These ISO DN 63 without spacer centering rings are made of Stainless Steel with durable Viton® (FPM) rubber O-rings and are used to seal both ISO-K and ISO-F vacuum flanges. These centering rings follow vacuum technology standards as described in DIN 28404 and ISO 1609. The centering ring which contains a rubber elastomeric O-ring forms the vacuum seal between the two mating ISO-K or ISO-F flange faces. ISO-K fittings are quick to assemble using claw clamps (please see figure - click to enlarge) and can be connected together in any orientation. The claw clamp hooks into a ridged groove in the back side of the ISO-K flange. ISO-F fixed flange vacuum fittings are different and have a bolt circle where bolts align the two flanges. These ISO 63 centering rings are commonly used to build process vacuum plumbing and systems for industrial standard and high vacuum applications. The ISO standard sizes for large LF flanges are ISO-F and ISO-K DN 63, 80, 100, 160, 200, 250, 320, 400, 500, 630, 800, 1000. These vacuum fittings are made with inner and outer rings of Stainless Steel with FPM rubber O-ring material.