The BSA B25 was a series of 250 cc (15 cu in) unit construction single-cylinder OHV four-stroke motorcycles made by the Birmingham Small Arms Company.

Bore and stroke of 67 x 70 mm was retained,[6] but a new alloy barrel and cylinder head were introduced featuring square fins.

[9] To improve performance the inlet tract was opened[7] up and bigger valves fitted[10] along with the newly introduced Amal Concentric carburettor.

[8] Developed from the competition C15s,[11] the all-welded frame had a single downtube that split into two to form a cradle under the engine.

[11] The output from the alternator was regulated by a zener diode with a large heat sink mounted between the front forks.

[10] Sharing many cycle parts with the 441 cc B44 Victor Roadster,[18][19] the model had a 2 US gallons (1.7 imp gal) fibre glass tank and a seat with a rear hump.

[6] Front forks were two-way damped and rubber gaiters fitted instead of the metal covers of the C15 and at the rear the springs of the shock absorbers were now exposed.

[10] Minor cosmetic changes included rubber knee pads on the tank and a chrome passenger grab-rail.

[25] The machine was updated with the oil bearing frame in 1971, but unlike the other B25 models retained its roadster styling.

[27] Since the late 1960s, BSA engineers had been working on a revamp of the existing range and development of new models.

[11] A new high-level lozenge shaped box silencer was fitted, which was designed to meet possible future more stringent noise laws.

The financial situation of the BSA Group forced a great reduction in the model range for 1972 and the 250s were discontinued.

Both variants had a steel 2 imperial gallons (2.4 US gal) petrol tank with a twist-off filler cap.

[34] Triumph Engineering had been bought by BSA in 1951 but had retained its own identity and had a separate model range.

(The "W" in the model designation stood for "woodsman")[37] The machine shared the chassis of the Starfire, but used 19" wheels front and rear, with a steel Triumph tank and side panels, high bars and a high level exhaust on the right hand side.

A 3 imperial gallons (3.6 US gal) tank was available, which because of its extra length required a shorter dualseat to be fitted.