Abstract: An anti-rotational fastening system consisting of a primary bolt or screw that is fabricated with an axial passage at the center of the drive head extending through the entire length. The threaded end has a tapered counterbore that is sectioned to provide radially expandable segments. The segments expand by means of an internally threaded conical shaped lock pin that is drawn into the tapered counterbore of the primary fastener by clockwise rotation of a secondary threaded device positioned in the axial passage. As these segments expand, clearance is reduced between the external threads of the primary member and the internal threads of the tapped hole or nut of the bolted connection. Frictional resistance to rotation is increased as the clearance between the threads decrease. The friction increases to a point where the fastening system is locked and resists all but deliberate loosening of the bolted connection.