1977 block. I used all the internal parts from the 70 and now have oil leaking out of the distributor. Its coming up the shaft and out of the cap. Is something different about the blocks or did the machine shop leave out a oil plug?? I have a HEI with the coil in the cap, the newer engine has HEI with external coil, could they be different? Looking at the old block, I can't see any reason for this to happen. I need help !! Joe

Put the newer HEI/external coil back in and see if you get a leak.Maybe an O ring is missing?
just a guess.

sounds like the Dist. Needs a rebuild The seals in it are bad To be leaking oil Out the top.
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Ps put the old one from the other block in a check for leaks If not more Leaks either leave that one in Or Get you a New one.

I did some more investigating and found the distributors are different. The internal coil HEI is .120" longer (from the mounting surface to the center of the gear) then the other HEI or my old points unit. This then put the lower bushing right in the oil flow from the gears meshing which act like a small oil pump. The gear and shim washers are worn at the top instead of the center, and the gear was digging into the block right above the oil pump. The seal and bushing are most likely worn as well in the HEI.

I am rebuilding the new style HEI so I can replace the bad unit. It came out of this engine so it should take care of the problem. At least I don't have to replace or remove the engine. Its a very tight fit in a '37 Chevrolet truck.

Thanks for the help, Joe