Post-War twins BSA

All BSA parallel twins were pushrod operated overhead valve machines. The A7 and A10 models were semi-unit construction until about 1953 and pre-unit construction thereafter. All A50, A65 and A70 models were unit construction.

The BSA A7 was a motorcycle made by Birmingham Small Arms Company (BSA) at their factory in Armoury Road, Small Heath, Birmingham in 1946. There were two versions of the A7, the original 495 cc (30.2 cu in) version, and an improved 497 cc (30.3 cu in) version launched in 1950. Although its name was changed to the Star Twin and later the Shooting Star the BSA A7 continued in production with minor modifications until 1961.
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. slogan 'It's time YOU had a BSA!'.

Engine
The 495 cc (30.2 cu in) twin cylinder engine produced 26 bhp (19 kW) and was capable of 85 mph (137 km/h).
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 had 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 such as the BSA C12/C15, BSA B40, Triumph 3TA and so 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 650 cc twin cylinder BSA A10. Although resembling the A7 500 cc 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 SS the new model had 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 72.6 mm and increasing the bore to 66 mm, giving a slight increase in capacity to 497 cc.
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 (22 kW) at 5800 rpm, with a top speed of just under 90 mph (140 km/h).

BSA Golden Flash
The BSA Golden Flash was a Birmingham Small Arms Company (BSA) motorcycle. The Golden Flash was also available in black and chrome, but it was the all-over gold paint scheme that gave it the name, and made it such a popular escape from post war austerity. Its development after the 1937 launch of the ground breaking Triumph Speed Twin, together with the need to pay off British war-debt, led to the two creating the post-war rise of the parallel twin engine layout, which was to dominate British design throughout the 1950s and 60s.

Development
In May 1948, Hopwood joined BSA. Briefed specifically to create a competitive parallel-twin, the internally designated A10 model was based on an earlier A7 design by Page and Bert Perkins. After BSA took over Triumph in 1951, Hopwood returned to Norton. Early Golden Flash A10s had frames with either no rear suspension, or plungers, and had a semi-unit engine and gearbox, with the gearbox bolted to the rear of the engine Launched in October 1949, the A10 Golden Flash was a new post-war design, with most of the difference to the A7 being in the engine. Increased to 650 cc (40 cu in), it encompassed revised castings for the cylinder head and rocker box, and a cast-in carburettor manifold. The frame was available in rear rigid format, but the more common option was the then new plunger suspension, specifically designed for overseas export.
BSA was a manufacturer who focused on machines for the working man, and so the design incorporated two practical use features: a hinged rear mudguard, designed to ease rear wheel removal; and a semi-unit engine and gearbox arrangement. The semi-unit power train enabled the primary chain to be adjusted via a slipper within the primary chain case.

Production
The large carry over of parts from the A7 had the advantage of greater reliability, as it minimised the risk of any new technical problems. Launched in a new form of gold colour, the combination of reliability and marketing made early exports possible, with 80% of production destined for the USA. The result was long delivery times for British customers, who were offered a more practical and less stand out black.
Although never designed as a fast machine, the Golden Flash was nonetheless fast for its time and competitive with the Triumph Tiger 100, achieving over 100 miles per hour (160 km/h) in tests in 1950, and covering a standing quarter mile (400 m) in under 16 seconds. Its gold colour also made it a marketing and sales success, out selling the red Speed Twin and the later blue-grey Triumph 6T Thunderbird, which was resultantly revised the following year into polychromatic blue.
However, BSA were concerned about a tendency for wear in the rear, plunger suspension, leading to uncertain handling. In 1954 it revised the model with a modern swinging arm, and the hinged mudguard was deleted. The semi-unit power train was also abandoned, making for a more involved procedure for adjustment of the primary chain, it offered benefits in the areas of the clutch and gearbox internals. The revisions also allowed for the launch of more sporting A10 Road Rocket. In 1956, alloy brake drums were fitted as standard which both reduced unsprung weight and increased stopping power.
Later transfer-style Golden Flash badge In 1957, the 40 bhp (30 kW) 105 miles per hour (169 km/h) BSA Super Rocket was launched for the 1958 season, with an Amal TT carburettor and high lift cam.
The final A10 development, and fastest in terms of power and performance, the 1962 BSA Rocket Gold Star, which fitted a tuned A10 Super Rocket engine into the well proven BSA Gold Star single frame. The result was a good handling fast bike that was a true classic. Current demand for the Rocket Gold Star, or "RGS", is such that non-RGS A10s are sometimes sold as originals, having been modified using suitable alternative parts.
With the launch of the new unit-construction (combined engine/gearbox) BSA A50 and BSA A65, the A7 and A10 were discontinued at the end of 1962 but the Rocket Gold Star continued into 1963 production.
All Post War Twins BSA Categories:

Model	Engine	First year	Last year	Notes
A7	500 cc	1947	1962	first parallel twin
A7ST	500 cc	1949	1954
A7SS	500 cc	1954	1962
A10 Golden Flash	650 cc	1950	1962	BSA's first 650 cc parallel twin
A10 Super Flash	650 cc	1953	1954	Limited edition for homologation for production racing in the USA
A10 Road Rocket	650 cc	1954	1957
A10 Super Rocket	650 cc	1958	1963	Amal TT 'racing' carburettor and new "357" full-race camshaft
A10 Rocket Gold Star	650 cc	1962	1963	Special - tuned Super Rocket in a Gold Star frame
A50	500 cc	1962	1970	Called "Royal Star" from 1966 onwards
A50C Cyclone	500 cc	1964	1965	Also A50 cc "Cyclone Clubman" for 1965 only
A50 Wasp	500 cc	1966	1968
A65	650 cc	1962	1966	Sometimes called "Star Twin"
A65R Rocket	650 cc	1964	1965	Sports model with 9:1 compression, separate headlight, and sports mudguards
A65T Thunderbolt	650 cc	1964	1972	Single carburettor. From 1971 had oil-bearing frame
A65L Lightning / Rocket	650 cc	1964	1972	Twin carburettors. From 1971 had oil-bearing frame
A65S Spitfire	650 cc	1964	1968	High performance model produced in Mk I, Mk II, Mk III and Mk IV versions
A65 Hornet	650 cc	1966	1967	Sometimes called "Hornet Scrambler"
A65F Firebird Scrambler	650 cc	1968	1972	Twin carburettors. From 1971 had oil-bearing frame, high level exhaust pipes on left hand side
A70L Lightning	750 cc			Limited edition for homologation for production racing in the USA
Fury	350 cc	1971	1972	Prototype only (never produced)