may be slidably reciprocated along the tool axis 27 and is also free to rotate within the cylinder 30 about the tool axis 27 as shown the middle portion 53 of the piston 50 is provided with tooling flats 61 as best seen in fig1 and 2 a diametric tongue 63 extends upwardly from the upper face 59 of the piston 50 the tongue 63 shown is looking at fig1 rectangular in cross section but is most easily formed by use of a rotating cutter so that the upper face 59 of the piston 50 is looking at fig2 arcuate other cross sections and machining methods may be used however and the upper face 59 of the piston 50 may be in a horizontal plane the upper face 65 of the tongue 63 is preferably in a horizontal plane as is hereafter explained the pin 70 has an externally threaded lower portion 71 which engages within the internal threads of the upper portion 31 of the cylinder 30 the middle portion 73 of the pin 70 has an outer diameter which is greater than the inner diameter of the upper portion 31 of the cylinder 30 so that when the pin 70 is fully threaded into the cylinder 30 the middle portion 73 of the pin engages the upper end of the cylinder 30 and.