Abstract: A piston machine has arcuately-curved, cooperating pistons and cylinders arranged on the periphery of a pair of coaxially mounted rotatable disks or arms. Swiveling movements of the disks, causing the pistons to move in the cylinders, are controlled by cams and connecting gears. The cams are fixed with respect to the disks. Each cam is scanned by a cam follower connected by an arm to a shaft journaled in one of the disks. A connector gear is affixed to the cam follower and connector gear shaft to drive the disk opposite the disk in which the connector gear shaft is carried. Gear segments on the external periphery of each disk engage interchangeably a driven shaft for power output where work is produced by the machine as in compressed air and heat engine embodiments disclosed or for power input where the machine does work on the compressible fluid as in a compressor embodiment.