Oring ( 5 3/4" Diameter ) for the 10" or 20" Sump of the large Heavy Duty "Big Blue" style filter housing. These take the large diameter ( 4 1/2" ) cartridges, and the length can be 10" or 20".
Our PN DM151122
Common "brands" include (Ametek, Plymouth, American Plumber, Pentek, and even Culligan OR-100).
Note: Only use foodgrade Silicone grease lubricant on this or any other o-ring ( otherwise o-rings will swell and shorten the life of the o-ring ).