Home > Features > Ariel 4G Square Four 1,000cc: 1937-1959

Ariel 4G Square Four 1,000cc: 1937-1959

Clement SalvadoriJanuary 28, 2016
Year/Model: 1951 4G Square Four Mark I; Owner: Mike Barber, Shell Beach, California.

Year/Model: 1951 4G Square Four Mark I; Owner: Mike Barber, Shell Beach, California.

The magnificent Squariel! This one-liter engine was, in effect, two overhead-valve parallel twins standing vertically, with the two crankshafts geared together and all four cylinders fed by a single carburetor.

1951 4G Square Four Mark I

1951 4G Square Four Mark I

This layout meant lots of low-end power and lots of torque, an excellent choice for pulling a large sidecar. A well-sprung solo saddle kept the rider happy, and the ride was, in a word, comfortable. In the minds of many vintage collectors, this is one of England’s great designs, on a par with the Vincent V-twin and the proliferating vertical twins from half a dozen manufacturers. It wasn’t ferociously fast, nor did it have much in the way of cornering clearance, but the ride of a gentleman did not need to raise the adrenalin. It was a dignified mount, a connoisseur’s machine.

The original design, done by the remarkable Edward Turner, first appeared in 1931 when he was employed by the Ariel Works, Ltd., in Selly Oak, Birmingham. Much would be changed over the next 28 years. The world economy was plummeting into an abyss when the Ariel 4F was introduced, a 500cc square four (51 x 61mm bore/stroke), the cylinders cast as a single piece, with a chain-driven overhead camshaft and a 3-speed transmission built in unit with the engine. Within a year, the size had been enlarged to 601cc, done by merely boring out the cylinders 5mm. The OHC engine was complicated and expensive to build, so Turner decided to redo it into a simpler OHV design in 1935.

1951 4G Square Four Mark I

1951 4G Square Four Mark I

Production problems arose, then Turner moved on to Triumph, and the OHV Ariel 4G did not appear until 1937. This 997cc motor had a bore of 65mm, stroke of 75mm, with one camshaft in the center operating the eight valves. A single Solex carb fed fuel to the combustion chambers and a magdyno provided both sparks and light. The engine, bolted into a cradle frame with no rear suspension, was lauded for its smoothness and its ability to accelerate in top gear from 15 mph to a top speed of more than 90. A single-row primary ran the power to a dry clutch and through a new Burman 4-speed box.

In 1939, the ride was even smoother when an optional frame was available, using an Antsey-link rear suspension, a mildly complicated plunger affair that effectively kept the rear chain in constant tension.
Then came the second war to end all wars, and production of the Squariel temporarily ceased, the factory turning out singles for military use. In 1944, with victory in sight, British businessmen were thinking about the post-war economy, and one move was that Ariel got bought up by the much bigger BSA group—with the assurance that Ariel Motors would stand by itself, production continuing at Selly Oak.

Year/Model: 1951 4G Square Four Mark I; Owner: Mike Barber, Shell Beach, California.

Year/Model: 1951 4G Square Four Mark I; Owner: Mike Barber, Shell Beach, California.

The first civilian bikes rolled out of the factory for the 1946 model year, and the 4G appeared with a girder fork and the choice of sprung or rigid rear. Rigid frames were still preferred by many who planned to attach a sidecar. By June of ’46, new oil-damped telescopic forks appeared, and these “elegantly simple” units stayed with Ariel models for more than 20 years. The iron engine was rated at 36 horsepower at 5,800 rpm, with a compression ratio of 5.8:1.

For 1949, a rather new version appeared, called the 4G Mark I. In an effort to both make the engine tolerant of the low-grade gas and to lighten the machine, the cylinder block was made of aluminum alloy, with liners pressed in. The cylinder head was also alloy, and incorporated both the exhaust manifolds and the rockerbox. Compression ratio was upped slightly to 6:1, and revs lowered to 5,400 rpm. The magdyno gave way to a coil with a car-type distributor and a 70-watt dynamo; advance and retard for the spark was now done by centrifugal mechanism.

1951 4G Square Four Mark I

1951 4G Square Four Mark I

Wheels were 19 inches at the front, 18 at the back, with the standard 7-inch narrow brakes at both ends, upped to 8 inches at the front for 1951. Wheelbase with the plunger rear was 56 inches, an inch and a half longer than the rigid. Dry weight, fully suspended, was said to be 434 pounds, a good 30 pounds less than its predecessor. Top speed for a solo rider was a shade more than 90 mph, some 20 miles less with a
sidecar attached.

One problem for the solo rider was parking the bike, as it had only a centerstand that pivoted just aft of the rear axle. A fellow with a sidecar had no problem, as he could just get off, leaving the Squariel standing by virtue of the third wheel. But a solo rider had only a rear-wheel stand, meaning he had to dismount gingerly and, holding the bike upright, work around to the back, lower the stand, and hoist the bike up using whatever he could grab on to—a lifting handle was added in 1951.

In 1953, the Mark II “four-piper” appeared, with bolt-on exhaust manifolds altered to accommodate two header pipes on each side. And the Anstey plunger was now standard and the rigid frame dropped. An SU carburetor was fitted, compression was raised to 7.2:1, revs to 5,600 and horsepower to 40. A dual-seat was offered, and a sidestand was added—a very sensible improvement. In an effort to give it a sportier look, a lightweight front fender was available. However, those looking to go fast wanted the high-revving parallel twins.

1951 4G Square Four Mark I

1951 4G Square Four Mark I

Development continued, but the Mark III with its Earles-type front fork, and the Mark IV, with a new chassis having a swingarm rear suspension, never made it beyond the prototype stages. And at BSA Cycles some not-very-bright light figured that the entire line of Ariel four-strokes, singles, twins and the 4G should be ended, and that the company should focus on brand new lightweight two-strokes. The last four-stroke Ariel appeared in 1958.

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