page_title: BSA A7
section_title: Engine
hierarchical_section_title: BSA A7 / Engine
caption_attribution_description: English: 1947 "long-stroke" A7 engine with bolted on gearbox
context_page_description: The BSA A7 was a 500cc motorcycle model range made by Birmingham Small Arms Company at their factory in Armoury Road, Small Heath, Birmingham. The range was launched in 1946 using a 495 cc long stroke engine. An improved 497 cc 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 The various A7 models continued in production with minor modifications until 1961/2 when they were superseded by the unit-construction A50 model.
context_section_description: The 495 cc (30.2 cu in) twin cylinder engine produced 26 bhp (19 kW) and was capable of 85 mph (137 km/h). The engine had a long stroke with a bore and stroke of 62 mm × 82 mm (2.4 in × 3.2 in). 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 66 mm × 72.6 mm (2.60 in × 2.86 in).
  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 Monoblock carbs were used.
caption: 1947 "long-stroke" A7 engine with bolted on gearbox
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