The rotor/impeller assembly for Rena SmartFilter 55 consists of the magnet and impeller blades. These should be replaced when you notice a significant slowing of filter flow or if your rotor/impeller is damaged due to hot running or particles clogging the pump intake. The rotor/impeller areas requires maintenance on a monthly basis to maintain optimum operation. NOTE-Impeller Blade Color Will Vary and you may or may not need the extra parts included. This rotor/impeller assembly is also used in regular water pump models S40, C40, S40T, C40T, RF900S, RF1100S.