Abstract: This valve actuator is primarily intended for use with rotary cut-off valves. It includes a housing which is attached to the valve body for receiving the valve stem and a longitudinal passage slidably mounting opposed piston elements. The piston elements include spaced shoulders which engage the remote end of a radial arm mounted within the housing and attached to the stem for rotating said stem. The radial arm includes a cam at the remote end having opposed cam faces alternately engaged by the piston elements and configurated to provide a minimum gap between the cam and the piston shoulders during the rotation of the valve between open and closed positions.