Ariel Motorcycles Square Four 4gii
The original 1931 Ariel Square Four was a 498cc model featuring single overhead cam valve operation.
The engine was enlarged to 600cc in 1932 by enlarging the bores 5mm but in 1939 a pushrod version was launched that was essentially a sleeved down version of the 995cc engine that Edward Turner designed in 1935 and which was launched for the 1937 model year to compete with the large displacement V-twins offered by BSA and Matchless.
To increase the rotating mass of the engine, the original small diameter geared flywheels connecting the crankshafts were replaced with larger diameter, overlapping flywheels and separate coupling gears, aiding the engine’s capacity to pull from low revs, particularly when laden with the extra weight of a sidecar. To demonstrate this enhanced pulling capacity and underscore the engine’s legendary smoothness, Turner had Freddie Clark accelerate a prototype 1,000cc model from 10mph to 100mph in top gear at Brooklands. In 1939, the Square Four was available with optional, Anstey-link, plunger-type rear suspension and it was upgraded to telescopic front forks in 1946.
The original 1,000cc engine design with cast iron cylinder barrels and head remained in production until it was superseded in 1949 by an all-alloy model, later designated the 4G Mk I-1000cc, when a newly-developed, four-exhaust-pipe cylinder head was introduced in 1953. The new, all-alloy model became known as the 4G Mk II-1000cc, or 4GII, and remained in production until 1959.
This example is one of 3,828 produced of that last Square Four model. It was acquired by NZ Classic Motorcycles in April, 2008, from a private owner in Alberta, Canada.