1957/1949 BSA/Ariel "Flash Four"
Engine: 997cc OHV air-cooled Square Four MkI, 65mm x 78mm bore and stroke, 6:1 compression ratio, 35hp @ 5,500rpm
Top speed: 90mph
Carburetion: Single Solex 26 AHD
Transmission: Chain primary, BSA multiplate clutch, BSA 4-speed gearbox
Electrics: 12v, battery/coil with distributor
Frame: BSA A10 Golden Flash dual downtube full cradle, modified
Suspension: BSA Telescopic fork front, two coil spring/damper units rear
Brakes: BSA/Triumph 8in (203mm) TLS drum front, 7in (178mm) SLS drum rear
Tires: 100/90 x 18in front, 110/90 x 18in rear
Weight (wet): 400lb (182kg)
Seat height: 32in (813mm)
Fuel capacity/MPG: 4.2gal/55mpg (est.)
It’s a situation familiar to many motorcycle wrenchers: In one corner of the garage is a frame left over from a previous project, and hiding under the proverbial workbench is a similarly orphaned engine.
In Lyle Whitter’s case, the engine was a 1949 alloy two-pipe Ariel Square Four Mk1, and the frame was a 1957 BSA A10 Golden Flash. And the question uppermost in Whitter’s mind: would the two go together?
The eventual answer was … yes — but not without some considerable workshop skill and patience.
The engine that Whitter used for his BSA-framed custom bore little resemblance to Edward Turner’s original design — except for the “Square Four” cylinder layout.
In the 1920s, Turner owned a motorcycle dealership, but his goal was to become a designer for one of the big British motorcycle makers. His calling card was an elegant and innovative design for a 500cc 4-cylinder motorcycle engine. Only Ariel showed any serious interest, offering Turner a job in the engineering and design office working under Valentine Page.
Turner conceived a unit-construction powerplant with two crankshafts of 61mm stroke coupled by central bevel gears, the rear crankshaft also driving the integral 3-speed transmission. Each crankshaft ran on two main bearings inboard of the flywheels with “overhung” roller bearing big ends and steel connecting rods outboard.
A chain spun the single overhead camshaft via a half-time bevel gear driven from the crankshafts. The crankcase was split horizontally, with engine oil stored in the “wet sump.” The iron cylinder block was machined for four 51mm bores and capped with an iron cylinder head. The result was a light, compact and relatively powerful engine. Turner’s prototype engine was fitted into the chassis of a production Ariel 250cc giving an overall weight of less than 300 pounds, but with 90mph capability. It was scheduled for production in 1931.
The Flash Four uses a GPS-based speedometer and a wireless tachometer.
In many ways, the prototype Square Four was a typical Turner design: ingenious and audacious but also underdeveloped. Performance would ultimately be limited by the flexibility of the crankshafts, with the big ends being supported only on one side.
The financial collapse of 1929 brought serious rationalization at Ariel. Turner was required to redesign his engine to use a conventional chain-drive Burman gearbox. Fitted in the chassis of Ariel’s 500cc “sloper” single, the resulting 4F/31 model displayed at the Earls Court motorcycle show in London in 1930 was considerably heavier (and slower) than the prototype. Within a year, capacity had been increased to 601cc by adding 5mm to the bore. Ariel won the 1931 Maudes Trophy, a reliability and endurance award, which included the 4F6 Square Four covering 700 miles in 700 minutes.
Turner completely redesigned his engine for 1936 as the model 4G. Gone were the overhung crankshafts, now supported by conventional outboard main bearings. The connecting rods became light alloy with plain bearing big ends. Straight-cut coupling gears for the two crankshafts were moved outboard of the main bearings. The camshaft was moved into the crankcase, which was now vertically split in line with industry practice, and the valves operated by pushrods. Capacity was boosted to 997cc by increasing bore and stroke to 65mm x 75mm. Turner’s light, sporting “cammy” 500 had turned into a portly 1,000cc cruiser and sidecar tug.
The Solex carburetor is made up of leftover parts from another project.
The 4G ran until 1949, during which time, the chassis’ girder fork and rigid rear were replaced by hydraulic suspension units at both ends. The plunger-type rear used an articulated linkage, designed by Ariel’s Frank Anstey, and was intended to maintain constant chain tension through its range of movement. Unfortunately, the Anstey link introduced numerous wear points that required frequent lubrication — otherwise rapid wear would occur, allowing the rear wheel to twist. Regardless, Ariel persevered with the Anstey link frame until Square Four production ceased in 1959.
1949 brought a significant upgrade with the confusingly named “Mk1” engine. Light alloy replaced the cast iron cylinder block and head, though the exhaust was still siamesed. The “2-piper” featured generous art-deco-style cooling fins around the cylinder head, and many consider this the most elegant of the Square Four engines.
The last of the Square Fours was the 1953-59 MkII “4-piper” with a redesigned top end featuring four separate exhaust headers.
Though overshadowed by Norton’s “Featherbed,” BSA was also at the forefront of motorcycle frame development in the 1950s. Traditional British motorcycle frames evolved from bicycle items and were made in a similar way. Mild steel tubes were assembled into iron lugs, then brazed into place in a furnace using a filler metal with a lower melting point, such as bronze or brass.
Making frames this way suited mass production and unskilled labor. But the cast lugs were expensive and heavy, and the advent of new materials and techniques allowed frame tubes to be welded together without lugs. Welding with low-temperature bronze also allowed thin-wall chrome-moly tubes to be welded without compromising their strength.
BSA’s welded frame also used a dual-loop cradle design like the Featherbed, but with a single, large diameter top tube and a bracing tube below, triangulating the steering head. The new frame was introduced on the Gold Star in 1953 and the “A” model BSA twins in 1954, giving the Goldie and top-of-the-range bikes like the Super Rocket their reputation for sweet handling.
Though always in the shadow of the Featherbed, the new BSA frame also became popular with specials builders, for example with a Triumph engine (“TriBSA”), or even for offroad use, like the VeloStar scrambler.
Shoehorning a Square Four engine into a Featherbed has been done before — even without modifying the frame. But a BSA frame?
Lyle Whitter is a fan of both BSAs and Ariels. His daily ride is a 1960 BSA A10 Golden Flash, and his Sunday bike is a 1953 Square Four MkII, which he rebuilt from a basket case. Along the way, while collecting parts for the Square Four restoration, Whitter bought a Mk1 Square Four engine — the all-alloy 2-piper.
The completely hand-done primary.
The frame for the Flash Four came from a complete A10 that Whitter had earmarked for restoration. But an inspection of the engine revealed it had been blown up and crudely repaired. That’s when the idea for the Flash Four popped up.
“I was lying in bed one night and I remembered. ‘Why don’t I try? I’ve got enough pieces to put this 2-pipe engine together.’
The bike uses a modern 12-volt electrical system.
“So I assembled (the engine and frame) loosely to get dimensions. I could see I had to cut the frame and put a belly in the tubing to accommodate the crankcase. I had tubing bent and then welded it in. And then started making mounts and aligning.”
Sadly, the ’49 2-pipe engine wasn’t in the best of shape.
“Most of the bolts were missing, but it was basically complete. It was pretty gross inside, you could see lots of water. It looked like it had been sunk,” Whitter says.
Ample finning on the “2-pipe” cylinder head.
Fortunately, the crankshafts had survived undamaged, and were appropriated for the 1953 MkII rebuild. The crankshafts now in the Flash Four were supplied by Draganfly Motorcycles in Bungay, England. The original crankshafts from the ’53 went back into Whitter’s inventory. Rebuilding the ’49 engine, though, required plenty of acquired experience with Square Fours and a generous supply of new and used parts — as well as Whitter’s expertise as a former marine engine fitter, which also came in handy.
“I poured my own main bearings just to try it out, to see if I could do it, and machined them myself.”
Four connecting rods left over from the ’53 were pressed into service after checking that they were in good shape. Whitter then installed a new set of cylinder liners, using a barbecue to heat the block. He then had the cylinder block “decked” (machined flat and square).
The BSA A10 gearbox.
“A friend of mine had a brand-new set of pistons that he’d got from New Zealand. But he wanted to use some older ones in his bike. So, he decided to sell them to me. They’re all standard bore.”
The valves and guides required another mix and match. Whitter had a set of valves in his ’53 that kept seizing. He replaced them, and the old valves were installed in the ’49 engine.
“So, I used the new valves on my ‘53, and the old valves, I put them in this one because their guides were a little bit more (loose), but it made it perfect.”
Similarly, the carb on the ’53 had been assembled from the best parts of two Solexes. Whitter used the leftover pieces to build a new carb.
“A few pieces went missing. I had to machine them up, put them together. And it actually runs very well with it.”
Mating the Square Four engine to the BSA transmission took some ingenuity. Whitter used a complete BSA A10 clutch and gearbox.
“I had an Ariel inner primary that I had to cut to adapt. The outer primary is completely hand done. Homemade. And lots of machining. That was a lot of work.”
Whitter also had to make some significant repairs to the BSA frame, including fabricating the engine and transmission mounts. The kickstand lug was missing, so that had to be re-welded. Whitter also converted the steering head for taper-roller bearings. The oil tank had to be modified to suit the Ariel’s particular number and size of oil lines, and to accommodate a filter. The exhaust headers and pipes came from a ’52 2-piper, and the front brake is a twin-leading-shoe unit from a 1968-1970 BSA twin. Whitter also notes the help he got from local master alloy fabricator and welder Peter Dent, who made the front fender and chainguard.
“I guess it becomes a process of, as you move forward, you find obstacles and you have to overcome them. You have to make sure everything fits. That’s the problem.”
Whitter wanted to include a modern 12-volt electrical system, and Draganfly recommended a unit from Iron Horse Spares in the U.K.. They supplied a 300-watt alternator and regulator/rectifier system that fits inside the original DC generator casing, complete with distributor drive.
Whitter is delighted with the Iron Horse product: “Superb machining. Just superb.”
Owner and builder Lyle Whitter aboard his Flash Four creation.
Whitter also fitted a GPS-based speedometer and wireless tachometer, but is chasing down some gremlins in the instruments. Otherwise the project is pretty much complete. Whitter admits he hadn’t been a big fan of the 2-pipe engine, preferring the appearance of the 4-pipe. So how does he feel about the finished project?
“As much as I didn’t like a 2-pipe engine at that time, I admire them now,” he says, noting that the 2- and 4-pipe engines “feel the same as far as torque goes.”
“To me, it’s an absolute delight of a bike to ride. Smooth, torquey and handles like a dream. It’s much smoother than my Golden Flash, and a nicer handling frame than the Square Four.”
And how does it compare to Whitter’s 1953 Ariel?
“It’s just a nicer-feeling bike. Just the way you can flick it into corners. But there again, I’ve got alloy wheels and nice tires on it.”
Any other comments? “Just a lot of thanks to friends that have helped with the project.” MC
Edward and Valentine: A Real Page Turner
Though both Val Page and Edward Turner were talented motorcycle engineers and designers, it’s hard to imagine two more different personalities — at least, based on contemporary reports. In his book Edward Turner: The Man Behind the Motorcycles, Jeff Clew admits “much has been written about his (Turner’s) irascible temperament, his intolerance and his autocratic style of management …” By contrast, Clew describes Page as “a modest, kindly man, a true gentleman in every sense of the word.”
And while their professional paths were bound to cross, Page seems to have tried to avoid working with Turner after their spell together at Ariel. Hired as designer and engineer in 1928 by Ariel, Turner worked under Page for four years before Ariel went bust in 1932. Page moved to Triumph, and Turner was installed as chief designer in the reborn Ariel company. When Ariel owner Jack Sangster bought the bankrupt Triumph company in 1936 and installed Turner as general manager, Page left for BSA. And when BSA bought Triumph in 1951, Page returned to Ariel (though it was now also a part of the BSA Group).
But whatever their personalities, between them Page and Turner were responsible for the most iconic of classic British motorcycle designs, including the Gold Star, World War II workhorse BSA M20, the Ariel 6/1 twin, Ariel Leader and Arrow (Page), and the Speed Twin, Red Hunter, Triumph Terrier and Cub, and, of course, Triumph Bonneville (Turner).