BSA A7

Development

Designed by Val Page, Herbert Parker and David Munro, the BSA A7 was the first of the BSA twin-cylinder motorcycles and was ready for launch in 1939, but the outbreak of World War II delayed the launch until September 1946 when hostilities 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

BSA A7 1948
The 495cc twin cylinder engine produced 26bhp and was capable of 85mph. 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 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. 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 featured a fixed gearbox, bolted to the back of the crankcase, and an internal tensioner for the duplex primary chain.This gave it the appearance of unit construction and pioneered the system later used in unit-construction engines (e.g. BSA C12/C15, BSA B40, Triumph 3TA and on). However, in 1954 a re-design reverted to the older system. 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 a single Amal remote float Type 6 until 1955 when it was upgraded to a 376 Monobloc.

BSA A7 'Star Twin'
In October 1949 BSA also launched the Bert Hopwood designed 650cc twin cylinder BSA A10. Although resembling the A7 500cc twin, it had a revised engine design and a new A7 soon followed, based on the A10 - in response to competition from the Triumph Tiger 100. Launched as the BSA Star Twin the new model featured twin carburettors and increased compression ratio. It also had the latest design of cylinder head with austenitic steel inlet and exhaust valves. The uprated engine was fitted to a plunger frame and finished extra chrome. Both models were produced with an option of rigid or plunger frames until 1954 and the introduction of a pivoted fork frame. The new frame also led to a separate gearbox to replace the bolted on version.

BSA A7 'Shooting Star'
In 1954 the Star Twin was redesignated the Shooting Star with a new swinging arm frame and the engine further developed by reducing the stroke to 72.6mm and increasing the bore to 66mm, giving a slight increase in capacity to 497cc.By the end of production in 1961 the BSA Shooting Star was the culmination of the development of the BSA A7, with a deep bottle green colour scheme with light green tank, mudguards and side panels, it had an alloy cylinder head, a duplex cradle frame with swinging arm rear suspension, full-width light alloy hubs and 8-inch drum brakes. Engine compression was upgraded from 6.6:1 to 7:1 and power was up to 30 bhp at 5800rpm, with a top speed of just under 90 mph.

Maudes Trophy
In 1952 three BSA A7's were entered for the Maudes Trophy and the International Six Days Trial, achieving 4,500 miles without problems and confirming the reliability of the design. All three bikes were randomly selected from the production line, picked up Gold medals and earned BSA the Team award as well as the Trophy. The three bikes were ridden by Brian Martin, Fred Rist and Norman Vanhouse. From Birmingham the team rode to Vienna then on through Germany, Denmark, Sweden and Norway before returning safely and with a clean sheet to Birmingham.

American speed record
Also in 1952 American BSA dealer Hap Alzina prepared a BSA Star Twin for an attempt on the American Class C speed record for standard catalogue motorcycles. The rules prevented major modification but Alzina was allowed to use 80 octane fuel, which together with a compression ratio of 8 to 1 enabled rider Gene Thiessen to achieve a two way record speed of 123.69 mph.

from wikipedia.com