he skirt 95 and sealing ring 97 into engagement on and with the top surface 62 of the stem section,`to sealtherewith, in the manner set forth above. lt is highly important and is to b e particularly noted that the diameter and effective cross-sectional area of the upper portion of the piston, in the first counterbore45 of the barrel, is greater than the diameter and etfectivecross-sectional area o