periphery of shaft portion 32 39 guide bodies 40 serve on the one hand to an additional centering of second piston part 32 within first piston part 31 and on the other hand as a stop by which in cooperation with rear guide collar 38 the full extension of telescoping piston 5 is limited in this embodiment the outer diameter of first piston part 31 must be larger than the inside diameter of spindle hole 8 of the respective working machine 7 the outside diameters of centering head 33 bushing 50 and front guide collar 39 on the contrary must be slightly smaller than the diameter of the spindle hole of the machine to permit second piston part 32 to reach a fully extended state to penetrate entirely into spindle hole 8 of machine 7 the outside diameter of centering head 33 slidably fits the hollow spindle 8 of machine 7 thereby it is made certain that even the remnant of the rod to be worked is radially guided at its rear end after this end has been advanced into spindle hole 8 of the machine so that undesirable oscillations are prevented the axial lengths of second piston part 32 bushing 50 centering head 33 and the arrangement of guide bodies 40 in piston part.