The 100760525 Centering Ring Assembly is designed for NW250 size ISO-MF or ISO-BF vacuum flanges and features a Viton® O-ring and 304 stainless steel construction for chemical resistance. These Series 76 centering rings are inserted between two same size ISO flange faces to provide the vacuum seal when joining them. The included Viton O-ring is recommended when high temperature, chemical resistance or long installed life make them beneficial. ISO Flanges and seals are designed to be tightened to metal to metal contact. It is important that the clamps or bolts not be over tightened or warping of the flanges could result. The torque rating for clamps and bolts is 7 to 10 pounds per foot.