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The Gap Years at BSA
In 1955 Bert Hopwood left BSA for AMC. He tookwith him his ideas about sports twins, triples and 4 cylinder modular engines to a company he hoped would be more receptive to his vision of motorcycle development.
In the immediate aftermath at BSA development of the 500cc twins stopped. With the Gold Star BSA already had a successful 500cc sports bike and didn't see the need to complicate their marketing with another. No significant changes were made to the 500cc twins for the next 8 years. |
For the 650cc twins, valve and carburettor sizes increased first for the Super Rocket in 1958, then the Rocket Gold Star. But as handsome as the Rocket Gold Staris, it's still just a Super Rocket in Gold Star clothing.
So after 1955, not much happened in the twins department at BSA and wouldn't happen until the new unit twins appeared in 1963.
But at Triumph, whose name was synonymous with twins, something unusual was happening in the US...
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The Triumph Threat
In the same way that US BSA dealers in the early 50's demanded bigger, faster bikes so did US Triumph dealers.
Racing in the US had to comply with AMA class 'C' regulations. i.e. must be based on production bikes.
So racers started with standard bikes but had to rely on their own skills and ingenuity to make them go faster.
This culture resulted in many skilled tuners, fabricators and machine shops in the US who offered tune-up parts and services for BSA and Triumph roadsters because the manufacturers didn't.
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So it wasn't an exceptional event when one pair of enthusiasts decided to cast their own twin carburettor head for a 650 Triumph twin with splayed inlet ports and called it the delta head.
What was exceptional was that they presented this to Triumph and asked them why they hadn't done this themselves. So Triumph did, producing the legendary Bonneville 650 twin that first appeared in 1958.
This caused a sensation in the US where a power hungry public could buy two cylinders, two carburettors, 650cc and styling that shouted 'speed'. It was an immediate success.
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A theory...
What follows is pure speculation, something that may only have been recorded in letters from US dealers and distributors. I have neither seen these letters or spoken to the dealers. BSA dealers, perhaps shocked by the impact of the Bonneville decided to do something similar. But rather than cast their own twin carburettor head they look at the old 54-55 bolt-on manifold heads and the fast, successful Daytona twins that had been fitted with twin carburettors. Perhaps they wondered if they could do something similar with the roadsters. Because most of the parts already existed they could, at great speed and with minimal outlay produce a kit of parts that BSA owners could fit to their 500 and 650cc road bikes to acquire something of the performance and glamour of the Bonneville.
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With the power of persuasion that comes from being BSA's biggest market they got permission from BSA to market these kits in the US as BSA approved accessories and issue them with new part numbers. The kits were sold until the early 1960s when the twin carburettor 650cc sports unit Lightning twin arrives.
That's the theory. But I have some some verbal evidence from
people who for US dealers that some
US Rocket Gold Stars and Spitfire scramblers had these kits fitted by the dealer
from new.
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