FIBER-SERT inserts are internally and externally threaded metal bushings that are installed in wood and other comparatively soft materials to provide strong, permanent threads for fastening. They cut their own threads as they are driven (turned) into previously drilled holes and the external flutes provide a balanced cutting action. There are no slots or holes through the wall of the insert so the inside threads stay clean. This permits the driving stud to extend within the whole length of the insert for accurate installation. The coarse, external threads are engineered for maximum resistance to backout and loosening.