A crank mechanism for a reciprocating compressor is described, which can be of the double acting type or of the type provided with opposite single acting cylinders (26A, 26B). The crank mechanism is made up of a crank shaft (10) that, through a connecting rod (12), transforms its rotary motion into reciprocating motion. The connecting rod (12) receives the motion from the shaft (10) and transmits it to a cross head (14) through a jack (16). The cross head (14) is in turn, normally, made up of two shoes, a lower one (42) and an upper one (40), guided by respective fixed counter-shoes (18, 20) and transmits its reciprocating motion to the piston (22, 22A, 22B) through a rod (24). One (40) of the sliding shoes is equipped with a surface for making contact with the tracks (18, 20) having different shape and/or size with respect to the contact surface of the opposite sliding shoe (42).