• Year of manufacture 
  • Motorcycle type 
  • Engine number 
    RD 1378
  • Lot number 
  • Condition 
  • Colour  
  • Location


1951 Ariel 995cc 'Square Four' 4G Mark 1
Registration no. UMM 491
Frame no. SC 1376
Engine no. RD 1378

• Believed 1 mile since last restoration
• Offered for re-commissioning
• No documents

Designed by Edward Turner, creator of the legendary Triumph Speed Twin, the first Ariel Square Four was shown at the Olympia Motorcycle Show in 1930. Unique at the time of its introduction, the 'Squariel', as it was swiftly dubbed, featured a single block of four cylinders and twin geared-together crankshafts with pistons phased at 180 degrees. The crankcase was split horizontally, unusually for a motorcycle engine of the period, while the vertical valves were operated by a single overhead camshaft. A supercharged example was raced at the Isle of Man TT in 1931 without success, which was hardly surprising as its makers had envisaged the model as more of a luxury tourer than an out-and-out sports machine.

Although launched as a '500', the Squariel was soon enlarged to 601cc with an eye on the important sidecar market, this bigger version being made available for the 1932 model year. In 1937 a total redesign saw it re-emerge as the Model 4G, with 995cc overhead-valve engine, making the Squariel an even more enticing prospect for sidecarists. Anstey-link plunger rear suspension became an option in 1939 but would not be offered again until 1946 when a telescopic front fork replaced the previous girder type. An exercise in weight shedding saw the cast-iron cylinder head and barrel replaced by alloy components for 1949, the revised model, now capable of 90mph-plus, being known as the 'Mark 1'. Introduced in 1953, the 'four pipe' Mark 2 with its redesigned cylinder head elevated the Square Four into the league of genuine 100mph motorcycles. Square Four production along with that of all other Ariel four-strokes ceased in 1959. To date, the innovative Ariel Square Four remains unique in motorcycling history; a true 'gentleman's motorcycle, this refined yet charismatic machine retains an enthusiastic and loyal following, and is highly prized by discerning enthusiasts.

This Mark 1 Square Four is offered for re-commissioning/restoration. The odometer reading is 1 mile, which is presumed to be the distance travelled since the machine's last restoration. There are no documents with this Lot.

Bonhams 1793
101 New Bond Street
United Kingdom
Contact Person Kontaktperson
First name 
Bonhams Collectors’ Car department