The BSA A7 was a 500cc motorcycle model range made by Birmingham Small Arms Company (BSA) at its factory in Armoury Road, Small Heath, Birmingham. The range was launched in 1946 using a long stroke engine. An improved version based on the BSA A10 engine was launched in 1950. The various A7 models continued in production with minor modifications until 1961/2 when they were superseded by the unit-construction A50 model.
Development
After the 1938 launch of the Edward Turner designed Triumph Speed Twin, BSA needed a 500cc twin to compete with the Speed Twin. Designed by Herbert Parker, David Munro and BSA's chief designer, Val Page, the BSA A7 was the first of the BSA twin-cylinder motorcycles. The outbreak of World War II delayed the launch and several prototypes were built during the war years. The model was finally launched in September 1946 when hostilities had ended.
The very first A7 off the production line was flown to Paris for the first motorcycle show after the end of the war. There was huge demand for affordable transport after the war and the simplicity of the A7 twin was helped along by the slogan 'It's time YOU had a BSA!'.
Engine
The twin cylinder engine produced and was capable of . The engine had a long stroke with a bore and stroke of . A single camshaft behind the cylinders operated the valves via long pushrods passing through a tunnel in the cast iron block. This system needed a considerable number of studs and nuts to fasten down the cast iron cylinder head and rockerboxes, many of them deeply recessed and requiring well-made box spanners or the then uncommon sockets. As with other British motorcycles of the period, this kind of set-up regularly led to oil leaks.
In October 1949 BSA launched the Bert Hopwood designed 650 cc twin cylinder A10. The engine in the A10 was a redesigned and improved version of the A7 engine. A 497 version of the redesigned engine was produced for the A7. Initially fitted in the new A7S Star Twin, it was also fitted to the base A7 model in 1951. The new engine had a shorter stroke with bore and stroke dimensions of .
Most motorcycles of this period tensioned the primary chain by drawing or rotating the gearbox backwards on a hinge with threaded rods, this was known as pre-unit construction. The first A7 had a fixed gearbox, bolted to the back of the crankcase, and an internal tensioner for the duplex primary chain. However, in 1954 when the swinging arm was introduced, the re-design reverted to the older system of a separate gearbox. The electrics (as was universal for larger British motorcycles of the period) consisted of two independent systems, the very reliable and self-contained Lucas magneto, with a dynamo generator to charge the battery and provide lights. Carburation was initially a single Amal remote float Type 6. Later Amal TT and then 376 Monobloc carbs were used.
Cycle Parts
The A7 was launched with a rigid frame and telescopic front forks. One useful feature was a hinged rear mudguard that lifted up to allow the rear wheel to be removed more easily.
Plunger rear suspension was offered as an option to the rigid frame from 1949. Concerned about the high wear rate of the plunger suspension that lead to poor handling a swinging arm frame was introduced in 1954 and the rigid and plunger frames discontinued.
Model Range
Overview of models
All models are swinging arm frame unless otherwise indicated.
Notes
A7
The A7 was launched in 1947 with a rigid frame and the "long stroke" engine with iron barrels and cylinder head. Plunger suspension was offered as an alternative from 1949. The engine was changed to the shorter stroke A10 based design in 1951. The rigid frame was discontinued in 1953 and the plunger in 1954. From 1954 the swinging arm frame was produced, and the name "Flash" given to the model in the US market. An alternator was offer as an alternative to the dynamo from 1961. Production ceased in 1962 when the unit construction A50 was introduced.
A7S Star Twin
In response to competition from the Triumph Tiger 100BSA launched the A7SS Star Twin. The new model had increased compression ratio. It had the later A10 based engine and the latest design of cylinder head with austenitic steel inlet and exhaust valves. The inlet manifold was separate from the cylinder head so twin carburettors could optionally be fitted. The uprated engine was fitted to a plunger frame and finished extra chrome. The model was produced with an option of rigid or plunger frames. In 1953 a new alloy head was fitted together with an Amal Monoblock carburettor. The model was discontinued in 1954 in favour of the "Shooting Star" model.