Classic Bike Guide
UNIT SINGLE DEVELOPMENT
The BSA unit singles replaced the venerable B31/33 pre-unit singles and were developed not from a BSA at all, but from Triumph’s successful Tiger Cub, which in turn had been developed from the 150cc Terrier. By redeveloping the Triumph’s bottom end and making the cylinder upright rather than at a slant, the Tiger
Cub concept became the utilitarian 249cc C15, a compact and useful – if uninspiring – mount that was BSA’s first four-stroke unit model, replacing the heavy and pedestrian C11.
The lightweight 15bhp C15 was developed into highly-successful scrambles and trials versions, as well as getting a power boost to become the Super Sports model SS80 – the 80 indicating optimistically that the bike could reach 80mph. With the new 250cc learner laws coming into force, the SS80 sold well to young riders, coming as it did with a tuned engine, roller big-ends, and lower handlebars.
BSA’s competition department was looking to replace the Gold Star as an off-roader. To compete in the 350 class, the C15 barrel was bored out as far as it would go, and fitted with bigger valves, a bigger carb, lumpy cam and high compression piston to take it to the 343cc. This became the basis for the relatively short-lived B40 that stayed in the catalogue for just five years. As an alternative to the heavy, old-style 350s offered by others, the BSA was ahead of the game by British standards and became a useful machine, being bought in its thousands by the military throughout the world, and it was popular as a sensible steed for commuters replacing the old B31. An SS90 sports model was produced but it didn’t catch the imagination of the speed freaks moving on from 250s.
BSA works rider Jeff Smith had found an off road-version developed by BSAs competition shop effective but not quite fast enough, so BSA had another go at the engine, managing to build a 420cc model by increasing the stroke. This won races, but there were issues. The bottom end was by now reaching the end of its mechanical abilities, not least because the bikes still used the C15 gearbox. In 1963 the BSA design team took the bottom end, fitted stronger gear pinions and redesigned the over-stretched crankcases to use ball and roller main bearings. The cast iron barrel was replaced with a light alloy casting with a chrome plated bore keeping the 420cc capacity, and this in turn became a 441cc engine. In 1964 this rather special competition machine, with Jeff Smith on board, won the World Motocross Championship in considerable style, winning seven of 14 races and coming second in another six, with one third place, thanks to his suffering a puncture.
The following year saw the engine improved again and created another winner, with Smith winning the world championship by the middle of the season; the last to be won by an OHV single.