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 lapping a locality at the side of the ring while so collapsed, until a circumferentially continuous portion-narrow in width and preferably line size-of the side is flat in the range of at least about 10 to 5 light bands. That is, the out-of-flatness of the circumferentially continuous portion, stated another way, is limited to a maximum range equivalent to about 5 to 10 light bands at standard inspection wavelength. One of the broadest methods here disclosed is the use exclusively of rings so carefully made and so lap inspected as above on a percentage basis that the inner peripheral edge at the side has all points thereon in coplanarity to a degree equivalent to the stated range to seal in absolute contact in a substantially continuous sealing ring.