The Ariel Square Four is one of the most ingenious engines ever to power a motorcycle, with literally four vertical cylinders in little more than the space for two. Its Achilles heel was overheating, which was never really solved until Suzuki applied liquid cooling to the same idea in the 1976 RG 500 Grand Prix bike – and that was a much simpler two-stroke.

Edward Turner (who would go on to design the ground-breaking 1938 Triumph Speed Twin) actually came up with the Square Four in 1928 and shopped it around Coventry where many of Britain’s leading manufacturers were located. Jack Sangster of Ariel bought the idea and in 1930 the Square Four (or “Squariel”) was launched at London’s Olympia show. It was built 500 cc and 600 cc versions with a single SU carburetor and a four-speed gearbox.

The Square Four paired two sets of vertical cylinders in one casting, with separate cranks, geared together so they revolved in opposite directions to minimize vibration. The first Ariel 4F Square Four models had a chain-driven overhead camshaft, and the smaller 500 cc engine was dropped in 1932 after 927 were built. The 600 cc would be built until 1940, and 2,674 were produced.

Sportsmen viewed the engine hungrily as a potential record breaker and both Howard Sikes and Ben Bickell tried supercharging the 500 cc unit for the Isle of Man TT and a Brooklands record. Neither one was successful, but Ariel did win the Maude’s Trophy in 1933, for 700 miles in 700 minutes. In 1936 the engine was completely redesigned as the 997 cc OHV Ariel 4G Square Four model, replacing small geared flywheels with larger versions that vastly increased torque at low rpm. It was built until 1948, with 4,288 sold.

After the Second World War, the Square Four gained telescopic forks and was advertised as “The World’s Most Wonderful Motorcycle”. It was pitched as a solo superbike and gained an alloy head and cylinder block in 1949 to become the Ariel 4G Mk 1 Square Four. The changes trimmed the bike by 33 lbs to 435 lbs total, and 3,922 were sold from 1949-53.

Overheating issues remained and were addressed in the final Ariel 4G Mk II Square Four, built from 1953-59. This gained a new cylinder head and four exhaust pipes, and power increased from 35 bhp to 45 bhp. In all 3,828 Mk II’s were built for a total of 15,639 before sales ceased in mid 1959. Ariel then focused on the pressed-steel frame, fully enclosed Leader and sporting Arrow, both of which used modified Adler 250 cc two-stroke engines.

Ariel Square Fours are a daunting proposition in anything other than excellent condition, due to their complexity and limited supply of parts. The later bikes are more successful in cooling, but Ariel never did build a rear swing-arm suspension, and the complicated plunger unit introduced in 1939 is hard on drive chains and requires frequent lubrication. Apart from overheating, another problem can be the oil sludge trap in the crankcase. It can plug up, with catastrophic results. The best solution for both issues is to fit a high volume oil pump and external oil cooler.

A well-tuned Square Four is easy to kick-start and a pleasure to ride. It’s very nicely finished, but it’s a gentle giant with a bad heart, so take it easy.