Abstract: The present invention relates to an apparatus for converting between reciprocating motion and rotational motion, and particularly involves such an apparatus used as a power output mechanism for an internal combustion engine. The end of a piston rod is connected to a cam device which follows a closed loop path defined by a cam track. The closed loop path includes a pair of generally parallel, substantially straight portions. The substantially straight portions of the closed loop path are disposed at an angle of greater than 90.degree., but less than 180.degree. to the longitudinal axis of the cylinder in which the piston travels.