Abstract: An air-cooled, reciprocating piston, internal combustion engine which includes at least one cylinder head which includes inlet and exhaust ducts having inlet and exhaust valves with stems, the stems extending parallel to one another, said valve stems being oriented with respect to a longitudinal axis of the engine such that a plane passing therethrough will lie at an acute angle to the longitudinal axis of the engine. The external walls of the cylinder heads which provide the inlet and exhaust ducts, together with a central area arranged between the ducts, form, in the cylinder head section facing away from the cylinder, at least one arcuate or S-shaped cooling air duct when positioned adjacent a similarly constructed cylinder head.