A piston unit of a differential cylinder, comprising a piston rod, a piston, a coupling ring, and a fixing ring, wherein the piston rod has an outer ring groove, wherein the piston has an axial bore (2.1) in which the piston rod is received, and it has an inner ring groove which is axially open on one side facing a piston head end (2.2), wherein the inner ring groove has a coupling ring section (6.1) and a fixing ring section (6.2) adjacent to it, and wherein the fixing ring section (6.2) has an undercut contour (6.3), wherein the outer ring groove and the coupling ring section (6.1) form a coupling ring groove space in which the coupling ring is arranged, wherein the coupling ring has a divided design and is radially enclosed by the coupling ring section (6.1), and wherein the piston is secured by the coupling ring in an axially positive-fit manner on one side on the piston rod in a tensile direction, wherein the fixing ring is arranged in the fixing ring section (6.2) between the undercut contour (6.3) and the coupling ring, wherein the fixing ring has a resilient design and in a relaxed position it engages in the fixing ring section (6.2) of the inner ring groove, characterized in that the fixing ring has a flattened radial cross section (4.2), wherein the flattened radial cross section (4.2) has a cross-sectional longitudinal axis (4.3) and a cross-sectional transverse axis (4.4), wherein the cross section (4.2) extends longer along the cross-sectional longitudinal axis (4.3) than along the cross-sectional transverse axis (4.4), wherein the cross section (4.2) along the cross-sectional longitudinal axis (4.3) has opposing planar boundaries forming a first and a second annular surface (4.5, 4.6), wherein the fixing ring is elastically resilient, wherein the fixing ring is designed such that it assumes a tensioned position when it is subject to force and a relaxed position when a force is not applied on it, wherein in a relaxed position, the cross-sectional longitudinal axis (4.3) is inclined relative to a ring plane (4.7) by an angle alpha, wherein the fixing ring can be tensioned to a tensioned position by means of a change in the cross-sectional longitudinal axis (4.3) about a circumferential ring axis, wherein in the tensioned position, the cross-sectional longitudinal axis (4.3) is inclined by an angle beta, and wherein the angle beta is greater than the angle alpha, and wherein the fixing ring secures the piston axially and in a positive-fit manner on one side on the piston rod in a return direction. comprising a piston rod, a piston, a coupling ring, and a fixing ring, wherein the piston rod has an outer ring groove, wherein the piston has an axial bore (2.1) in which the piston rod is received, and it has an inner ring groove which is axially open on one side facing a piston head end (2.2), wherein the inner ring groove has a coupling ring section (6.1) and a fixing ring section (6.2) adjacent to it, and wherein the fixing ring section (6.2) has an undercut contour (6.3), wherein the outer ring groove and the coupling ring section (6.1) form a coupling ring groove space in which the coupling ring is arranged, wherein the coupling ring has a divided design and is radially enclosed by the coupling ring section (6.1), and wherein the piston is secured by the coupling ring in an axially positive-fit manner on one side on the piston rod in a tensile direction, wherein the fixing ring is arranged in the fixing ring section (6.2) between the undercut contour (6.3) and the coupling ring, wherein the fixing ring has a resilient design and in a relaxed position it engages in the fixing ring section (6.2) of the inner ring groove, characterized in that the fixing ring has a flattened radial cross section (4.2), wherein the flattened radial cross section (4.2) has a cross-sectional longitudinal axis (4.3) and a cross-sectional transverse axis (4.4), wherein the cross section (4.2) extends longer along the cross-sectional longitudinal axis (4.3) than along the cross-sectional transverse axis (4.4), wherein the cross section (4.2) along the cross-sectional longitudinal axis (4.3) has opposing planar boundaries forming a first and a second annular surface (4.5, 4.6), wherein the fixing ring is elastically resilient, wherein the fixing ring is designed such that it assumes a tensioned position when it is subject to force and a relaxed position when a force is not applied on it, wherein in a relaxed position, the cross-sectional longitudinal axis (4.3) is inclined relative to a ring plane (4.7) by an angle alpha, wherein the fixing ring can be tensioned to a tensioned position by means of a change in the cross-sectional longitudinal axis (4.3) about a circumferential ring axis, wherein in the tensioned position, the cross-sectional longitudinal axis (4.3) is inclined by an angle beta, and wherein the angle beta is greater than the angle alpha, and wherein the fixing ring secures the piston axially and in a positive-fit manner on one side on the piston rod in a return direction.