BSA Motorcycles Zb34 Competition 1952
Following a disastrous attempt at the Isle of Man TT races in the 1920s, BSA withdrew from competition until 1946, when it launched a competition version of the 350cc B31, the B32.
That was followed in 1947 by the 500cc B34. In 1949, Gold Star versions were introduced featuring modifications to the bottom end and alloy heads and barrels.
The Competition model B32 and B34 engine numbers were prefixed with X(B) in 1946 and 1947 but became YB for 1948 and then ZB from 1949 through to 1952 after which they became BB. The Gold Star models were all prefixed ZB from 1949 to 1952 and featured an alloy cylinder head and barrel. From 1950, there were also Competition versions with an all alloy engine carrying the suffix A so that engine numbers were ZB32A and ZB34A. Throughout the ZB engine period, the frame numbers on bikes of both capacities were designated ZB31. No suffix indicated a rigid rear frame whereas ZB31S indicated plunger rear suspension.
The Competition B34 engine retained the bore and stroke of 85mm x 88mm and the bottom end of the B33 engine but featured a steeper head angle and a compression ratio of 6.8:1 to take peak output to 23bhp at 5,500rpm. An upswept exhaust pipe was standard fitment although a down-swept pipe was an optional extra. Plunger rear suspension was available from 1949 as an optional extra. They were normally supplied without lighting equipment although Lucas magdyno or alternative lighting was available to order at an extra cost.
The B34 model remained in production from 1947 to 1957 by which time it had accumulated an impressive list of trials and scrambles successes and carried the BB34A engine number. The Gold Star version also continued through to 1957 as the DB34GS. It was superseded by the DBD34GS, which was introduced in 1956.
This particular ZB34, with iron head and barrel and equipped with plunger rear suspension and a down-swept exhaust, carries engine number ZB34-4002, indicating that it was the second 1952 ZB34 engine produced. The frame number is ZB31S-17090. It was acquired for the NZ Classic Motorcycles collection via a US auction house in May, 2008. It had recorded only 1,309 miles since a comprehensive restoration and rebuild.