1953 Ariel 4G MKII Square Four 
Claimed power: 45 hp @ 5,500 rpm
Top speed: 100 mph
Engine: 997cc air-cooled OHV “square” four, 65mm x 75mm bore and stroke, 7.2:1 compression ratio
Weight: 425 lb.
Fuel Capacity/MPG: 6 ga. (22.7 ltr.)
Price then/now: $950 (est.)/$15,000-$25,000 

Ariel Square Four owners are used to being quizzed about the unique cylinder arrangement enjoyed by their air-cooled engines, the question most often posed being, “Don’t the rear cylinders overheat?” Savvy “Squariel” owners are ready with the answer. “Yes, but when they do that, we put them on the front …”

Although the limitations of Edward Turner’s compact power unit were recognized fairly early on in its life, it was still good enough to remain in production for 28 years, longer than many more famous designs. When Ariel owner BSA Group pulled the plug in 1959, the Square Four had become a rather portly touring machine that, though still capable of relatively high speeds, was being outclassed in the performance and handling stakes by newer British 650cc twins. But it wasn’t always that way. Like many touring bikes (Honda’s Gold Wing comes to mind), the original Square Four was designed with performance in mind.

Page and Turner

In 1928, Edward Turner was a motorcycle dealer in Peckham, London, with a dream of manufacturing bikes of his own. He had already designed and built two versions of the 500cc single-cylinder “Turner Special,” the first with a gear-driven overhead camshaft and the second using a vertical bevel-drive shaft and face cam to operate the valves.

In his attempts to find a manufacturer among Britain’s bike makers, Turner fetched up at the offices of Ariel Works Limited in Birmingham, where, instead of taking on Turner’s special, they offered him a job in the design department working under Valentine Page. What had impressed Ariel boss Jack Sangster was Turner’s second design, a unique 4-cylinder sketched out, goes the legend, on the back of a pack of cigarettes. (It’s likely Turner had already produced engineering drawings, but the smoke-pack story has stuck.)

Turner’s creation bristled with fresh ideas and ingenuity. The 4-cylinder 500cc engine used two crankshafts connected by helical gears with a chain-driven overhead camshaft. The cranks were transversely mounted, with one set in front of the other and each crankshaft driving a set of two pistons. The pistons were literally in a “Square Four” arrangement. The rear crankshaft’s helical gear also transferred power to the integral 3-speed transmission.