Motorcycle Restoration Projects

A long-serving Ariel Square Four


This Ariel Square Four has been owned by Kevin T since 1966. An ex-Victoria Police bike, it’s had a chequered career; being raced as a solo machine, then its engine was transplanted into a racing outfit, before being returned to the original solo frame.



Before we get into the details of Kevin’s machine, let’s look at some Ariel Square Four background. There’s more Ariel info in our Motorcycle Brands section.

The Square Four engine was designed by Edward Turner in 1928 and powered successive Ariel Square Four models between 1931 and 1959. 

Turner’s engine, with its two transverse crankshafts, was essentially a pair of across-frame, parallel twins geared at their flywheels to counter-rotate. The four-cylinder cast-iron block mounted a single head, with an overhead camshaft. 

When he touted the design around the UK’s motorcycle-manufacturing halls,Turner’s radical design was too much for BSA, but was adopted by Ariel. 



Edward Turner was no dope and his concept of counter-rotating crankshafts cancelled out the gyro effect of each. Also, piston BDC and TDC positions were calculated to provide inherent diagonal balance, so the result was a smooth, torquey engine.

The first Ariel Square Four 4F was shown at the Olympia Motorcycle Show in 1930, in chain-driven, overhead-camshaft 500cc form and with a hand-change, four-speed Burman gearbox. 

Over the next four years the Cammy engine, as it was called, grew to 600cc and developed a reputation for overheating its rear cylinders. Before he left Ariel for BSA in 1936, Val Page redesigned the Square Four engine that was released in 1937 as the 600cc 4F and new 998cc model 4G.




Val Page dropped the overhead-camshaft design, so that he could redesign the cylinder head with a central air-cooling valley. On either side of that channel were valve rockers, operated by pushrods from a crankcase-mounted camshaft. At the same time, the head was liberally finned. 

Post-War, in 1949, the Ariel Square Four Mark I sported a new aluminium-alloy barrel, cylinder head and crankcase, replacing the cast-iron ones. Weight was reduced by around 14kg and cooling was further enhanced. 

The 1949 engine produced 35bhp (26kW) at 5500rpm and was capable of  propelling the 435lb (197kg) machine to 90mph.



In 1953, the ‘four pipe’ 997cc Ariel Square Four Mk II was released, with separated barrels, a re-designed cylinder head with four exhaust pipes from two cast-aluminium manifolds and rocker-boxes combined with the inlet manifold. 

A redesigned frame provided clearance for a high-mounted, car-type, SU carburettor. The 40bhp (30kW) Square Four bike weighed 425lb (193kg) and was capable of 100mph (160 km/h). Sadly, Ariel bit the dust in 1959.


Kevin’s Ariel Square Four MkII



Kevin was 16 years old when he bought this bike at a Victoria Police auction, in 1966. (His mother didn’t know until much later.) He raced it pretty much as bought, minus the Police bolt-on kit that included crash bars front and rear.

After one race, a pit visitor, who introduced himself as ‘Phil’, asked Kevin how fast he’d been going down the main straight. Kevin said he reckoned around 103mph and Phil said he could get Kevin 64-percent more power out of the Square Four.

The precision of this claim should have alerted Kevin to the fact that this bloke probably knew a fair bit about engine tuning, but he was in his late-teens and we all knew everything at that age, didn’t we?

Phil suggested considerable engine mods, including rotating the cylinder head 180 degrees, so that the exhaust pipes ran straight aft, rather than curling forward first and suggested making and fitting precisely dimensioned megaphones to the pipes that Kevin realised Phil was a fountain of knowledge.

The anonymous ‘Phil’ turned out to be none other than Phil Irving!



Kevin discovered that his bike was different from non-police bikes in having a double-row timing chain for the camshaft, generator and distributor. The sprocket on the generator/distributor shaft was resized as a special mod for police bikes, to increase generator revs and amps. That was necessary to power police electrical extras, including radios, lights and sirens.

The changed distributor socket made timing tricky if a standard-issue distributor and drive gear was fitted to a police-issue engine – a fact that Kevin found out the hard way!



For 1968, Kevin embraced outfit racing at Philip Island, using the Ariel engine in a Norton Featherbed frame, with a Triumph coil-swing arm rear suspension up front and hydraulic drum brakes, plus Yamaha discs at the rear. In its final form the engine was good for 130mph at 7000rpm.

In 1972, Kevin stopped outfit racing and put the Square Four engine back into the bike frame. 

“When I say I put the engine back into the bike, it was a bit like grandad’s axe that’s had two new heads and four new handles, but is still grandad’s axe,” Kevin explained.

“It was bored 80-thou oversize and racing had taken its toll on some of the engine bits, but most of them were still intact.

“Of course, the head needed to be turned around once more, to put the exhaust pipes in front of the engine, but I left the race-ground camshaft in place, so it’s a tad lumpy at idle.

“On the plus side, it goes lot better than a stock Square Four MkII !”



In 1973 the bike was given nine coats of black baked enamel, with pin-striping, by Viv Bognor in Melbourne.

Kevin rode the Ariel for recreation in the intervening years to 2007, at which time he decided to rebuild the engine. Given that modern oils have detergent qualities that strip carbon buildup and block oil drillings, he fitted an after-market, spin-on oil filter kit to the Square Four, replacing the original gauze token filter.

Kevin said that his experience with Square Four alloy engines showed that seizures put down to the problems that iron ones had – overheating of the rear cylinders – was actually caused by modern oils displacing carbon flakes from the engine components and this debris blocked the crankshaft oil drillings. That led to big end seizure and subsequent destruction.



Time had wreaked havoc on Kevin’s steel fuel tank and a careful scan of the accompanying photos reveals some rust pinholes around the base of the tank. A replica tank was on order for fitment in late-2022 and new seat upholstery had already been fitted.

When we checked out Kevin’s Ariel Square Four it still had open pipes and made the most wonderful sound, but how do the neighbours like it, we wondered?

“They’re fine about the noise when I’m working on the engine, but I have some appropriate mufflers coming for it, so I can ride it around the streets,” said Kevin T.

“It’s a lovely thing.” 

He’s right, you know.
















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