Abstract: An internal combustion engine comprising a piston slidably disposed for rectilinear reciprocal movement within a cylinder. A drive rack pivotally mounted to the piston carrys a continuous internally facing row of teeth. A drive gear nonrotatably keyed to a drive shaft and internally adjacent to the continuous row of teeth engages a portion of the drive rack teeth. A runner guides the drive rack such that the drive gear cooperatively associates with the drive rack teeth to convert rectilinear movement of the reciprocating piston into rotary movement. One or both of the drive gear and the drive rack are shaped in accordance with the variable forces exerted on the piston during its motion so as to apply torque more evenly to the crank shaft.