BMW R 51

The BMW R 51 was a two-cylinder BMW motorcycle with a 500 cc boxer engine of 17 kilowatts, built from 1938 to 1941. BMW presented it on 18 February 1938 at the Berlin motor show with the R 61, R 66 and R 71 as a new series of large motorcycles with rear suspension, four engine variants in one chassis covering 500 to 750 cc. The R 71 was a side-valve 750 of slightly lower power with high torque, suited to sidecar work, while the R 51 was the sporting 500. Its predecessor was the R 5. The engine has two chain-driven camshafts for the overhead valves, and like the R 5 it is carried on through axles in the frame, with the silencer clamped directly to the engine axle and frame.