History: From Rigid 500 To Britain's Rarest 750

Oil-in-Frame 1971 BSA A70 has to be Britain's rarest production 750cc parallel twin.

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.

The 650cc A10 arrived in 1950, designed in three months by Bert Hopwood who had recently joined BSA from Norton. Subtle external changes, such as a single rocker box casting instead of separate inlet and exhaust covers and a symmetrical timing case fronted by the familiar piled arms logo, turned out to be minor aspects of a thorough engine redesign. Squarer 70 x 84mm bore and stroke dimensions than Perkins's A7 gave a 646cc capacity.

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.