Includes upgraded rotor with O-rings and spacer to reduce chamber volume. This rotor assembly comes with 5 each check valves, 1 each purge stand pipe, all O-rings and seals, all nuts, bolts and screws (comes fully assembled). The column seals are made by O-rings. This rotor assembly has the bank slow drains. This can be used in a Biolytic 3900. This cannot be used in an ABI or original designed 3900.