Once the realisation their Gold Star was past its scrambles best had hit BSA, the search for its replacement was on. A couple of C15S models were prepped for Jeff Smithwas that, being lighter and more manoeuvrable than the bigger bikes, the lack of power wasn’t so noticeable. A company the size of BSA has many resources available and the comp department drew on as much as they could. To increase the capacity as much as possible a bore and stroke of 79mm x 86mm giving 420cc was arrived at using as much ‘B40’ as possible… except the con rod was too short… the B40 stroke is 70mm… however the experimental department still had a couple of rods available from an engine which hadn’t made production. These rods were for an engine of 79mm x 86mm bore and stroke; even better, the forging dies were still available to make more. The result was the 420cc ‘B40’ (pictured above); naturally a lot more went into the eventual world championship-winning machine but as the main feature is about a B40-inspired bike we’ll stop there.
