[5] The early engine with "two transverse crankshafts"[2] was essentially a pair of 'across frame' OHC parallel twins joined by their geared central flywheels, with a four-cylinder block (or Monobloc) and single head.

The first Ariel Square Four 4F was shown at the Olympia Motorcycle Show in 1930,[7] in chain driven overhead-camshaft 500 cc form.

[6] In 1932, the cylinder bores were enlarged by 5 mm to give a capacity of 601 cc, specifically to accommodate owners who wanted a sidecar.

[9] This model was used for the Maudes Trophy test, covering 700 miles (1,127 km) in 700 minutes, followed by a timed lap of 87.4 mph (140.7 km/h).

[8] In 1946, the plunger rear was available again, and oil damped telescopic front forks replaced the previous girder type.