Use the AquaPure 63597-174C o-ring for sealing the Aqua-Pure water filter housing cap to the Aqua-Pure water filter housing sump to prevent leaks, The AquaPure 63597174C oring is 3, 5"" in Diameter, The Aqua-Pure 63597-174C oring can be used with several of the AquaPure housings including AP11, AP11S, AP11T, AP12S, AP12T, AS26T, AP51T, AP101, AP101S, AP101T, AP102S, and AP102T, SKU: CWD1534