Pentek Filter Housing O-Ring
Marty - Arizona (Sunday, January 22, 2012)
You'll want to replace this o-ring with every filter change, although merchants who sell the filter cartridges never mention it. Once you remove the old filter for a change, and the existing o-ring pops out (which it will!), you'll never get the old one to re-seat in the groove because it will have deformed and expanded in size.