A swinging arm Square 4
The Ariel Square Four – a brief background
The original Square Four engine was designed by Edward Turner in 1928. The early engine with “two transverse crankshafts” was essentially a pair of ‘across frame’ OHC parallel twins joined by their geared central flywheels, with a square four-cylinder block and a single head. With a chain driven overhead-camshaft 500 cc early Square Fours used a hand-change, four-speed Burman gearbox. In 1932, the cylinder bores were enlarged by 5 mm to give a capacity of 601 cc, specifically to accommodate owners who wanted to attach a sidecar.