Abstract: An expander member has a screw-threaded trailing shaft portion and a leading expander portion which is preferably of one piece with the shaft portion and diverges forwardly from the same. An expansion sleeve slidably surrounds the shaft portion of a part of its length, and an actuating member has a front end and a tapped bore into which the shaft portion is threaded so that, when the actuating member is turned in requisite direction, the front end engages the expansion sleeve and pushes the same onto the expander portion of the expander member.