1934 4F Square Four 600. Overhead camshaft. Two-pipe. 4-speed Burman gearbox. Hand change. Horizontally split crank case. Forward facing carburettor. We think this is by far the prettiest of a beautiful bunch. In 2011, Bonhams sold this example in Las Vegas, USA for £33,997 ($45,000) inc premium.


Ariel Square Four

4F | 4G | Mk1 | Mk2 | 1931 - 1959. 500cc, 600cc, and 1000cc, OHV air-cooled fours


Ariel Square Four panel tank


Ariel Square Four panel tank

Smiths speedometer, 8-day clock, Eureka oil pressure gauge, inspection light and a screw-on fuel filler cap. Ariel was a quality manufacturer famous for excellent finishes. For decades, this bike was the flagship. Other manufacturers looked on with envy.


Ariel Square Four for 1931 - OHC 500cc model


OHC Ariel Square Four

Physically small, this original 1931 engine nevertheless had a huge impact and grew from 500cc, to 600cc, to 1,000cc. Edward Turner is said to have shoehorned the prototype engine into a 250cc Ariel frame. It's also said that Turner had envisaged a unit construction engine, but Burman supplied gearboxes for other models in the range. So Ariel, for political and/or economy reasons, made some design concessions and the engine became a pre-unit. But true or false? We don't know for sure.


Ariel Square Four 4G 1000cc


▲ Ariel 4G

1000cc two-piper for 1939. Pushrods had replaced the original overhead camshaft. Suspension is still a conventional girder front fork with the (optional) dubious Anstey link set up at the rear. These Ariels, with their beautiful panel tanks are handsome touring bikes. Fuel economy is around 40 - 50mpg. Top speed is around 90mph in the right conditions.


1000cc Ariel Square Four for 1939


▲ 1000cc Ariel 4G

Two-piper (primary side). Understanding the variants is complex, and it's a mistake to think of the "Squariel" as a single basic design. There were three engine sizes (nominally 500, 600 and 1,000), an overhead cam or pushrods, magneto or coil ignition, hand-change or foot-change transmission, different exhaust port configurations, rigid or plunger frames, girders or teles, and more. The bike was as changeable as the English weather, and it weathered it all with aplomb. British class on wheels.


Ariel Square Four 600cc engine cutaway


▲ Ariel Square Four engine cutaway

This 1939 600cc engine was built around an aluminium crankcase with an iron barrel and an iron cylinder head. The original OHC design had now given way to four pushrods. A retrograde step? Technically, yes. But this engine was derived and downsized from the enlarged 1,000cc unit (see main text, right hand column). On the prototype, the crankshaft coupling gears were internal and located between the front flywheels. But on the production engines, the crank coupling gears are part of the primary drive. The single carburettor, which was at the front of the engine on earlier Fours, was moved to the rear. Carburettors were Amal, Solex, or SU. But many owners have retro-fitted Amals. Staying cool was a problem throughout the production run causing induction gases to also run hotter than ideal.

Ariel Square Four 4F engine graphic


▲ 600cc Ariel 4F

Cast iron (pushrod) cylinder head. A single carburettor was sufficient to feed all four cylinders. Exhaust gases were routed along either side of the cylinder head to the header pipes. Later Squariel top ends were cast in an aluminium alloy with detachable exhaust manifolds. Oil starvation due to crank sludge trap blockage can wreck one or both rear cylinders. The solution? Keep the oil clean, and keep it flowing through regular usage.


Ariel Square Four 4F restored


▲ Ariel Square Four 4F

600cc. 1939. This beautifully rebuilt bike was restored by Terry at the excellent Ariel Motorcycle Club of North America who also supplied various images for this feature. The Americans, it seems, love the Squariel even more than we Brits. But then, most of the bikes went Stateside and therefore command a larger fan base.


Ariel Square Four 4G Mk1


▲ Ariel 4G

Capable of 100mph, this 1951 995cc, two-pipe Fourster, with its cast iron head & barrel and sprung saddle bridged the gap between the pre-war girder-forked bikes and the modern telescopic forked Squariels with twin seats and all-aluminium alloy engines. In 2012, Bonhams sold this bike for £10,350 including premium.

Ariel Square Four Mk2 1000cc


▲ Ariel Square Four 4G Mk1

1953. Still running with Anstey link rear suspension (designed by Frank Anstey), but a telescopic fork has supplanted the girder. This all aluminium alloy two-pipe engine gave the Squariel a fresh, modern, streamlined look with 40hp to play with, but for metallurgical reasons they have a reputation for blowing head gaskets. The fix is a superior Otto gasket. A headlight cowl arrived in 1956. In 2010 Bonhams (which supplied this image) sold this prime example for £12,075 including premium. Prices have since fluctuated, but there's little doubt that this gentleman's tourer will always be an investable motorcycle subject to the usual international economic whims and caprices. But why invest when you can ride?

Ariel Square Four Mk2 engine cutaway schematic


▲ All aluminium Ariel Square Four

In 1953 the 4G Mk2 997cc four-piper arrived. Finally this motorcycle could crack the "magic" ton, and it could do it with aplomb. The Mk2, as it was called, would stay in production for the next decade, but it was fighting a losing battle against sales, and manufacturing costs had risen hugely. Note the piston positions: two up, and two down.


1952 Ariel Square Four Mk2 engine 1000cc

▲ 1000cc 4G Mk2 Square Four

This cutaway engine was prepared for the 1952 Earl's Court Motorcycle Show. Production would start the following year. In 2015, this engine was sold by auction house Bonhams. The Mk2, it's said, will pull from 10mph to maximum speed in top gear. We've ridden a couple of Mk2 and a 4F, but never tried that. But certainly, the engine walks the walk, and torques the torque. See Sump Classic Bike News January 2015 for details of this cutaway display exhibit.



Ariel Square Four Mk2 four-pipe

▲ Four-pipe Ariel Square Four engine

1958. There's something of the jet age reflected in these beautiful all-aluminium Mk2 997cc engines. Edward Turner's vision of a smooth, reliable and stylish multi-cylinder tourer had lasted nearly 30 years. The Honda Goldwing of its day? Perhaps, but we prefer to put this motorcycle in a class of its own.



Ariel Square Four 4G Mk2 specifications

Square four, air-cooled, OHV, pushrod, 4-stroke
997cc (60-cubic inches)
65mm (2.56-inches)

Stroke: 78mm (2.96-inches)
Compression ratio:
SU carburettor
Max power:
40hp @ 5500 rpm (claimed; also quoted as 42hp @ 5,800rpm)
Final drive:
Single down tube into twin cradle
Front suspension:
Telescopic fork, hydraulically damped
Rear suspension:
Plunger (Anstey link)
Front brake:
8-inch drum, full-width hub
Rear brakes:
7-inch drum (also quoted as 8-inch), cast iron
Front wheel:
3.25 x 19-inch

Rear wheel: 4.00 x 18-inch
Dry weight:
Fuel capacity:
5 gallons (imperial)

Oil capacity: 1 gallon (imperial)
Maximum speed:



Ariel Motorcycle Club of North America


Ariel Motorcycle Club of North America. These guys, especially Terry, get a special thanks for letting us use and rework some of their images for this feature. This looks like a great club with a great site. A must for Ariel Square Four fans. Check it out.






Ariel T-shirt



Square Four designer


Ariel 4F Square Four

A single camshaft, indirectly chain-driven from the right side of the crankshaft, is located in the cylinder head between the front and rear cylinders. Via another chain, that camshaft also drives the generator and magneto. The oil pump is a gear type.

The valves are opened with rocker arms actuated by the camshaft lobes. Conventional valve springs keep those poppets under control in the usual way.


SU carburettor for Ariel Square Four


A single carburettor feeds all four cylinders and is located between the exhaust header pipes (see immediately above) which no doubt heavily compromises gas flow.

These early two-pipe 4F engines, like other Fours in the range, are arranged with exhaust ports exiting each side of the engine, whereby each exhaust serves two cylinders; one front, and one rear. The exhaust manifold are cast integrally with the cylinder head. Therefore, they're no detachable.

The bore and stroke is, respectively, 56mm x 61mm. The power output is around 23 - 25hp @ 5,600rpm.

Overall, it's a beautifully compact and technically smooth engine design, but with a tendency towards overheating the rear cylinders.

The clutch is a conventional wet multi-plate unit that transfers the power to a four-speed Burman gearbox. Gear-changing is by hand. Final drive is via the usual single row chain.

The frame is rigid. The front fork is a girder. The rest of the cycle parts are typical of the era, including the well-equipped and attractive "panel" tank.

The 7-inch drum brakes were never anything other than typical of the age, but by modern standards they're poor. The wheels are 3.25 x 19-inches front and rear.


Ariel Square Four prototype engine




The prototype Square Four engine, note, differed markedly from the production version (see the image immediately above).

This 498cc unit featured overhanging cranks, meaning that there are no "outside" main bearings (think bicycle pedals again). The cranks, it appears, were connected via straight-cut gears which would have significantly increased engine noise.



It was all a good starting point, but the concept was soon revised (largely by the hugely competent Val Page, image immediately above, who did so much for Ariel, BSA and Triumph) and never saw production. Page's contribution to the Ariel Square Four is generally underrated.


4F Square Four


600cc Square Four


In 1932, the 500cc Squariel continued to be produced (927 were built) but it was joined by a new 600cc variant (actually 601cc). To achieve this, the cylinder bores of the engine were opened 5mm, but the stroke remained the same.

The extra power was broadly welcomed, not least by "sidecar men", but the engine ran slightly less smooth and slightly hotter. Nevertheless, it was still a great power unit, and although bike sales were still relatively low at 2,675 units, the 600 4F was eagerly received.


600cc Ariel Square Four engine cutaway


Gear-shifting, incidentally, was now by foot rather than hand. However, the big news that year was that Ariel had run into financial trouble which necessitated a major restructuring by Sangster. His strategy was a little brutal, but it worked and the company became more streamlined, albeit smaller.


4G Square Four

4G Mk1 Square Four


4G Mk2 Square Four


Further information



Copyright Sump Publishing 2016