Abstract: A plunger (4) for joining an inner workpiece (7) to an outer tubular workpiece (2) is disclosed. The plunger includes an expansion element (9) which engages the inner workpiece and crimps it to the outer workpiece. The expansion element (9) is an annular cup-shaped resilient element which is mounted on a part (21) which is movable relative to a body (8) and to a bottom plate (16). The body (8) and its bottom plate (16) are provided with guide surfaces which are spaced apart to provide a space through which the expansion element may pass upon movement of member 21. The guide surfaces constrain the resilient expansion element to move into pressure applying engagement with the inner workpiece.