Abstract: An improved fastener assembly includes helical internal and external thread convolutions. A generally helical insert having a plurality of turns is disposed between the internal and external thread convolutions. This insert is resiliently twisted or flexed from a first condition to a fully deflected condition under the influence of torsion forces applied to the insert by the internal and external thread convolutions. In one embodiment of the invention, the insert has four side surfaces. When this insert is in the first condition, one pair of side surfaces is disposed in abutting engagement with the leading flanks of the internal and external thread convolutions. At this time the other side surfaces of the insert are spaced apart from the trailing flank surfaces of the thread convolutions. As used herein, the leading flank surfaces of the thread convolutions are the flank surfaces which, when the thread is about to be assembled with a mating thread, face the mating thread.