I have since pulled the head and the jug. The sleeve and piston are scored on two sides. There is no debris in the crankcase. It appears to be from a lean run, although, the engine was running rich at the time. I was using the last of my one gallon can of gas. Could it be that the oil content was low? I use 87 octane and 50:1 Klotz. The engine is two years old and has been urn extensively. When I cleaned the screen in the carb and switched from a 20x8 Menz to a 20x6 Mejlic, the rpm went up to 8,000. I wonder if this is too much rpm? I was advised, by users of this engine, that they were running Mejlic 20x6 on profile 3D planes.