Abstract: A fitting having members threaded together to a desired position is provided with an arrangement for locking the members against relative rotation so as to maintain the members within a predetermined angular range of rotation relative to each other which encompasses the desired position. The locking arrangement includes longitudinally extending recesses in the outer surfaces of the fitting members which are axially aligned, and an insert which is fitted in a set of axially aligned recesses to lock the members against relative rotation. The recesses are keyhole shaped. The insert is also keyhole shaped in cross section, or is cylindrical. The insert may be made of suitable plastic such as nylon, or may be metallic. When the insert is of a material that is resiliently deformable, it may be laid over and then pressed into the recesses. A metallic or other less deformable insert, such as a rod or wire segment of safety wire, piano wire, etc.