Abstract: A screw threaded member having a thread with at least two portions of different form, one portion being of standard form and the other portion, a locking portion, having its root and crest offset from the standard lead, the offset of the crest being in the same direction as and greater than the offset of the root, the flanks of the locking thread being correspondingly displaced and the flank on the side of the thread which is displaced away from the standard pitch line being bulged outwardly. The root offset may be between one third and two thirds of the offset of the crest. The offsets may be in either direction but are preferably in that direction which moves the lead flank away from the standard pitch line.