A piston unit of a pull 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 hole (2.1), in which the piston rod is received, and it has an inner ring groove which is axially open on one side towards a piston crown side 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 fixed by the coupling ring axially in a positive-fit manner on one side on the piston rod in a pulling 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, and wherein the fixing ring secures the piston axially in a positive-fit manner on one side on the piston rod in a return direction, characterized in that the fixing ring has a flattened radial cross section (4.2), that the flattened radial cross section (4.2) has a cross-sectional longitudinal axis (4.3) and a cross-sectional transverse axis (4.4), that the cross section (4.2) extends longer along the cross-sectional longitudinal axis (4.3) than along the cross-sectional transverse axis (4.4), that the cross section (4.2) has opposing planar boundaries along the cross-sectional longitudinal axis (4.3) which form a first and a second annular surface (4.6, 4.7), that the fixing ring is elastically resilient, that the fixing ring is designed such that it assumes a tensioned position under the action of a force and a relaxed position when a force is not applied on it, that in a relaxed position, the cross-sectional longitudinal axis (4.3) is Inclined by an angle alpha relative to a ring plane (4.5), that 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, that in the tensioned position, the cross-sectional longitudinal axis (4.3) is Inclined by an angle beta, and that the angle beta is greater than the angle alpha. 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 hole (2.1), in which the piston rod is received, and it has an inner ring groove which is axially open on one side towards a piston crown side 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 fixed by the coupling ring axially in a positive-fit manner on one side on the piston rod in a pulling 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, and wherein the fixing ring secures the piston axially in a positive-fit manner on one side on the piston rod in a return direction, characterized in that the fixing ring has a flattened radial cross section (4.2), that the flattened radial cross section (4.2) has a cross-sectional longitudinal axis (4.3) and a cross-sectional transverse axis (4.4), that the cross section (4.2) extends longer along the cross-sectional longitudinal axis (4.3) than along the cross-sectional transverse axis (4.4), that the cross section (4.2) has opposing planar boundaries along the cross-sectional longitudinal axis (4.3) which form a first and a second annular surface (4.6, 4.7), that the fixing ring is elastically resilient, that the fixing ring is designed such that it assumes a tensioned position under the action of a force and a relaxed position when a force is not applied on it, that in a relaxed position, the cross-sectional longitudinal axis (4.3) is Inclined by an angle alpha relative to a ring plane (4.5), that 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, that in the tensioned position, the cross-sectional longitudinal axis (4.3) is Inclined by an angle beta, and that the angle beta is greater than the angle alpha.