The invention relates to a reciprocating engine comprising at least one piston (7) housed in a cylinder (2), said piston being provided with at least one piston ring (10) arranged in an allocated piston ring groove (8), which has ends pieces (12, 13) that engage with one another in the form of a groove and a spring and form an outer peripheral surface cooperating with a running surface located on the cylinder side which traverses the periphery without any gaps. The invention aims at making it possible for said motor to operate as long as possible with little wear and enabling high operating facility. This is achieved in that at least one end piece (12) is provided with at least one radial through recess (18) whose open cross section is influenced by the mutual relative movement of the end pieces (12, 13) that engage with one another.