Cord O-ring for tank replacement | Fits Sta-Rite system 3 Modular Media SM series pool and spa cartridge filter model S7M120, S7M400, S8M150, S8M500 | Compatible with Sta-Rite System 3 SD series pool and spa D.E. filter model S7D75, S8D110 | Fits with Sta-Rite System 3 Modular SMD series pool and spa D.E. Filter | Measures 21-inch size