Ihave a Magnum engine that was doing the same thing. Pulled the head and it did not have a gasket. Added a gasket and no change. Messed with it for a while and got to wondering if it was possible that the leak was coming from between the crank case and sleeve. A very small amout if silicone on the outside of the sleeve solved the problem. Just to make sure it was not a fluke I removed the silicone and the leak returned. Reapplied silicome and no more leak. Believe it or not.