1932 Ariel Square Four Rare overhead cam model
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Ariel Square Four The Iconic Four-Cylinder British Motorcycle

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Few machines in the history of British motorcycling carry the same aura of engineering audacity as the Ariel Square Four. Introduced in the early 1930s and produced in various forms until 1959, the Square Four combined distinctive looks, powerful performance, and mechanical ingenuity, earning its place as one of the most iconic motorcycles ever built in the UK. This feature includes a brief history of the Ariel Square Four along with a price guide of some realised prices at auction.

1932 Ariel Square Four Rare overhead cam model
A rare 1932 Ariel Square Four with its distinctive overhead cam 600cc ‘square four’ engine sold for £11,000 at H&H Classics’ 26 March 2025 sale at the National Motorcycle Museum. Designed by Edward Turner and initially rejected by BSA, the Square Four became Ariel’s flagship model. This early example, restored and fitted with a new crank, oil pump, and period tyres, is supplied with a dating letter and V5C. Highly advanced for its time, the Square Four’s unique layout and rarity—especially in OHC form—make it one of the most collectible British motorcycles of the pre-war era.

Designed by the legendary Edward Turner, who would later go on to create Triumph’s famed parallel twin, the Square Four was unlike anything on the road at the time. Its name comes from the innovative layout of its engine: four cylinders arranged in a square formation—essentially two parallel twins geared together. This compact and symmetrical design allowed for a relatively narrow engine, unusual for a four-cylinder, and contributed to the bike’s distinctive character.

1955 Ariel Square Four Mk2 1000cc 4-pipe model
This authentically restored 1955 Ariel Square Four Mk2 1000cc ‘4-pipe’ model sold for £6,000 at H&H Classics’ 26 March 2025 auction at the National Motorcycle Museum. Featuring Edward Turner’s unique square four engine—two parallel twins geared together—the Mk2 was the final and most refined version, boasting overhead valves and four distinctive exhaust pipes. Supplied with receipts, a green logbook and a V5C, the bike remains a symbol of post-war British engineering. Smooth, powerful and stylish, the Mk2 Square Four is prized by collectors for its usability, originality, and place in Ariel’s 30-year legacy of motorcycle innovation.

The original version of the Square Four was launched in 1931 with a 500cc overhead camshaft engine. It was promoted as a smooth, torquey tourer, ideal for riders seeking comfort and refinement rather than outright speed. Though it offered a unique selling point in its four-cylinder layout, the early models were known to suffer from overheating in the rear cylinders, due to limited airflow.

Ariel responded to these issues over time. In 1936, the engine was redesigned as a 600cc overhead-valve unit, improving reliability. The most famous and enduring version, however, was the Ariel Square Four 1000, introduced in 1937. With its 997cc capacity, the model became known for its effortless cruising power and luxurious ride. By the post-war era, the Square Four had become a gentleman’s touring motorcycle—fast, refined, and unmistakably British.

1954 Ariel Square Four Mk2
This nicely presented 1954 Ariel Square Four Mk2 1000cc sold for £5,600 at H&H Classics’ 26 March 2025 auction at the National Motorcycle Museum. A later version of Edward Turner’s innovative design, the Mk2 featured an alloy head and four exhaust pipes to improve cooling and performance. Restored to a good standard in 2015 with supporting receipts, it was recently recommissioned and is reported to start easily and run well. Supplied with a current V5C, this classic combines vintage charm with practical usability, making it a fine example of Ariel’s advanced four-cylinder engineering from the golden era of British motorcycling.

The final iteration, the Mark II Square Four, launched in 1953, featured alloy cylinder heads and barrels, helping to reduce the weight and improve cooling. It also had a new cylinder head design with four separate exhaust pipes, giving the bike an aggressive, muscular look and a satisfying growl. With around 40 horsepower and a top speed near 100 mph, it was a strong performer in its class, especially for a touring bike.

Despite its many strengths, the Square Four faced increasing competition from faster, lighter, and more affordable twins by the late 1950s. Its complex engine and relatively high production costs made it less viable in a changing market. Production came to an end in 1959, and Ariel itself would cease motorcycle manufacturing entirely in 1965.

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