formed by cords extending from the upper end of groove 139 to the upper end of 46 of the guide portion 138 there being circumferentially spaced elongate arcuate sections 47 between the relief portions 44 having the same external diameter as the upper piston portions 37 and assisting in guiding the hammer piston 20 in its reciprocation along the inner wall of the housing section 14 as described herein below when the hammer piston 20 is at the lower end of its stroke as shown in fig1 a and 1b a flow control piston corner 50 at the upper end of the piston portion 37 is spaced below the upper housing flow control corner 31 allowing air in the housing above the piston 20 to flow down through the passages 44 and into the internal circumferential exhaust groove 32 around the upper piston portion 37 then through the lower portion of the groove 32 and the grooves 39 and 136 into radial exhaust ports 51 formed through the hammer piston below its intermediate piston wall 52 that communicate with a elongate central piston cavity 53 into which an exhaust tube 54 extends upwardly from the anvil 18 the tube forming a continuation of exhaust passage 53 and communicating with an exhaust passage 55 through the anvil.