This manual covers the 400 SX, 450 SX, 400 MXC, 450 MXC, 400 XC, and 450 XC motocross and off-road motorcycles produced between the 2000 and 2007 model years. These machines are renowned for their performance-oriented design and are built for competitive off-road use. The heritage in motorsport is evident in the engineering quality and reliability of these models, which were developed to excel in demanding racing environments. The robust construction and potent single-cylinder, 4-stroke engines provide riders with the power and agility needed for motocross and enduro disciplines. This comprehensive workshop service repair manual provides detailed information essential for the maintenance, repair, and troubleshooting of your motorcycle. It covers all major systems, including the engine, transmission, chassis, suspension, brakes, and electrical system, with specific attention to components like the top end, crankshaft, clutch, and final drive. The manual includes step-by-step disassembly and assembly instructions, torque specifications, diagnostic procedures, and wiring diagrams, all presented in clear English. This resource is designed to empower owners and technicians to perform professional-level service, ensuring the optimal performance and longevity of these capable off-road machines.