Abstract: An engine with a cylinder assembly defining a cylinder chamber therein in which is reciprocally mounted a piston on a support tube slidably projecting on opposing ends of the cylinder chamber. A drive shaft is rotatably mounted through the support tube and a camming arrangement connects the piston and drive shaft to convert the reciprocal motion of the piston into rotary motion of the drive shaft. A pair of roller and guides alignment mechanisms are connected to opposite ends of the support tube externally of the cylinder assembly to prevent the piston from rotating but permit reciprocal motion thereof.