A rotary plug valve has a plug portion which is rotatably fitted within a valve body. This plug portion of the valve is joined by a stem or shaft portion which extends out of the valve body to interact with an actuator and transmit torque to the plug portion. Pressure needs to be applied to the plug portion to insure a proper closure. The invention relates to a mechanism to provide such pressure adjustment that uses only one screw that is positioned on the front of the valve cover for easy access. The force generated by the screw is transposed into a vertical force by a cam. The multiple faces of the cam are used to transmit the force vertically and will maintain the mechanism in a straight position.