This manual covers the YZ450 and YZ450FR models produced between 1998 and 2009. Known for its potent 4-stroke, liquid-cooled, single-cylinder engine, typically around 449cc and delivering impressive horsepower and torque, the YZ450 has been a cornerstone of its off-road racing legacy. Throughout these model years, advancements were introduced, including the adoption of aluminum frames, enhanced suspension technologies, and refined engine tuning, solidifying its reputation for engineering quality and performance in the competitive motocross and off-road racing circuits. This comprehensive factory service repair manual provides detailed information essential for the maintenance and repair of your YZ450. It covers a wide array of systems, including engine mechanicals, drivetrain, chassis components, and electrical systems, offering step-by-step procedures for routine maintenance, diagnostics, and complex repairs. The manual is designed to empower owners and technicians alike, enabling them to understand, maintain, and effectively service these high-performance machines, thereby ensuring optimal performance and longevity.