Chipstractor--You may already know this, but I am going to blab on about it anyways. Conventional wisdom has it that a properly set up hit and miss engine shouldn't require a cooling fan. Why---Because on the strokes where it is "missing" it is pumping ambient air temperature through the cylinder and taking the heat away. However--that being said, I have two or 3 model water cooled hit and miss engines, and all of them will heat the water enough to see steam coming off it if you let them run long enough. Even if not required, a fan adds a bit of eyecandy to any engine.---Brian