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.[1] The engine was essentially a pair of 'across frame' OHC parallel twins joined by their geared central flywheels,[2] with one four cylinder block (or Monobloc) and one head.[3] The idea for the engine was rejected by BSA, but adopted by Ariel. Thus it became the Ariel Square Four.
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.[4] The Square Four was now a genuine 100 mph motorcycle.[5]
In 1954 Ariel built prototypes of a Mk3 with Earles forks, but the model was never put into production.[3]
In 1959 Square Four production, and that of all other Ariel four-stroke models, ceased
1958 4G Mk II Square Four Specifications
| Engine | 997 cc, 4-stroke, square four, air-cooled, OHV, 8-valve |
| Bore/Stroke | 65.0 mm x 75.0 mm |
| Compression Ratio | 7.2:1 |
| Max Power | 45 bhp @ 5500 rpm |
| Max Torque | |
| Fuel System | Single SU carburettor |
| Lubrication | Double gear pump |
| Ignition | 6V 20A/h battery, coil ignition |
| Transmission | 4-speed |
| Final Drive | Chain |
| Overall Length | 82 inch |
| Overall Width | 32 inch |
| Seat Height | 30 inches (787 mm) |
| Wheelbase | 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) |
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