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BSA A10 series
The BSA A10 series was a range of 646 cc (39.4 cu in) air-cooled parallel twin motorcycles designed by Bert Hopwood and produced by Birmingham Small Arms Company at Small Heath, Birmingham from 1950 to 1963. The series was succeeded by the A65 unit construction models.
BSA, then the largest UK motorcycle manufacturer, was falling behind in the parallel-twin race after the introduction of the 650cc Triumph Thunderbird. Although BSA had a 500cc parallel-twin, the BSA A7, they needed to develop a 650 to remain competitive.
Bert Hopwood served an apprenticeship under designer Val Page at Ariel. In 1936, Hopwood moved to Triumph, where he worked under Edward Turner to develop the 1937 Triumph Speed Twin. The innovative Speed Twin became the exemplar of the parallel twin engine layout for British motorcycles in the 1950s and 1960s. In April 1947 Hopwood joined Norton to design the Norton Dominator engine.
In May 1948, the factory enticed Hopwood after only a year at Norton, and he was commissioned to create a competitive BSA 650cc parallel-twin.
Launched in October 1949, Hopwood's A10 Golden Flash drew heavily from the A7 design by Page and Bert Perkins. The A10 was increased to 650 cc (40 cu in) by using a 70mm bore and 84mm stroke. It had a revised alloy rocker box and cast-iron cylinder head, plus an integral manifold for the single Amal carburettor. A single camshaft behind the cylinders operated the valves via pushrods passing through a tunnel in the cast iron block. A magneto was located behind the cylinders, driven by a train of gears, to provide ignition. A dynamo was positioned in front of the cylinders, driven by a chain. Lubrication was dry sump with oil being distributed by a mechanical pump located inside the timing cover.
The crankshaft was a bolt-up 360-degree item (both pistons went up and down at the same time). Main bearings were roller drive-side and a white metal plain bush on the timing side. The conrods were of the split type with shell type big ends. The crank-cases were vertically split.
A semi-unit gearbox meant the duplex primary chain was adjustable via a slipper tensioner within the primary chain case. The A10 was so closely based on the A7 that it used many of its well-proven components, and this large carry-over of parts from the A7 promised greater reliability, with minimal risk of new technical problems.
With the introduction of a swinging arm frame in 1954, the semi-unit gearbox was abandoned for a separate "pre-unit" item. The new design had a modified clutch, new gearbox internals and a single row primary chain. The primary chain was adjusted by moving the gearbox, which caused the rear chain to requiring re-tensioning.
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BSA A10 series
The BSA A10 series was a range of 646 cc (39.4 cu in) air-cooled parallel twin motorcycles designed by Bert Hopwood and produced by Birmingham Small Arms Company at Small Heath, Birmingham from 1950 to 1963. The series was succeeded by the A65 unit construction models.
BSA, then the largest UK motorcycle manufacturer, was falling behind in the parallel-twin race after the introduction of the 650cc Triumph Thunderbird. Although BSA had a 500cc parallel-twin, the BSA A7, they needed to develop a 650 to remain competitive.
Bert Hopwood served an apprenticeship under designer Val Page at Ariel. In 1936, Hopwood moved to Triumph, where he worked under Edward Turner to develop the 1937 Triumph Speed Twin. The innovative Speed Twin became the exemplar of the parallel twin engine layout for British motorcycles in the 1950s and 1960s. In April 1947 Hopwood joined Norton to design the Norton Dominator engine.
In May 1948, the factory enticed Hopwood after only a year at Norton, and he was commissioned to create a competitive BSA 650cc parallel-twin.
Launched in October 1949, Hopwood's A10 Golden Flash drew heavily from the A7 design by Page and Bert Perkins. The A10 was increased to 650 cc (40 cu in) by using a 70mm bore and 84mm stroke. It had a revised alloy rocker box and cast-iron cylinder head, plus an integral manifold for the single Amal carburettor. A single camshaft behind the cylinders operated the valves via pushrods passing through a tunnel in the cast iron block. A magneto was located behind the cylinders, driven by a train of gears, to provide ignition. A dynamo was positioned in front of the cylinders, driven by a chain. Lubrication was dry sump with oil being distributed by a mechanical pump located inside the timing cover.
The crankshaft was a bolt-up 360-degree item (both pistons went up and down at the same time). Main bearings were roller drive-side and a white metal plain bush on the timing side. The conrods were of the split type with shell type big ends. The crank-cases were vertically split.
A semi-unit gearbox meant the duplex primary chain was adjustable via a slipper tensioner within the primary chain case. The A10 was so closely based on the A7 that it used many of its well-proven components, and this large carry-over of parts from the A7 promised greater reliability, with minimal risk of new technical problems.
With the introduction of a swinging arm frame in 1954, the semi-unit gearbox was abandoned for a separate "pre-unit" item. The new design had a modified clutch, new gearbox internals and a single row primary chain. The primary chain was adjusted by moving the gearbox, which caused the rear chain to requiring re-tensioning.