Abstract: To permit dry operation of a piston within the cylinder, the cylinder is made of metal, and the piston is made of polytetrafluorethylene; the piston, preferably, is formed with an annular ring groove extending downwardly from an end face thereof to leave a circumferential lip, the groove having a spring located therein biasing the lip outwardly against the cylinder wall, to provide for sealing contact of the lip against the cylinder wall, which is additionally reinforced by pressure of the gas being compressed by movement of the piston within the cylinder, tending to expand the lip against the piston surface, at the time when the compression is highest, and sealing is the most desired.