if it does it after a romping I would actually be more inclined to say guides. The seals don't expand and contract nearly enough to make them only leak on hard drives. They would have a constant blow by from my experience

for 40 bucks though I would try the seals first. Didn't you just build this motor? You didn't do all the guides/seals etc...or did you just build the bottom end and left the head alone?

oh and lube them.