Abstract: A self-locking threaded connecting device comprising a male component (11) with a recess locking thread type (31) and a female component (12) with a flexible thread segment (21). Tension from the alignment of the internal flexible thread segment urges projections (22) of the flexible thread segment into the recesses (32) of the recess locking thread and thereby prevent the threaded fastener from loosening and coming apart in response to axial pressure optionally combined with vibration, expansion, contraction or other movement. The projections and recesses are shaped so that the faces glide by each other and unlock with the application of adequate torque. The two threads are designed to work together. The female component when used with a standard male component without recesses provides resistance to inadvertent loosening.