Abstract: An externally screw threaded member of the self-locking, prevailing torque type has a series of convolutions of a standard thread form, and a locking series of convolutions of the same thread pitch as the standard series but of wider section and with incomplete thread crests. When the member is mated with an internal thread of standard form complementary to the standard series, the locking series of convolutions have an interference fit with the internal thread. In consequence of the interference fit one or both flank portions of each convolution of the locking series are caused to be swaged to some extent towards the crest region.