This Replacement filter cartridge has the technical specifications as Unicel # C-4609, Filbur # FC-3725 and OEM # PC-8. This 8 inch long and 4 ¾ inch wide filter cartridge has open hole of 1 ¼ inch at its top while 2 7/16 inch hole at the bottom. By installing this filter cartridge you get clean and clear water for your spas and pools. Very easy to install this filter cartridge is mad of high quality materials.