an external combustion engine wherein a plurality of radially disposed piston cylinders are arranged around the axis of an output drive shaft . pistons are reciprocal in the cylinders and actuate rollers carried by radially extending slide bearings , the rollers engaging in an eccentric cam groove formed in a flywheel fixed to the drive shaft . the cylinders have closed inner and outer ends and are provided with valve means for applying pressurized gas to opposite ends of the pistons at different times in a manner to provide power to a majority of the pistons at all times during rotation of the drive shaft . the engine can alternatively operate as a pump by applying motive power to the shaft .