Abstract: A self-locking adjustable screw comprising a housing having a generally tubular axial bore extending between openings in the axial opposed first and second ends thereof, a generally tubular downlock element coaxially disposed in the axial bore, the element including a co-axial bore extending between a closed end and an opening in a driving end, screw threads operatively interconnecting the element with the wall of the axial bore for translating rotation of the element into axial movement, a lock pin disposed for axial movement in the co-axial bore, a spring disposed between the closed end of the co-axial bore and the lock pin for biasing the lock pin toward the opening of the co-axial bore, a pin in slot structure for limiting the axial movement of the lock pin, a lock member movably disposed in a radially extending bore interconnecting the co-axial bore with the axial bore such that the lock in its normal position places the lock member in engagement with axial grooves in the wall of the axial bore to preven