The screw bolt assembly has a bolt sleeve (1) to give an adjustable mounting. A contact surface (7), at the head of the screw bolt sleeve (1), has a shortened and conical surface aligned away from the sleeve wholly around the outer edge (12) of the sleeve head or around a peripheral segment. The contact surface forms an angle (v) of 2-10 deg against a plane (13) which is at right angles to the longitudinal axis of the screw bolt sleeve. The assembly incorporates a support disc of polypropylene.
