I bought an O'Donell head for my O.S. 15, put it on and had nothing but problems. The engine would not idle, but at full throttle it ran like a champ. I even used the recommended glow plug, a McCoy mc59 still same symptoms. I was also using fresh fuel. After thinking about it for awhile, I came to the conclusion that the compression was too much. The instructions said to use the head shim from the engine, I did that. I think they should have mentioned using two shims instead. The other thing that sucks is that the head does not have a slot cut into it for flow through. I have since put my stock O.S. head back on and low and behold, no problems. Maybe I will buy another shim and give it a try later. Does anyone know who makes a good aftermarket head for O.S. engines?