Abstract: The metallic body of the nut is formed with a groove into which is inserted an elongate flexible compressible insert which may be rectangular, hexagonal or circular in cross-section and which may be made of, for example, nylon or a fibrous material. After use, the insert is removed from the groove, is indexed about its longitudinal axis to present an unmutilated face or portion and is then put back into the groove with said unmutilated face or portion facing radially inwardly.The groove may be continuous or it may be interrupted by an end stop in order to prevent movement of the insert along the groove during screwing-on of the nut; in such a case, the insert length is less than the circumferential length of the groove.An extraction hole, extending either radially or axially of the nut through the nut body into the groove, facilitates removal of the insert from the groove for indexing or replacement.