This invention relates generally to a refrigerant compressor, and more particularly, to an improvement in a piston assembly for a refrigerant compressor for use in a vehicle air conditioning system. 
Generally, in piston type refrigerant compressors, the piston is slidably mounted inside a cylindrical liner formed by a casting process which takes into account the resistance to wear and durability of the compressor.  This cylindrical liner must be placed inside a compressor housing formed of an aluminum alloy during a die casting process.  Since the cylindrical liner must be inserted within the compressor housing during the die casting process, the weight of the cylindrical liner cannot be reduced below a predetermined amount which thereby increases the cost of manufacturing the compressor housing and the cylindrical liner. 
One attempt to resolve the above disadvantages has been to form the cylindrical liner of an aluminum alloy rather than by casting.  In this construction of the compressor, the weight and cost of the compressor housing is reduced but other disadvantages occur.  For instance, the piston ring of the compressor, which is generally disposed on the outer peripheral surface of the piston to improve the sealing between the cylinder chamber and the crank chamber in the compressor housing, is generally formed of a high hardness material.  Since this high hardness piston ring contacts the cylindrical liner, heavy wearing of the cylindrical liner occurs.  Thus, it is not desirable to use a high hardness piston ring with an aluminum alloy cylindrical liner.  Instead, a resinous piston ring is used with an aluminum alloy cylindrical liner to reduce wearing of the cylindrical liner. 
Nevertheless, when an aluminum alloy cylindrical liner is used with a resinous piston ring in a wobble plate type compressor of the type described in U.S. Pat.  No.  Re.  27,844 and shown in FIG.  1, during the reciprocating motion of piston 27', the lower edge of one side of the piston often contacts the inner surface of cylindrical liner 12'.  This contact occurs because each connecting rod 28' in the above wobble plate type compressor is connected to a wobble plate at some angle to the center line of the cylindrical liner.  Accordingly, during the reciprocating motion of the piston within the cylindrical liner, the lower end portion on one side of the piston usually is pushed toward the inner surface of the cylindrical liner, contacts the cylindrical liner and causes abnormal wearing of the cylindrical liner.