A piston-operated refrigerant compressor provided with pistons fitted with piston rings and reciprocating in cylinder bores of a cylinder block having a central boss portion formed in an inner end face thereof confronting a crank chamber for receiving a cam or swash plate mounted on a drive shaft, and the central boss portion of the cylinder block defining therein a plurality of axially projecting lip portions formed at a portion of each bore end of the cylinder bore for permitting the diameter of the piston ring to be compressively reduced during inserting of the piston into the corresponding cylinder bore at the assembling stage of the refrigerant compressor either in cooperation with a piston-assembling jig provided with guide bores to promote a smooth insertion of each piston together with the piston ring into the corresponding cylinder bore or without cooperation of the piston-assembling jig.