BSA A10
BSA A10 Background & History
The BSA A10 was the company’s entry into the new and hotly-contested 650cc heavyweight motorcycle market in 1950. Triumph, who led the way from just before World War 2 all the way into the mid 1960s, had shocked the world (the motorcycle world, anyway) three times in a row. First was the introduction of the world’s first ‘modern’ vertical twin, the 500cc 1938 Triumph Speed Twin. Shock #2 was a hot rod version the following year, with the 1939 Triumph T100 Tiger. The war intervened, civilian production halted for the war effort, but at war’s end, Triumph was back at it harder than ever. With the rest of the industry trying to catch up during this time, and the world hungry for motorcycles (or any other form of cheap transportation). By 1949, BSA, Norton, Royal Enfield, Matchless/AJS and Ariel all had 500cc vertical twins of their own. Triumph wasted no time in upping the ante with Shock #3. The bored and stroked their 500 out to the world’s first mainstream 650, the 1950 Triumph 6T Thunderbird, and the whole arms race started up again. Now, everyone was scrambling to get a 650.
This 1952 BSA A10 Golden Flash has the optional plunger rear suspension. Note the sprung saddle to help smooth the ride.
BSA Needs a 650 to Keep Up
BSA was the first to follow Triumph in the creation of their own vertical twin in 1946. They had been working on it before the war and continued development and testing during the war. By war’s end, they were ready. The 500cc 1946 BSA A7 was received well and did well in the market. 500s sold well in England, where roads were narrow and gas was scarce. But in the all-important US market, that was increasingly becoming the bread-and-butter for the British motorcycle industry as a whole, “too much was ever enough”. The Yanks wanted speed, power and style. BSA felt it was losing the vertical twin battle without a 650 to compete with Triumph.
This 1954 BSA A10 Golden Flash was the first year for the swing arm rear suspension.
BSA A10 Engine Design
Bert Hopwood was one of the top motorcycle designers in England at the time, having served under the legendary Val Page at Ariel, then under the genius Edward Turner, working with him on the design of Triumph’s 500 vertical twin. He moved over Norton for a while, but BSA enticed him to come over and redesign the 500cc BSA A7 into a proper 650. The original BSA A7 had been designed by Val Page and Bert Perkins. These early A7 engines were considered ‘long stroke’ engines at the time (they all had long strokes by today’s standards) with a bore and stroke of 62mm X 82mm, and displacing 495cc. When Hopwood set to redesigning the engine for 650 duty, he wanted it to be less undersquare (small bore, long stroke) and in doing so, he laid the groundwork for a completely redesigned BSA A7 engine, now with a bore and stroke of 66mm X 72.6mm for a capacity of 497cc. The new engine powered every BSA A7 until the end of its production in 1963.
This is a 1959 BSA A10 Road Rocket, one of the fastest bikes on the road at the time.
More BSA A10 Engine Design
But the whole goal was to build a winning 650 and they achieved that. The new BSA A10 engine had a bore of 70mm and a stroke of 84mm giving a total displacement of 646cc. The A10 launched with a cast iron barrel and head with removable aluminum rocker boxes. The iron head also had a cast-in intake manifold mounting a single Amal carburetor. The single camshaft was gear-driven and mounted behind the cylinders and actuated the valves via four pushrods that passed through a tunnel cast into the cylinder barrels. And additional gear was added to this gear train to turn the Lucas magneto behind the cylinders, which provided reliable, powerful spark; and a chain-driven Lucas dynamo in front, powering the lights. The 360-degree crankshaft rode on a roller main bearing on the primary side and a white metal plain bushing on the timing side. There was no center main bearing, just a huge center flywheel. The connecting rods split at their big ends with with shell-type plain bearings. The crankcases split vertically and were joined to the gearbox and primary chain case with a system of brackets, thus the name non-unit construction.
The A10 garnered quite a reputation in off-road events, especially desert racing. This is a 1960 BSA A10 Spitfire.
BSA A10 Chain Adjustment
Early BSA A7s had the gearbox attached directly to the crankcase with primary chain adjustment accomplished with a slipper-type chain adjuster. When that bike got the swing arm frame in 1954, the whole thing was changed over to normal non-unit fashion, meaning the engine and gearbox were independent of each other, held in place by brackets. Primary chain adjustment was done now by moving the gearbox rearward to tighten the duplex primary chain, which at the same time loosened the final drive chain, which then needed to be readjusted. This earlier system was applied to the BSA A10 also, but also only until the adoption of the swing arm frame in 1954. From then on the BSA A10 would require rotating the gearbox to adjust the now single-row primary chain. The A10 used many of the A7 components, which were already proven.
The A10 in its final year. BSA took the occasion to pull out all the stops when naming this bike. This is a 1962 BSA A10 Rocket Gold Star. Barber Motorsports Museum, Leeds AL.
BSA A10 Midlife Improvements
n 1955, BSA introduced a new cast aluminum cylinder head on the A10 Road Rocket, which also featured a new cylinder barrel with a thicker base flange (1/2-inch instead of 3/8), which became known as ‘think flange’ cylinders. At the same time, the rod journal diameter was increased from 1.46” to 1.68”, which became known as ‘big journal’ cranks. It took until 1958 for these changes to show up in the BSA A10 Golden Flash.
The engine resides in a 1960 BSA A10 Spitfire, and shows the alloy head which arrived in 1958.
BSA A10 Goes out with a Bang!
The BSA A10 continued to improve and develop throughout its life, which ended in 1962 when BSA shifted all its engines over to unit-construction. The BSA A10 was replaced by the new, unit-construction 650cc BSA A65. The BSA A10 helped to establish, especially in the all-important US market, that BSA was a purveyor of high performance motorcycles. At the end of the BSA A10’s life BSA was at the height of their greatness, being the largest producer of motorcycles on the planet, successful not just at motorcycles, but also cars, trucks, busses, agricultural and industrial equipment, military vehicles, bullets, bombs and more.
The BSA A10 was one of the best selling motorcycles on the market, but things had to change. Lucas Electrics (aka: "The Prince of Darkness" because the lights don't always stay on), supplier of electrical components to the entire British motorcycle industry announced that they would cease production of their magnetos and dynamos at the end of 1962. The industry had to scramble to redesign their engines to work with the new system: alternator and coil. BSA and Triumph used this as an excuse to take their big twins unit-construction. When the smoke cleared in 1963, BSA launched their new unit-construction 650, the BSA A65. It was a fine bike for the times, but never enjoyed the success of the A10 it replaced. Meanwhile, cracks were starting to form and within a few short years BSA would struggle just to survive. The 1960s weren’t good to BSA.
BSA A10 Books
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By Roy Bacon, 56 pages illustrated A fascinating treatise on BSA's legendary pre-unit twins by British motorcycle expert Roy Bacon. |
More BSA Motorcycle Pages
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