Ariel Square Four: Collectable

Date 26.8.2014

Presented by
  • Motorcycle Trader


Ariel Square Four


Although a four-cylinder motorcycle is almost the norm today, it wasn’t always so. Before World War II, inline fours with the engine lengthwise were considered unwieldy and across-the-frame fours too wide. One day in late 1928, a London motorcycle dealer named Edward Turner sketched on the back of a cigarette packet the idea of two parallel twins side by side.

Turner made a trip to the Midlands trying to sell the idea, and while BSA nearly bought it, it was Ariel that provided Turner with a small drawing office and the resources to see his idea to fruition. Turner went on to arguably become the most influential person in the British motorcycle industry and his Square Four, or ‘Squariel’, a legend during its own lifespan in production.

The first Square Four appeared in 1931 and was a cast-iron 500 with a chain-driven single overhead camshaft, but the basic layout of cylinders at the corners of a square with two 180-degree crankshafts geared together and contra-rotating set the pattern for the future.

To make the engine more suitable for sidecar use, a 600cc version was added in 1932 but, as there were so many problems with the initial design, the engine was completely updated for 1936.

Although the new engine retained the two-by-two cylinder format, in most other respects it was totally revised. Instead of the earlier overhead camshaft, the valves were now pushrod-operated.

The crankshafts were coupled by outboard gears on the left side and the big-end bearings changed from roller to white metal in light-alloy connecting rods. The capacity was increased to 1000cc and the weight reduced. The Square Four continued until the outbreak of war largely unchanged, except for a sprung hub that became an option in 1939.


Resurrected after World War II, the Square Four initially retained the cast-iron 65 x 75mm, 997cc engine of the pre-war Squariel. But the addition of telescopic forks and the compensated-link plunger rear springing saw the first post-war models weighing more than 225kg.

Ariel figured it was time to shed some weight and for the 1949 Mk I (shown here) the old cast-iron cylinder block and head were scrapped and replaced with alloy castings. The claimed weight saving was an optimistic “half a hundredweight” (25kg), but the real saving was closer to 15kg.

The redesigned alloy cylinder head now included 20 fixing points instead of 12 to guard against the earlier version’s head gasket leaks. It incorporated the rocker boxes and inlet and exhaust manifolds. The alloy castings also improved cooling and, with a 6.0:1 compression ratio to cope with the 72-octane “pool” petrol, and an automotive-style, bi-starter Solex carburettor, the output was 34.5hp at 5400rpm.

Coil ignition, with a car-type distributor, and a large 70W separate dynamo replaced the Lucas Magdyno. As on the pre-war examples, lubrication was dry sump and the gearbox a four-speed Burman.

The new engine also improved the Squariel’s legendary characteristic: its acceleration. The rotary twist grip provided unusually delicate throttle control. This combined with relatively low flywheel weight and four cylinders gave instant response. The lighter engine also contributed to marginally improved handling and liveliness but the engine was always too much for the frame.

The Mk I frame was part cradle and, with an oil-damped telescopic fork and the option of an undamped plunger link sprung hub, the weight was 197kg. Rigid-frame versions weighed 187kg. Rolling on a 1422mm wheelbase, the Mk I Squariel was a surprisingly compact machine for its capacity. The tyres were also quite large for the day at 3.25 x 19 inches front and 4.0 x 18-inch item at the rear.
With production spanning 27 years, Ariel’s Square Four was never a mainstream motorcycle.

Revered for its smoothness, comfort and acceleration, the Squariel was always expensive and appealed as a status symbol rather than regular transportation. Ultimately, a lack of development resulted in its demise, but now the Square Four is acknowledged as one of Britain’s finest classic motorcycles.


• Price new £228.12.0. Rear springing £19 extra
• Price now $16,000-$25,000


Ariel Square Four

• Early Square Fours didn’t take to tuning and although Ben Bickell managed to lap Brooklands at more than 175km/h, his supercharged 500 kept blowing cylinder head gaskets and he never finished a race.
• Similar problems befell Somerville Sykes in the 1931 Senior TT. His blown 500cc Square Four produced 40hp but he retired with a blown cylinder head gasket.
• The four-pipe Mark II Square Four appeared in 1954, lasting until 1958. All Square Fours had plunger rear springing. Two prototypes with swingarm rear suspension were constructed but not put into production.
• After production had officially ended, George and Tim Healey started making their own spares and, in 1973, released the Healey Square Four with an Egli-designed frame. Only 20 Healeys were manufactured, the last in 1977.
• Ariel Square Four engines were produced under licence in Canada and used in pairs to power helicopters.

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