Culligan OR-233 O-Ring of 3-Inch Diameter, Small, Black Inlet/out filter housing O-ring For use with the Culligan US-600 Under-Sink Drinking Water Filter For residential use The OR-233 O-Ring has been branded under Ace Hardware, American Plumber, Ametek, Culligan, Pentek, US Filter Plymouth Products. Use with Model No.US-600, Model No.SY-2300 and Model No. SY-2650 The OR-233 rubber o-ring gasket is 3" in diameter and is a Buna-N o-ring. The OR-233 o-rings are used to seal the housing sump to the housing cap to prevent leaks. The OR-233 oring should be replaced every two filter changes or if there is damage to the OR-233 oring.