The BSA D1 125cc. engine unit began production in 1948 for export only. The design had been taken from the German firm DKW as part of the war reparations (reperations). The engine proved itself, and with small motorcycles becoming (becomming) ever more popular in Britain, BSA decided to build a complete motorcycle around the D1 engine. In October 1948 the BSA Bantam was released, the bike had telescopic forks, a rigid rear end, direct electrics, shovel front-mudguard and fishtail silencer. It (She) had three gears, a maximum speed of around 50 mph, good brakes for the time and fuel economy of over 100 miles to a gallon of fuel, interesting when compared to the lower efficiency (effeciency) of similar modern bikes.

The lighting was supplied by a 27 Watt Wico-Pacy Geni-mag generator, direct electrics meant no battery, and that the engine would have to be running for the main lights to work. A small battery was included to operate a smaller pilot bulb in the headlamp when parked. Ignition was supplied from the same unit, in the form of a simple Magneto, this also eliminated the need for a battery. Although the bike had its fair share of faults, it sold well and became a common sight on the Roads of Britain. One thing BSA had not expected was its introduction into competition events. Owners modified their bantams, fitting non-standard sprockets and wider handlebars. The Bantam was one of the first bikes to be used in this way, trials before that had been more professional events run by substantially larger bikes.

There were very many versions of the BSA Bantam model, which was produced from 1948 till 1991. The version 125 D1 was the first one. This model became the most successful for the company. Total more than half a million units were sold. After World War II England got as a compensation for destructions all pre-forms and documents of German producer DKW. Thus the BSA Bantam became the copy of the DKW RT125.