Abstract: The present invention provides an improved drive mechanism for reciprocating engines primary adapted for converting reciprocating motion of a piston within a cylinder to rotary motion which includes a piston and piston leg assembly which moves solely axially within the piston chamber with two rack gear members secured to the sides of the piston leg. A power driveshaft and an idler driveshaft are positioned on opposite sides of the movable piston leg and include a plurality of gears thereon positioned adjacent to each piston chamber for receiving driving movement therefrom.