the half and extends in a uniform diameter major portion 100 axially within the half each recess terminates in an annular rim 102 within which a reduced diameter opening 104 is formed externally each half includes an external annular shoulder 106 opposite the central face 98 the shoulder 106 has an enlarged outer diameter referring again to fig8 the halves 90 92 are assembled with central faces 98 in contact and with valve locating pin 95 in recess 97 of the half 90 aligned circumferentially with the locating pin 97 of the half 92 the halves are held against a shoulder 108 of the piston rod 26 by mechanical fastener such as a nut 110 threaded on the end of the piston rod 26 an annular temperature resistant wear band 112 encircles the halves 90 92 and is held between the shoulders 106 of the halves 90 92 the wear band 112 extends outward of the shoulders 106 to contact the inner wall of the inner cylinder 16 in the embodiment illustrated three valve recesses 96 and three cylindrical cavities are alternately formed in each piston half 90 92 together these recesses form six piston valve chambers three piston valve chambers such as chamber 114 in fig8 are occupied by piston compression valves such as valve 116 three piston valve chambers such as chamber 118 are occupied by piston recoil valves such as valve 120 as in fig8 no fluid flows through the piston.