1951 Ariel Square Four

There’s a special kind of reverence reserved for the bikes that refuse to play by the rules. The ones that, despite the passage of time and technological leaps, still work in our modern world. And few machines embody that spirit quite like the Ariel Square Four. This isn't just any classic Brit bike; this is the 1951 Ariel Square Four 4G Mk I that our own editor, Dean Larson, took a memorable jaunt on—an experience he described as "feeling like the bike equivalent of an old 911." It’s an opportunity you wouldn’t pass up.

The story of Ariel is one of British motorcycling royalty, but the Square Four is its most curious jewel. It all started with the brilliant mind of Edward Turner in 1928, who sought to design a high-output, long-haul touring engine without the bulk of traditional fours. His solution? A genius piece of packaging: two parallel twins, one behind the other, sharing a common crankcase and single cylinder head. The counter-rotating cranks and four pistons resulted in a wonderfully smooth alloy lump—a theory that holds true, even today.

Ariel took on Turner's unique concept, evolving it from the initial 498 cc OHC 4F in 1930. By 1936, overheating issues led to the larger, more robust OHV 995 cc 4G model. This particular bike is the sought-after Mark I, which debuted in 1949 with a significant upgrade: alloy cylinders and a cylinder head, reportedly shedding 15 pounds and further improving performance and cooling.

This 1951 Ariel Square Four (4G Mk I) is not a museum piece—it’s a runner and rider, documented in Dean Larson's 'Parallel x2: Thoughts From the Saddle of a 1951 Ariel Square Four' etched into the pages of Bike EXIF. Finished in burgundy with contrasting gold pinstripes, it presents as a highly desirable piece of old iron.

This Square Four is offered with a clean Wisconsin title in the seller’s name. Its quirks—the squeak from the brakes, the grind from the gearbox if you're lazy with the shift, the slightly-too-close ergonomics—are not flaws, but rather the charming idiosyncrasies that connect you to the dedicated souls who built and preserved this 72-year-old classic. It’s a bike that demands to be ridden, and rewards that effort with a feeling you simply won’t get from anything with a catalytic converter or ABS.

Mileage: 2,440

Title: Clean in Owner's Name

Condition: Restored

Engine and performance

Engine: 997cc, air-cooled, overhead-valve (OHV), square four

Power: 35 bhp @ 5,500 rpm

Carburetor: Single Amal concentric

Compression ratio: 6.0:1

Top speed: Over 90 mph

Drivetrain

Transmission: 4-speed manual

Final drive: Chain

Clutch: Multi-plate, wet

Chassis and suspension

Frame: Tubular single loop with a twin cradle

Front suspension: Hydraulic telescopic fork

Rear suspension: 2-plunger system

Wheels and brakes

Front wheel: 19-inch (3.25 x 19-inch tire)

Rear wheel: 18-inch (4.00 x 18-inch tire)

Front brake: 8-inch full-width drum

Rear brake: 7-inch drum

Electrical

Updated to 12V charging system

Other specifications

Dry weight: 430 lbs (claimed)

Fuel capacity: 4 gallons

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