These replacement elastomeric o-rings are used (in conjunction with a centering ring -- not included) for sealing ISO flanges. The ISO flange (also called Large ISO) is used for HV chamber ports and is available in two distinct styles; ISO-K which uses clamp fasteners and ISO-F which uses nut and bolt fasteners. O-Rings available in Viton (fluorocarbon), Buna-N (nitrile rubber).