Water Filter Housing Replacement O-Ring Set Compatible with Ametek WWHC-2 / American Plumber WWHC-2. We recommend consumers keep a spare o-ring on hand to avoid urgent situations. This set of two o-rings will accommodate filter housings which require large diameter cartridges. These cartridges have an approximate diameter of 4.5 inches. The o-ring itself actually measures approximately 5.465 inch (inside diameter). An o-ring facilitates a water tight seal between the water filter sump and the water filter head. The most common o-rings used in water filtration are made from Buna N material. Buna N is also commonly referred to as nitrile. This is the ideal choice for cold water filtration because they are inexpensive and have superior performance characteristics.