A mechanism for varying the stroke length of an internal combustion engine during each cycle of operation includes a gear set with a first gear non-rotatably mounted to the engine block and a second gear having teeth formed on an inner surface thereof meshing with the first gear to achieve a uniform mechanical crank arm and a variable cam arm for producing a varying length of piston reciprocation throughout the overall stroke cycle of the engine. The orientation of the crank arm and the cam arm relative to the axis of piston reciprocation is selected for causing the crank arm and the cam arm to cooperatively produce a positive torque on the crankshaft at the top dead center position of the piston. The gear set is also selectively configured and dimensioned to achieve a predetermined ratio of the length of the cam arm to the length of the crank arm.