Big Twin: These pulleys are milled from a solid chunk of steel and are 23% lighter than the stock cast iron piece; Baker recommends long-life pulleys for performance applications where 100,000 mile+ pulley life is required; Designed for long distance touring applications with mild to moderate engine performance upgrades. Sportster: Stock Sportster/Buell transmission pulleys are made of cast iron; Replace your stock 27 tooth pulley with this 26 for an instant 4% increase in torque to the rear wheel; These pulleys are lighter and stronger than stock; Less weight translates to reduced flywheel effect and quicker revs.