Abstract: In a self-locking screw nut comprising a central body having a central screw hole and an outside hexagonal upper portion and a downwardly tapering lower portion, and a ring nut having an inner taper matching that of the lower portion of the central body and adapted to be applied thereto, the improvement comprising a slit extending axially over the entire length of said central body and another slit extending from the top of said hexagonal portion to part of the length of said downwardly tapering lower portion to ensure uniform distribution of the forces of deformation acting on the screw nut in its position of use.