Fits Big Twin equipped with 22 or 32 amp charging system The alternator rotor features a fully encased set of magnets that have no way of working loose like the OEM rotor is known to do Instead of having just six long magnets like the OEM rotor, this rotor has 12 short magnets to match the number of laminated windings found on the stator Replaces OEM 29957-81B and 29957-85A