The Culligan® OR-233 O-Ring fits the following water filters: Culligan® models US-600A, SY-2000, SY-2500, SY-2300, SY-2300S, SY-2650, RO-3000 and RO-3500. The o-ring is used to seal the housing canister to the housing cap to prevent leaks. Replace your o-ring every third filter change or when there is damage to prevent leaks.