Aqua Kleen AK-4031 Unicel Replacement Filter Cartridge for Swimming Pool and Spa 37 sq. ft., 5" OD, Open Hole: 2 3/8" Top, Open Hole: 2 3/8" Bottom, 13 5/16" Long, AK-4031 Exact Fit for Unicel: C-5635 Exact Fit for Pleatco: PJ37-IN-4 Exact Fit for Filbur: FC-1437 Guaranteed Perfect Fit - Extremely Durable - Wash & Reuse - Most Energy Efficient - Withstands High Pressures & Temperatures