This high-speed pulley is specifically designed for use on the Model A Ford alternators and comes with a special nut for installation. Driving the alternator at 2.6 times the engine speed, this pulley enables the alternator to continue to charge down to slower speeds and reduce initial alternator turn-on RPM. The pulley can be used to retrofit alternators equipped with originally installed large, slow speed pulleys. Not required for any Nu-Rex production alternators. This is a genuine Nu-Rex product.