This manual covers the BMW R51, R61, R66, and R71 models, primarily produced in the late 1930s and early 1940s. These motorcycles represent a significant era in BMW's development of horizontally opposed twin-cylinder engines, renowned for their engineering quality and durability. The R series models are distinguished by their robust construction and performance for their time, making them highly sought after by collectors and enthusiasts. This comprehensive resource details their mechanical intricacies, reflecting BMW's long-standing legacy in motorcycle manufacturing. This Full Factory Service Repair Manual provides detailed guidance for the maintenance, repair, and rebuilding of your BMW motorcycle. It includes extensive coverage of the engine, including the top end, lubrication system, and removal/installation. You will also find in-depth information on the clutch, crankshaft, transmission, final drive, wheels, tires, brakes, suspension, steering, frame, and the electrical system. With hundreds of pages and detailed illustrations, this manual is an indispensable tool for DIY enthusiasts and professional mechanics alike, ensuring the longevity and optimal performance of these classic machines.