Abstract: Methods, apparatus, and improved structure having as their common basis the sequence of collapsing a piston ring in a fixture approximating engine bore diameter, and separately lapping a major portion of the ring side and a remaining locality --narrow in width, of band size and preferably line size-- of the side of the ring while it is so collapsed, until circumferentially continuous portions of the side are flat in the range of at least about 10 to five light bands. That is, out-of-flatness of the two circumferentially continuous portions, stated another way, is limited in each case to a range equivalent to about five to 10 light bands at maximum, at standard inspection wave length.

Abstract: Methods, apparatus, and improved structure having as their common basis the sequence of collapsing a piston ring in a fixture approximating engine bore diameter, and separately lapping a major portion of the ring side and a remaining locality -- narrow in width, of band size and preferably line size -- of the side of the ring while it is so collapsed, until circumferentially continuous portions of the side are flat in the range of at least about ten to five light bands. That is, out-of-flatness of the two circumferentially continuous potions, stated another way, is limited in each case to a range equivalent to about five to ten light bands at maximum, at standard inspection wave length.