The Ariel Square Four: four cylinders in a square, English eccentricity turned cult classic

Iconic modelsEngland 1931-1959Four-cylinder

Designed in 1928 by Edward Turner, then twenty-seven years old, the Square Four lined up four cylinders in a square around two crankshafts geared together. Unveiled at the London show in 1930, it went from 497 to 601 cc, then to a full litre in 1937 under Val Page's redesign, with 38 bhp. The Mark II of 1953, all-alloy and crowned with four exhausts, pulled without a snatch from 10 to over 100 mph. Its Achilles heel remained the cooling of the rear cylinders. Just over 15,600 examples left Selly Oak before production stopped in 1959.

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An idea born in 1928 in the head of a young engineer

In 1928, Edward Turner is twenty-seven and runs a small motorcycle business, Chepstow Motors, in Peckham, south London. Between two repair jobs, he draws an engine that exists nowhere else: four cylinders arranged in a square, that is two parallel twins set side by side one behind the other and linked by their toothed central flywheels. Legend has it that the first sketch was traced on a cigarette packet; the workshop drawings, obviously, came afterwards. The idea is bold: to fit four cylinders into a package barely larger than that of a twin, with near-perfect balance as the payoff.

BSA turns the project down. Ariel, based at Selly Oak on the outskirts of Birmingham, accepts it. Turner is taken on there and works in the drawing office under chief draughtsman Val Page, alongside a very young Bert Hopwood, and under the eye of Jack Sangster, the man who would soon buy the firm. The prototype became reality in the autumn of 1930, when the machine was unveiled at the Olympia show in London as a 1931 model. The British press got carried away, and journalist Dennis May, writing as “Cyclops” in Motor Cycling, christened it the “Squariel”, a contraction of Square and Ariel. The nickname would stick to it for twenty-eight years. At the time, the big prestige British machines were still called Brough Superior, and the very idea of a production four-cylinder was pure exotica, barely more than forty years after the first stirrings of the motorcycle.

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Two crankshafts, four pistons, a single block

Technically, the Square Four is neither a transverse inline four nor a “square” in the manner of the racing two-strokes. It is a single block of four cylinders, topped by a single cylinder head, inside which turn two transverse crankshafts geared together. The pistons work in pairs set at 180 degrees: the reciprocating forces cancel each other out almost entirely and the gyroscopic effects of the two shafts, spinning in opposite directions, neutralise one another. The result is a smoothness of running that the British singles and twins of the day could not come close to.

On the first version, the valve gear is handled by a chain-driven overhead camshaft, a very modern solution at the time but costly to produce. The gearbox is a four-speed Burman, separate, with hand change. The whole thing sits in a thoroughly conventional motorcycle frame, largely shared with the Sloper 500, which makes the Squariel visually discreet: only the wide head and the four spark plugs give the layout away. Long before the Japanese sixes such as the Honda CBX 1000 or the German extravagances of the Münch Mammut, England had therefore offered the multi-cylinder road bike to a clientele that was asking for nothing of the sort.

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1931-1940: from 497 to 601 cc

The 4F model of 1931 displaces exactly 497 cc, with a bore of 51 mm for a stroke of 61 mm. Only 927 examples would be built in two seasons: the production machine had put on weight compared with Turner's prototype, which Val Page had to simplify to make it manufacturable, and its performance disappointed those in a hurry. Ariel reacted fast. From the 1932model year, the bore gained 5 mm and capacity climbed to 601 cc (56 × 61 mm), with a crystal-clear commercial target: the side-car, back then the real family car of working-class Britain. This version would last until 1940, at 2,674 units.

The 600 also served as a sporting showcase. Ariel entered it in an observed trial for the Maudes Trophy, where it swallowed 700 miles in 700 minutes, with one timed lap completed at 87.4 mph. For a heavy tourer built for travelling, the exercise was enough to impress a public used to wheezy singles. This was the golden age of the British industry: Ariel, Norton, Triumph and BSA supplied a good share of what rolled on the roads of the Empire, and each marque cultivated its own mechanical peculiarity.

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1937: Val Page takes the Squariel up to a litre

Turner's overhead camshaft is elegant, but complicated, expensive to make, and it runs hot. The redesign is generally credited to Val Page, who dropped the ohc in favour of pushrod overhead-valve gear, driven by a single camshaft housed between the two crankshafts, simpler and better ventilated. Started in 1936, the new range reached the catalogue in 1937 with a brand-new model: the 4G of 995 cc (65 × 75 mm), often rounded up to 997 cc in the catalogues. With a compression ratio of 5.8:1, it develops around 38 bhp at 5,500 rpm and, above all, considerable torque.

This is where the Square Four's real reputation was born. The torque is such that the machine can be ridden on a single gear, so to speak, never calling for the gearbox, for a top speed that goes beyond 90 mph solo — some twenty less with a side-car attached. A grand-touring machine before its time, spiritual ancestor of what the Honda Gold Wingwould become, and an ideal side-car machine — a use that fans of theUral will understand straight away. The 4G would be built to the tune of 4,288 examples up to 1948. In the meantime, the Anstey-link plunger rear suspension appeared as an option in 1939, and the hydraulic telescopic fork replaced the girder in 1946.

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Mark I and Mark II: alloy and four exhausts

In 1949, Ariel replaced cast iron withlight alloy : aluminium cylinder block and head melt away nearly 14 kg (some thirty pounds) on the scales. This version, retrospectively christened the Mark I, claims around 35 bhp at 5,500 rpm for a dry weight of about 197 kg, and goes beyond 90 mph. Collectors nickname it the “two-piper”, because of its two exhaust outlets. 3,922 machines would leave the works this way between 1949 and 1953.

Then comes the most desirable of them all. The Mark II of 1953 received a redesigned head, cylinders more clearly separated to let the air through and, above all, four separate exhausts, two on each side, fed by two cast aluminium manifolds, which earned it the nickname “four-piper”. With the compression ratio raised to 7.2:1, power reaches around 40 bhp and the machine becomes a genuine 100 mph machine: in 1956, The Motor Cycle recorded 102 mph — more than 164 km/h — on a 193 kg Mark II, noting that it pulled “without a snatch” from 10 mph up to that maximum, in top gear alone. 3,828 Mark IIs would be built. The Mark II remains today the most sought-after of the British four-cylinder machines.

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The chronic flaw: the rear cylinders

There is a price to pay for this layout. An air-cooled engine needs the airflow to reach every cylinder; but in a square, the two front cylinders screen the two rear cylinders, which cook. On the overhead-camshaft version, the phenomenon is made worse by a one-piece block with few openings. Under hard use — heavy load, side-car attached, slow traffic in hot weather — the rear pair climbs in temperature, with the classic consequences: detonation, oil that suffers, a tired head gasket.

Every redesign tried to answer it: the switch to overhead valves in 1937, light alloy in 1949, more widely separated cylinders and four exhausts in 1953. The trouble was clearly reduced, never entirely cured, and it is still today the first thing any Squariel buyer looks at — even if “four-piper” owners are quick to call the reputation exaggerated. The Vincent Black Shadow had its own demons, the twins from Norton had theirs: British excellence always cost you something somewhere.

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Turner at Triumph, Ariel at BSA: the end of an adventure

The story of the Square Four is inseparable from that of its creator. In 1932, after Ariel's bankruptcy and then its purchase by Sangster, Turner became head of the marque's drawing office. In 1936, Jack Sangster bought Triumph, then in difficulty, renamed it Triumph Engineering Co. and installed Turner there as managing director and chief designer. As early as July 1937, he unveiled the Speed Twin 500, the parallel twin that would set the tone for the whole of British production for thirty years and lead straight to the Bonneville T120. The father of the square is therefore also the father of the Triumph twin : two opposing philosophies, one single man.

On the Ariel side, Sangster sold the marque to BSA in 1944, then handed Triumph over to the same group in 1951, before taking its chairmanship in 1956. Inside a conglomerate already turning out singles and twins by the tens of thousands, the Squariel became an anomaly: expensive to build, made in small batches, condemned by a price — close to 350 pounds at the end of its career — that the market no longer followed. In 1959, Ariel dropped the four-stroke and switched entirely to the 250 cc Leader and Arrow two-strokes to face the Japanese wave. In vain: the Ariel name disappeared from motorcycles, and the last machine to carry it would be the curious Ariel 3 tricycle of 1970.

In all, 15,639 Square Fours were built in twenty-eight years. In the 1970s, the Healey brothers would draw one final derivative from it, the Healey 1000/4, of which only 28 complete examples came out between 1971 and 1977. The British motorcycle, for its part, carried on its shift towards café racers and a speed culture to which the Squariel, a hushed tourer, never belonged. Today, coming across a square four in full song at Goodwood is still a small event.

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Summary table

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FAQ — Frequently asked questions

• Why is the Ariel Square Four nicknamed the “Squariel”?
  The nickname comes from the British journalist Dennis May, who wrote as “Cyclops” in Motor Cycling. As early as 1930, he contracted Square and Ariel into “Squariel”. The name stuck for the whole twenty-eight years of the model's career.
• Who designed the square engine, and what is his link with Triumph?
  Edward Turner, born in 1901, dreamed up the engine in 1928 while running a motorcycle business in Peckham. Turned down by BSA, the project was taken up by Ariel. Turner became head of Ariel's drawing office in 1932, then joined Triumph in 1936 as managing director and chief designer; there he unveiled the Speed Twin 500 in July 1937.
• Which capacities did the Square Four have?
  Three: 497 cc for the 4F of 1931, 601 cc from the 1932 model year, then 995 cc (65 × 75 mm) for the overhead-valve 4G of 1937, a capacity kept by the Mark I of 1949 and the Mark II of 1953. It is that same 995 that the catalogues and the advertising often round up to 997 cc.
• Why did the rear cylinders run hot?
  The engine is air-cooled, and in a square layout the two front cylinders screen the airflow meant for the two rear cylinders. The switch to overhead valves in 1937, to light alloy in 1949, then to more widely separated cylinders and four exhausts in 1953 eased the problem without ever removing it.
• Why did production stop in 1959?
  Ariel was sold to BSA by Jack Sangster in 1944. Inside a big group, the Square Four, complex and built in small batches, cost too much to make and sold for close to 350 pounds. In 1959, Ariel dropped the four-stroke for the 250 cc Leader and Arrow two-strokes; the name then disappeared from motorcycles, the Ariel 3 tricycle of 1970 being the last machine to carry it.

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