Abstract: In the preferred form a locking plate comprises a small rigid sheet of metal having a longitudinal axis of symmetry along which a generally rectangular central aperture extends. The central aperture has transversely opposed internal side edges and longitudinally opposed arcuate end edges. The central aperture defines a pair of spaced apart longitudinal side straps bent upwardly from the plane of the locking plate and a pair of spaced apart transverse arcuate end bases. Each base plate is formed with an arcuate slot with the slots having a common center of curvature located at the axis midpoint and subtending a predetermined central angle. The central aperture internal side edges juxtaposed diametrically opposite side flanks of the hex-head enabling a fastener to be received in each arcuate slot and an underlying one of a pair of diametrically opposed threaded bores so as to lock the hex-head in its tightened position.