My 250 didn't have lock washers. What type of gasket are you using? The 250 has a lot of weight pulling on the gasket, so its not surprising they leak. When I change gaskets or manifolds, I hold the weight up with one hand and tighten all the uppermost bolts first, then the lower bolts. Even though they are almost straight across, this seems to help mine seal better and having two manifolds with flat faces really helps. If you have the two separated, snug them to the head first, then to each other, then tighten to the head, then tighten to each other.
Joe
