With 633 cc (38.6 cu in), it was the largest and most powerful side-valve engine in the model range and with plenty of low end torque was mostly used to haul sidecars.

Mechanically, 'the power output, about which Norton Motors tended to be cagey, was lowly, 14 bhp for 1951'.

[1] Approximately 4700 of the nearly 100,000 military bikes made by Norton during WW2 were Big 4 sidecar outfits.

The first Big 4 machines were civilian versions starting in 1907 with a bore and stroke of 82 x 120 mm.

The sidecar was mounted on four leaf springs, and had two friction disc shock absorbers at the back.