Norton Big 4

The Norton Big 4, officially the Model 1, was a large side-valve motorcycle that Norton made in various forms between 1907 and 1954. Its 633 cc engine was the largest and most powerful side-valve unit in the Norton range, and with plenty of low-speed torque it was mostly used to pull a sidecar. The name came from its rating of four tax horsepower, and the real output was modest, about 14 bhp in 1951. The first machines were civilian models with a bore and stroke of 82 by 120 mm, and in 1948 the stroke was shortened to give 596 cc. In the Second World War about 4,700 of Norton's nearly 100,000 military machines were the WD Big 4, a sidecar outfit with a driven sidecar wheel used for reconnaissance and carrying ammunition. Side-valve singles ended in 1955.