Abstract: Apparatus including a reciprocating piston driven by a connecting means operating within a slider for driving a piston in a reciprocating motion. The slider is mounted in a crank case housing and includes guides for maintaining the orientation of the piston relative to surrounding cylinder walls. The cylinder is attached to the crank case housing at one end and attached to a cylinder head at the opposite end. The cylinder head includes an intake valve and an exhaust valve for controlling the flow of fluid into and from the piston chamber. To drive the piston a crank shaft is rotatably attached to a connecting means that rotates within the slider and connects thereto. Fastened to the crank shaft is a flexible coupling for coupling the crank shaft to a source of driving power.