Pentek 151122 O-ring is used for Big Blue / Heavy Duty Housings by Culligan, Ametek, Pentek, US Filter, Plymouth, Kleen Plus, and American Plumber. O-ring is 5 3/4" in diameter and can be used in place of the OR-100 by Culligan. Buna-N FDA grade O-ring. This O-Ring is used in all Big Blue housings by Pentek with 1" and 1 1/2" Inner / Outer Diameters, EXCEPT the Big Clear Housing. This O-Ring replaces the W10-OR.