1958 saw the beginning of a completely new approach to engine/transmission design with the introduction of unit construction machines, replacing the somewhat antiquated 250cc C12. The first of this new breed of motorcycle, the 250cc C15 Star, was an instant success. It had a clean, uncluttered appearance and provided reliable and economical transport.
This change from the old arrangement of separate gear box and engine was also accompanied by a redesigned frame which proved a considerable improvement on the C12. The new 250 was based on the Triumph Tiger Cub 200cc, with a vertical cylinder and contemporary styling. It proved a winner in many ways, but especially as this was the largest capacity solo machine that a learner rider in the UK could have until they had passed their driving test. Given that so many of the wee beasties were put into the hands of beginners who no doubt thrashed them mercilessly, this may account for their reputation of exploding gearboxes, dearly departed valve gear, fragile big-ends and a slippery crutch clutch.
In 1961 the Super Sports model (SS80) was added to the range. This had a tuned engine with roller big-ends, and lower handle bars to give a sleeker look, but it could still blow up the gearbox.
In 1965 the distributor was moved to the right hand side cover which gave an enhanced appearance, a style adopted across the BSA range.
During 1967 the 250cc C15 was replaced by the B25 Starfire and C25 Barracuda models, which featured a quickly detachable rear wheel and 12 volt lighting. Although this was an improvement on the C15 engine with tougher internals and castings, it was susceptible to big-end failure due of its higher state of tune and automotive-style big-end shell bearings.
Trials and Scrambles versions were available from 1959 and became very popular particularly among competition riders, due to their excellent power to weight ratio, handling qualities, high torque and reliability.
Source: Adapted from a page at the Cedric Norman Archive
