Specifically designed for F5J, this is the Mega 16/25/4CE F5J. It is a 3 Phase, 6 pole, permanent magnet inrunner, with a hardened steel, 5mm shaft riding on high quality ball bearings. The lightweight rotor is loaded with neodymium magnets and wrapped in kevlar and then dynamically balanced. The kevlar hold the magnets in place, withstanding RPMs in excess of 35,000. The hand wound stators and tight tolerances of the rotor / stator all contribute to its 86% efficiency rating.