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BSA Bantam 125

BSA Bantam (1951)
Years in production: 1948-63 Engine type: piston-ported
two-stroke single Bore and stroke: 52 x 28mm Capacity: I23cc Compress/on ratio: 6.5:1 Power: 4.5bhp @ 5000rpm Top speed: 50mph

One of the most successful lightweights produced in Britain, the Bantam was actually 'borrowed' from a pre-war German design.
Before World War 2 the German manufacturer DKW was one of the great innovators of two-stroke technology. Until then most two-strokes had used a cumbersome 'defector piston'. DKWs version used a symmetrical, flat-top piston - together with clever transfer porting - to direct the gas flow for better power and reliability. As part of the reparations of war, German designs were offered to the Allies. One such was the DKW RTI25.

The design was turned down by Villiers, but was rapidly adopted by BSA. The same design was also taken up by the other Allies. Harley-Davidson, Voskhod and WSK all made their own variants - in the latter cases, for decades later. Even Yamaha borrowed the design.

Although the original design had the gearchange on the left in European, rather than British fashion, BSA simply reversed the drawings to make a mirror image of the DKW with a 'conventional' gear-change. Inside the egg-shaped engine castings was a three-speed gearbox in unit with the engine, which had a pressed-up full-circle flywheel.

The capacity was 123cc. Lubrication was by petroil mix, fed by a tiny carburettor with integral air filter. Ignition and lighting were by a Wipac flywheel magneto fitted to the left of the crankshaft. Lighting was direct, with no battery fitted.

The new BSA was announced early in 1948, and initially called the Dl it was going to be supplied as a proprietary engine unit only. By October, it had gained the Bantam name. Producing 4'/2bhp, it was good for about 50mph on a miserly 120mpg, and despite its £260 price tag, the model sold as fast as BSA could produce it. A version designed for trials or scrambles was also catalogued, while some owners modified machines for lightweight racing.

In 1950 the Bantam was improved and updated with an optional plunger rear suspension. In 1954 a larger ISOcc version called the D3 was introduced. It had heavier forks, plunger springing as standard, and a larger front brake, among minor variations. It gained a new swinging-arm frame in 1956, but this was dropped in 1957.

The Bantam continued in production until August 1963, with only a few minor adjustments to electrics, engine and frame. It sold some 20,000 a year and was much-loved by owners all around the world. A workhorse whose duties included telegram deliveries, commuting and farming in the Australian bush. Examples soldiered on for more than a decade as a mainstays of some rider training schemes. Meanwhile, derivatives of the original would continue to be produced as late as 1971.