Abstract: An internal combustion engine, a fluid motor or a pump includes a cylinder block enclosing an elongated cylindrical bore. A double headed piston is slidably mounted within the bore. The piston and cylinder block are rotatable relative to each other about the longitudinal axis of the bore. Rotation is imparted by a sinusoidal cam and a cam follower. Reciprocation of the piston within the bore is accomplished along with corresponding rotational movement of the cylinder block. Opposed ends of the cylinder block include openings which are aligned with and periodically communicate with exhaust and intake chambers. Porting collars are slidably mounted to the cylinder block and are stationary relative to the cylinder.