Abstract: This invention relates to direct acting rotary actuator cylinders wherein the linear motion of the piston is directly converted to angular motion by a means completely enclosed within the cylinder, which means comprises a slidable partitioning body engaging the circular cylindrical cavity in the cylinder in an air-tight sliding relationship and a circular cylindrical rod coaxially and rotatably disposed in said circular cylindrical cavity having a mid-section with a noncircular cross section of a diametrical dimension greater than the diameter of said circular cylindrical rod.