By the early 1950s, BSA (Birmingham Small Arms) had become the world’s largest motorcycle manufacturer, building and selling up to 75,000 machines per year. This claim was, in part, accomplished by following Triumph’s lead in building, right after the Second World War, a 500 cc vertical-twin using a simpler and less costly pushrod OHV engine. While some pre-war design work on the BSA vertical-twin had been carried out by Val Page, most of the detailed design work on the A7, as the post-war 500 twin was called, was engineered by Herbert Perkins, BSA’s chief designer at the time.
Introduced in 1946, the first A7s were modest machines with low compression and a single carburetor. They differed from the 500 Triumphs in that they were of a semi–unit construction design, so-called…