A device for protecting motor-vehicles against theft comprising a freely rotatable sleeve covering the polygonal head of a wheel nut or screw, into which sleeve a cylinder lock engages rotatably, which lock is provided with at least one tumbler entering the inner wall of a cylinder lock location cavity of said polygonal head which projects beyond the circumference of said cylinder lock. In the locking position, said tumbler engages behind a cooperating locking shoulder which is located on an annular body inserted into an axial chamber of said polygonal head and locked in position thereto.