Improved rotor position control for rotary machines allows for positioning mechanisms which are more compact, relative to the stroke of the machine. A crank having a center shaft which extends through the rotor chamber is rotatably mounted in a forward in wall of the chamber. An eccentric shaft of the crank is rotatably mounted by the rotor and is aligned with rotor axis. A stationary gear extends into the rotor chamber and is rigidly mounted in a forward in wall of the chamber. A crank web is connected to the center shaft rearward of the stationary gear. The eccentric shaft passes through the rotor gear and is connected to the crank web forward of the rotor gear. The rotor gear has a pitch radius greater than that of the stationary gear by a ratio of the number of lobes of the rotor divided by the number of lobes minus one. A re versing gear is rigidly mounted to the crank web and has a center axis offset from that of the rotor by a distance equal to the stationary gear pitch radius plus the reversing gear pitch radius.