BSA, then the largest UK motorcycle manufacturer, was falling behind in the parallel-twin race after the introduction of the 650cc Triumph Thunderbird.

In May 1948, the factory enticed Hopwood after only a year at Norton,[2] and he was commissioned to create a competitive BSA 650cc parallel-twin.

[3][4] Launched in October 1949, Hopwood's A10 Golden Flash drew heavily from the A7 design by Page and Bert Perkins.

[1] 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.

Lubrication was dry sump with oil being distributed by a mechanical pump located inside the timing cover.

Main bearings were roller drive-side and a white metal plain bush on the timing side.

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.

[5] 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 frame was available in rear rigid format, but the more common option was plunger suspension, adopted for the export market.

This resulted in long delivery times for British customers, who were offered the model only in black.

[26] Its gold colour proved a marketing success, outselling Triumph's Speed Twin and 6T Thunderbird.

In 1951, BSA Chief designer, Bert Hopwood, went on a fact-finding tour to America and talked to dealers and riders.

The swinging arm frame and alloy head Road Rocket were planning stages but some way off production.

Hopwood decided to make a faster odel by bolting on performance parts to the existing model.

[27] The first Super Flash was shipped to America in February 1953 and American Motorcycling magazine give it 'Motor of the Month'.

[31] Unlike the Golden Flash, the inlet manifold was separate to the cylinder head[7] enabling twin carburettors to be fitted if required.

These engine modifications raised the power output to 40 bhp (30 kW)[31] and a top speed of 109 mph (175 km/h).

The special was well received and the BSA management decided to put the concept into limited production.

[20] The later (1961–1963) 9:1 compression Super Rocket engine was used with a 357 Spitfire camshaft and a 1 5/32" bore Amal Monobloc carburettor which gave 46 bhp (34 kW) as standard.

Options such as an Amal GP2 carburettor, siamesed exhaust pipes and a close-ratio RRT2 gearbox could increase this to 50 bhp (37 kW) – and add 30% to the price.

The Spitfire Scrambler was introduced in 1957 at the request of BSA's West Coast distributor, Hap Alzina, to meet a need in US desert racing to deat the dominant Triumph twins.

[42] A shorter seat was fitted in 1959 and the exhausts were high level on the left hand side.

[44] Although they already had a 500cc twin, the Val Page designed model KH500,[45] the management did not want the expense of developing a 650cc machine.