frustoconical shape and the piston has a complementary frustoconical groove the ring is a parallelogram in cross section application_number US 39606103 A description referring to the figures wherein like numerals indicate like or corresponding parts throughout the several views a compressor incorporating the invention is generally shown at 10 in fig3 a typical compressor includes a swash plate 12 to reciprocate a plurality of pistons 14 with each piston 14 disposed in a cylinder bore 16 the swash plate 12 is connected to a driveshaft 18 that is in turn connected to a motor not shown the motor rotates the driveshaft 18 and the swash plate 12 to reciprocate the pistons 14 in the cylinder 16 vehicle air conditioning systems generally include a compressor that compresses and superheats refrigerant the refrigerant exits the compressor and flows to a condenser and then to an expansion device from the expansion device the refrigerant enters an evaporator and then returns to the compressor to begin the cycle again a compression chamber 20 is defined on the top of each piston 14 this chamber 20 is used to compress the refrigerant during the upward stroke of the piston 14 in order to achieve adequate compression of the refrigerant it is important to achieve a tight seal between the piston ring 22 and the cylinder bore 16 as best shown in fig1 and 2 the cylindrical piston 14 has an annular groove 24 in the outer circumference the groove 24 is defined by an upper side wall 26 and a lower side wall 28 that are spaced apart by a bottom 30 and extend into the outer circumference of the piston 14 the upper side wall 26 has a greater radial depth into the circumference.