Aqua Kleen AK-3021 Unicel Replacement Filter Cartridge for Swimming Pool and Spa 50 sq. ft., 4 15/16" OD, Open Hole: 2 1/8" Top, Open Hole: 2 1/8" Bottom, 17 3/8" Long, AK-3021 Exact Fit for Unicel: C-4349 Exact Fit for Pleatco: PAE50 Exact Fit for Filbur: FC-6310 Guaranteed Perfect Fit - Extremely Durable - Wash & Reuse - Most Energy Efficient - Withstands High Pressures & Temperatures