Culligan OR-38 Undersink and RV Filter O-Ring-- (Package Of 3) OR-38 Culligan Undersink and RV Filter O-Ring Quality Replacement parts by Professor Foam Replace your O-Ring with every 3rd filter change to prevent leaks 3 5/8 inches (outside diameter) Fits Culligan under sink and RV filter systems including US-550, US 560, SY-5167, and SY-5197