the chamfered surfaces 42 45 define a guide passage 46 which extends along the circumference of each piston 26 the cross sectional area of the guide passage 46 increases toward the discharge port 32 the maximum width of the chamfered surface 45 is slightly greater than the width for example 0 5 to 1 0 mm of the chamfered surfaces 41 42 of the embodiment of fig1 to 4 the volume of the space when the piston 26 is at the top dead center position or the volume of the dead space is smaller than that of the embodiment of fig1 to 4 in addition to the advantages of the embodiment of fig1 to 4 the embodiment of fig7 and 8 has the following advantages the width of the chamfered surface 45 decreases at locations that are farther away from the discharge port 32 thus compared to the embodiment of fig1 to 4 the compressor of fig7 and 8 has a smaller dead space which improves the compression efficiency the guide passage does not need to be formed along the circumference of the end face of the pistons 26 for example as shown in fig9 and 10 a groove 48 may be formed on the piston end face to define a central guide passage 49 to conduct gas in the compression chamber 36 to the discharge port 32 in the embodiment of fig9 and 10 the ports 31 32 are radially inside the wall of the cylinder bore 25 the groove 48 extends along a diametral line connecting the.