Abstract: A modified hypocycloid gear arrangement is incorporated as a transmission converting rotary motion into linear motion or vice versa in any form of prime mover, such as a gas engine, internal combustion engine, or any form of pump or other energy converter, in place of a conventional crank-connecting rod transmission element; specific embodiments are disclosed, relating to single, twin, flat-four, vee-8 and other single and multi-cylinder piston engines and motors and their adaptions, e.g. to vibration-free surgical power saws and torpedo propulsion units, due to the improved mechanical efficiency and precise engine balancing allowed by transmission arrangemenets according to the invention disclosed.