By the start of WWII, Birmingham's largest motorcycle manufacturer had developed their answer to Triumph's ground breaking Speed Twin, launched the previous year. But six years of global conflict delayed the launch of BSA's 495cc A7 parallel twin - it was finally ready for production in September 1946.
Designer Herbert Perkins laid out the long stroke 62 x 82mm engine which featured a 360-degree built up crankshaft, fitted with steel connecting rods running on plain lead bronze big end bearings. A single camshaft behind the cast iron cylinder block and head operated both inlet and exhaust valves. Main bearings consisted of a ball journal on the drive side and a white metal lined plain bush on the timing side, through which passed the big end oil supply. A separate four speed gearbox was bolted in semi unit construction to the back of the crankcase.
An unsprung full cradle frame with twin downtubes was fitted with an oil damped telescopic front fork. Interchangeable 19in wheels - both quickly detachable - had seven inch drum brakes. Overall weight was 385lb fully equipped.
A one-piece rather than a built up crankshaft was supported on its drive side by a roller main bearing - changed from the original ballrace - but the timing side plain bush was retained.
Aluminium alloy connecting rods reduced bearing loadings. A year later Hopwood similarly revised the A7. Its 66 x 72.6mm dimensions were also much squarer, although the capacity was unchanged.
But don't worry, your choice doesn't have to include the rare 650cc Rocket Gold Star. Less than 1800 of this now highly collectable pre-unit BSA twin were made during a short 1962-1963 production run. With cycle parts based on the Gold Star singles, an RGS could now cost you over £8000. But any of the more common twins in good condition can be bought for a third of this sum. And as they rolled out of Small Heath in their thousands, there are still plenty of smart survivors to choose from.
oil-in-frame 1971 BSA A70 has to be Britain's rarest production 750cc parallel twin.
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