History: From Rigid 500 To Britain's Rarest 750

BSA twins adopted unit construction in 1962. But little had changed inside the new crankcase

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.

A plunger sprung frame was an option from 1949 and was fitted to the A10 from the start. The all welded swinging arm frame became available on home market twins from 1955 - on export models the previous year. Shrewd BSA had already fitted the new frame to their B series roadster singles.

Extending its use cut costs and speeded up production, with the benefit to the customer of improved handling. The twins also adopted the single's separate gearbox and thereby a pre-unit engine layout. A lengthened primary chaincase, still in aluminium alloy, also signified the change.

Aluminium alloy cylinder heads were fitted to sporting versions from 1955. A year later, Ariel type full width wheel hubs - also in alloy with die cast seven inch brake plates and fulcrum adjusters - became a standard fitment. They were replaced in 1958 by BSA's own cast iron hubs.

By adopting unit construction in 1962, BSA again followed their Coventry rival's design lead. Triumph had first used combined crankcase and gearbox castings on their 1957 350cc Twenty One parallel twin with its enclosed 'bathtub' styling.

Many enthusiasts believe that unit construction made BSA's twins look podgy and overweight. Yet despite their chubby engine casings, bulbous petrol tanks and massive side panels, the new models weighed up to 31lb less than the pre-unit twins. And with a two inch shorter wheelbase they had quicker steering.

Inside the egg shaped crankcase halves little had changed, although the separate magneto and dynamo had been replaced by coil ignition and an alternator. Primary drive was by triplex rather than a single row chain. The new 500 twin - the A50 - had 65.5 x 74mm dimensions, while the 650cc A65 was just oversquare at 75 x 74mm - both models sharing the same stroke.

A drive side ball journal bearing replaced the pre-unit engine's roller race, while larger crankpins and timing side bush helped to strengthen the bottom end. An aluminium alloy cylinder head included an integral rocker box.