A damper assembly comprising: a main tube disposed on a center axis and extending between a first end and a second end defining a fluid chamber (11, 12) for containing a working fluid; a main piston slidably disposed in said fluid chamber (11, 12) dividing said fluid chamber (11, 12) into a rebound chamber and a compression chamber with said compression chamber being between said first end and said main piston and said rebound chamber being between said second end and said main piston; a piston rod disposed on said center axis and attached to said main piston for moving said main piston between a compression stroke and a rebound stroke; an additional piston attached to said piston rod adjacent to said main piston for providing additional damping force; characterized by said additional piston including a body having an upper member and a lower member axially spaced from one another and defining a groove between said upper member and said lower member; a sealing ring disposed in said groove between said upper member and said lower member for movement in said groove with said sealing ring being radially expandable in response to a working fluid pressure to engage said main tube during said compression stroke and said rebound stroke; and said sealing ring including an outer surface facing said main tube, an upper surface facing said upper member, and a lower surface facing said lower member and at least one annular collar (8431, 8432) extending outwardly from said sealing ring for forming a locking engagement with said upper member and said lower member to limit the radial expansion of said sealing ring, wherein said at least one annular collar (8431, 8432) includes a rebound collar extending annularly outwardly from said lower surface toward said lower member for engaging said lower member, wherein said lower member defines a lower recess facing said sealing ring for receiving said sealing ring and allowing said sealing ring to form said locking engagement with said rebound collar during said rebound stroke. a main tube disposed on a center axis and extending between a first end and a second end defining a fluid chamber (11, 12) for containing a working fluid; a main piston slidably disposed in said fluid chamber (11, 12) dividing said fluid chamber (11, 12) into a rebound chamber and a compression chamber with said compression chamber being between said first end and said main piston and said rebound chamber being between said second end and said main piston; a piston rod disposed on said center axis and attached to said main piston for moving said main piston between a compression stroke and a rebound stroke; an additional piston attached to said piston rod adjacent to said main piston for providing additional damping force; characterized by said additional piston including a body having an upper member and a lower member axially spaced from one another and defining a groove between said upper member and said lower member; a sealing ring disposed in said groove between said upper member and said lower member for movement in said groove with said sealing ring being radially expandable in response to a working fluid pressure to engage said main tube during said compression stroke and said rebound stroke; and said sealing ring including an outer surface facing said main tube, an upper surface facing said upper member, and a lower surface facing said lower member and at least one annular collar (8431, 8432) extending outwardly from said sealing ring for forming a locking engagement with said upper member and said lower member to limit the radial expansion of said sealing ring, wherein said at least one annular collar (8431, 8432) includes a rebound collar extending annularly outwardly from said lower surface toward said lower member for engaging said lower member, wherein said lower member defines a lower recess facing said sealing ring for receiving said sealing ring and allowing said sealing ring to form said locking engagement with said rebound collar during said rebound stroke.