Aqua Kleen AK-4001 Unicel Replacement Filter Cartridge for Swimming Pool and Spa 50 sq. ft., 5 11/16" OD, closed w/ string loop Top, Open Hole: 2 1/8" Bottom, 10 3/8" Long, AK-4001 Exact Fit for Unicel: C-5300 Exact Fit for Pleatco: PJW50 Exact Fit for Filbur: FC-1320 Guaranteed Perfect Fit - Extremely Durable - Wash & Reuse - Most Energy Efficient - Withstands High Pressures & Temperatures