Abstract: An internal combustion engine is disclosed having opposed, axially aligned, annular reciprocating and rotatable pistons connected through radially disposed crankshafts to a drive shaft which extends coaxially of the opposed pistons. The crankshafts are so supported to permit rotation about the axis of the drive shaft when a clamping brake is released, as during an idle condition. Planetary cam plates are rotated by and in timed relation to the crankshafts and serve to control valve and ignition operation for four cycle operation.