The spring is held in the bore (10) at its end furthest from the screw head and part (9) and base (8). The spring extends right along the bore and is dimensioned so that on assembly its end (2a) near the head (5) closes off flush with the bore margin and is closed off above by the screw head. The spring end (2b) furthest from the screw head bears on a shoulder projecting radially inwards off the wall of the bore. The spring is fixed to the screw shaft (3) so as to rotate round the axis (4) of the screw and the spring end at the head is turned radially inwards so as to engage the axially operating undercut part (3a) of the screw shaft. This undercut part is formed of part of the shaft which stands radially clear of the head (6) over an area limited by a projecting shaft ring segment or plug.
