1949 BSA 500 Star Twin
When Triumph released the Speed Twin in 1937, it threw down the gauntlet to the rest of the British motorcycle industry, forcing it to follow suit. Triumph’s twin was immediately successful. It looked similar to the ubiquitous single, was lighter and only slightly more expensive. BSA, Triumph’s main competitor, wasted no time in developing its own twin, and had a development machine running by 1939. War intervened and the first production BSA twin, the 500cc A7, appeared late in 1946.
Like many British designs, the A7/A10 had its roots in an earlier design, in this case Val Page’s Triumph Model 6/1 of 1933. The 360-degree twin, with a single gear-driven camshaft at the rear of the crankcase operating all four pushrods and a four-speed gearbox bolted in semi-unit form to the rear, set the form that would remain basically unchanged for the engine’s life.
The first A7 was known at BSA as ‘Mr Perkins’ baby’, after the designer Howard Perkins. With a cast-iron barrel and cylinder this was a long-stroke design, the dimensions of 62mm x 82mm providing 495cc. Allowing easy serviceability, the A7 featured a forged, onepiece crankshaft with a bolted-on flywheel, a gear-driven magneto behind the engine and a chain-driven dynamo mounted at the front. The cylinder head was a one-piece, castiron casting, designed with large openings between the cylinders and two separate wells for the pairs of inlet and exhaust valves separated by a lateral air space.
Most A7s were sold with a rigid frame and it soon earned a reputation as a solid and reliable machine, albeit with moderate vibration. But the early A7 lacked excitement and, while it proved exceptionally reliable, there was no way the 19.9kW (27hp) A7 was going to beat Triumph’s new Tiger 100.
Despite the austere times of the UK in the 1940s, BSA announced a sports version of the A7 for 1949, known as the Star Twin. Although only lowoctane ‘pool’ petrol (something akin to kerosene in octane value) was available at the time, the compression ratio went up to 7.5:1 (from 6.6:1) and, with a pair of Amal Type 275 carburettors, the power went up to 22.8kW (31hp) at 6000rpm.
The Star Twin was fitted as standard with plunger rear suspension with a star badge on the tank to distinguish it from the standard model. The rest of the machine was fairly orthodox for the time: a duplex frame, 19in wheels front and rear, 7in singleleading-shoe brakes, and 6V Lucas electrics. The weight was a moderate 173kg and the Star Twin could be punted quite briskly along the empty backroads of the late 1940s.
But, somehow, the BSA engine didn’t really respond particularly well to twin carburettors. The top speed of 135km/h was no faster than the single-carb A7 and the culprit was the cylinder head. A small bore allied to a deep combustion chamber saw the engine displaying poor fuel burning characteristics, limiting power. Thus the Star Twin as featured here only lasted two years: 1949 and 1950.
Determined to take on Triumph’s new 650 Thunderbird, BSA lured Bert Hopwood from Norton to redesign the A7. This was primarily to permit an increase in capacity to 650cc but Hopwood also sought to cure the combustion problems of the earlier design. A shorter stroke allowed the engine to rev more freely and a larger bore, allied to a slight downdraft angle from the single carburettor, offered more efficient cylinder filling and combustion. The resulting A7 (and A10) was a brilliant design. Not quite as fast as the Triumph twins, they were quieter and more oil tight. The robust design
1949 BSA 500 Star Twin
• There were two versions of A7 twins. The first version was released in 1946 and displaced 495cc. This lasted until 1950 when a new, shorter-stroke 497cc version replaced it. By 1949 the 500cc A7 had grown to the 650cc A10. The A7 and A10 twins ran through until 1962, when they were replaced by the unit-construction A50 and A65.
• Although twin-carb conversions were later available, the A7 Star Twin was the only BSA A7/A10 to feature twin carburettors as stock. Even the final sporting A10, the 33.8kW (46hp) Rocket Gold Star, only had a single Amal TT monobloc carburettor.
• One of the problems with the twin carbs was they ran without air cleaners and had an unfortunate tendency to catch fire unless gauze covers were fitted to the bellmouths. The early Star Twin also had a habit of ‘running on’ after it had been switched off. In 1954 the Star Twin was redesignated and repackaged as the Shooting Star. This lasting until 1961. Ariel, bought by BSA in 1944, also produced the Huntmaster from 1953 until 1959 with a thinly disguised A10 engine in an Ariel frame.
• The A10 lived on long after its demise in England in the form of the Kawasaki W-series, originally the Meguro K1. Kawasaki offered the W3 through until 1975 – not bad for a design that first arrived
Want to know more?
* There is a gallery of pictures here:
* This is an interesting story of a Star Twin:
* Here is a BSA forum, with a special section for the long-stroke A7:
* This is a useful A7/A10 resource:
* There’s more info on the A7 here: