These adapters are KF-25 to 1 1/8-20 thread size, Welch vacuum pump Inlets, for direct drive model pumps 8907, 8912, 8917, 8920, 8925 all serial numbers. They are commonly used to make connections between Welch vacuum pumps and ISO-KF Flanges. The ISO-KF flanges are often called quick flange fittings because they are quick to assemble. A circumferential clamp and centering ring form the vacuum seal between the connecting flanges (please see top right figure - click to enlarge). The centering ring contains a rubber elastomer o-ring. The standard sizes for quick flange fittings are KF-10, KF-16, KF-25, KF-40, and KF-50 with flange sizes NW-10, NW-16, NW-25, NW-40, and NW-50, respectively. They are made of stainless steel.