Abstract: This invention relates to mechanism for converting linear motion to rotary motion without the use of a crank or crankshaft. Two circular members which may be provided with teeth are driven simultaneously in opposite directions by a chain, belt or rack which is in turn connected to a piston reciprocating in a linear path. The invention is particularly adapted to vapor engines sometimes referred to as expanders. It also comprises both electrically and mechanically actuated valve motions, including a reverse means and means for varying cut-off.