This cord O-ring for tank is designed to use with Sta-Rite system 3 Modular Media SM series pool and spa cartridge filter model S7M120, S7M400, S8M150, S8M500 and also 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 and also fits with Sta-Rite System 3 pool and spa sand filter model S7S50, S8S70. Measures 21-inch size.