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Ariel Square Four

1958 Ariel Square 4Square Four1959 Ariel Square Four

Edward Turner conceived the Square Four engine in 1928. At this time he was looking for work, showing drawings of his engine design to motorcycle manufacturers. The engine was essentially a pair of 'across frame' OHC parallel twins joined by their geared central flywheels, with one four cylinder block (or Monobloc) and one head. The idea for the engine was rejected by BSA, but adopted by Ariel. Thus it became the Ariel Square Four.

The Square Four is one of the most charismatic British bikes built after the great leveller that was the second world war. A good ride on a good example reveals the gentle and forgiving nature of he big engine. The litre motor pulls very well almost from any speed, and in fact it is perfectly possible to pull away fairly smoothly in top gear alone. The top speed was never as high as might have been expected from an engine of this capacity, and indeed Ariel did not advertise the bike with the sporting pretensions of the other litre bike from Vincent. Instead they depicted tweedy and immaculate young things piloting their Squariel through a field of bemused lambs, in one particularly mystifying advert; the sad fact is that the lambs would have run a mile at the sound of an approaching Four. Not that the exhaust was particularly noisy by the standards of the time - it wasn't - but the clattering of even a low-mileage engine was enough to give ample warnings of your approach.

The big four's steering was a source of involvement too. No effortless featherbed this one , but a steady bike all the same. The machine's lengthy wheelbase and low-slung weight helped it negotiate long, fast sweeping curves with some aplomb, but at the same time those same features stopped it being the most flickable of models through tight corners. And as with the rest of the range, the experience of a well-worn rear suspension could be a speech-loosening one.

In 1953 a Mk 2 version was offered, with the big engine's inherent cooling problems finally addressed by the inclusion of not only an alloy head and barrel but by a redesign of the cylinder head to incorporate two exhausts on each side. One of the reason's for the Square Four's relatively low power output was the difficultly of effectively cooling the block and overheating traditionally went hand in glove with oil leaks. Flange distortion and blowing head gaskets.

The final version also featured a full-width alloy front brake, an oil tank of giant but distinctive dimensions, which contained a full gallon of lubricant (to aid cooling) and one of the most handsome styling jobs carried out on a British motorcycle.

Its braking could still be a little marginal, its handling was never the best and the chorus from the engine would awaken the most deaf of heavy sleepers but the Ariel Square Four is still one of the greatest British motorcycles.

Ariel Square Four 4F (1931 - 1936)

The first Ariel Square Four 4F was shown at the Olympia Motorcycle Show in 1930 in chain driven overhead-camshaft 500 cc form. 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. This model was used for the Maudes Trophy test, covering 700 miles in 700 minutes, followed by a timed lap of 87.4 mph. (In 1923 a Mr George Pettyt, of Maudes Motor Mart, had donated a "challenge trophy" for the ACU to award each year for the most meritorious, observed endurance test for motor cycles, known as the Maudes Trophy.)

Ariel Square Four 4G (1936 - 1949)

The Cammy engine gained a reputation for overheating the rear cylinder heads, so in 1936 the engine was completely redesigned, emerging as the 1937 OHV 995 cc model 4G. In 1939 Ariel's patented Anstey-link plunger rear suspension became an option.

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

Ariel Square Four Mark I (1949 - 1953)

In 1949 the Ariel Square Four Mark I saw the cast-iron cylinder head and barrel replaced by alloy head and barrel. This saved about 30 lb in weight. The 1949 machine weighed around 435 lb dry, produced 35 bhp at 5500 rpm. The Mark I was capable of 90 mph-plus.

Ariel Square Four Mark II (1953 - 1959)

In 1953, the four pipe 997 cc Ariel Square Four Mk II was released, with a separate barrels and a re-designed cylinder head, with four separate exhaust pipes. The Square Four was now a genuine 100 mph motorcycle.

In 1954 Ariel built prototypes of a Mk3 with Earles forks, but the model was never put into production.

In 1959 Square Four production, and that of all other Ariel four-stroke models, ceased.

1958 4G Mk II Square Four Specifications

997 cc, 4-stroke, square four, air-cooled, OHV, 8-valve
65.0 mm x 75.0 mm
Compression Ratio
Max Power
45 bhp @ 5500 rpm
Max Torque
Fuel System
Single SU carburettor
Double gear pump
6V 20A/h battery, coil ignition
Final Drive
Overall Length
82 inch
Overall Width
32 inch
Seat Height
30 inches (787 mm)
56 inches (1422 mm)
Dry Weight
425 lb / (197 kg)
Suspension Front
Telescopic Forks
Suspension Rear
Twin link-and-plunger units
Brakes Front
8-inch (203 mm) sls drum
Brakes Rear
8-inch (203 mm) sls drum
Tires Front
3.25 x 19 inches
Tires Rear
4.00 x 18 inches
Fuel Tank Capacity
5 imp gallons (23 litres)

Square Four Production

Years Produced
Mk I
All Models

Pre-War Ariel Square Four Model Designations

  • 1931-1932: 498 cc 4F/31 Square Four.
  • 1932: 498 cc 4F Square Four.
  • 1932-1936: 601 cc 4F/600 Square Four.
  • 1937-1948: 995 cc 4G Square Four.
  • 1939: 599 cc 4F/600 Square Four.