1971 BSA B50SS: The Other Gold Star

Was the Gold Star name sullied by the BSA B50SS, an overgrown commuter bike?

| January/February 2014

1971 BSA B50SS
Claimed power: 34hp @ 6,200rpm
Engine: 499cc air-cooled OHV single, 84mm x 90mm bore and stroke, 10:1 compression ratio
Top speed: 80mph (est.)
Weight (dry): 310lb (141kg)
Fuel capacity/MPG: 2.5gal (9.5ltr)/45-55mpg
Price then/now: $1,300 (approx.)/$2,500-$4,500

When BSA announced that its 1971 range would include a new 500cc unit-construction single called the “Gold Star,” it provoked outrage among traditional British motorcyclists. How could they sully the name of the mighty Goldie — the most successful British clubman’s race bike and favorite of the café racer crowd — by attaching it to an overgrown commuter bike?

Of course, BSA wanted to capitalize on the first Goldie’s reputation on racetracks in the U.K. and on the Grand National circuit in the U.S. But the association was in name only. The original Gold Star was developed from the 1930s Empire Star into a potent AMA Class C racing package, with the street versions sporting much of the race bike’s performance potential. The 1971 bike had grown out of BSA’s prosaic 250cc Star, itself a development of the Triumph Tiger Cub. But perhaps the older Goldie’s fans were being a little unfair. Jeff Smith won two world motocross championships (in 1964 and 1965) with the B50’s immediate predecessor, the BSA 441cc Victor, and John Banks narrowly missed giving BSA two more world motocross titles in 1968 and 1969 with the factory prepared 500cc B50MX. So why was so much scorn heaped on the unit-construction bike? Was it the humble origins? Or BSA’s hubris in usurping the famous Gold Star name?

BSA Origins

All BSA unit-construction singles can trace their roots to Edward Turner’s 150cc Triumph Terrier of 1953. The Terrier grew into the 200cc Tiger Cub, and the basic design, albeit with an upright cylinder instead of the Triumph’s forward tilt, was developed into the BSA C15 Star of 1959 with cylinder dimensions of 67mm x 70mm. Taking the bore out to 79mm, the C15 begat the 340cc B40 in 1961. Meanwhile, in BSA’s competition shop experiments began with stretching the B40’s stroke, first to 85mm for 420cc and finally to 90mm for 441cc, with the intention that it would replace the heavier and bulkier DBD34 Gold Star engine in the factory motocrossers. It was the unit-construction 441 engine, when installed in a lightweight oil-bearing frame, that powered Jeff Smith’s two 500cc FIM Motocross World Championships.

A difficult 1966 season with a titanium-framed Victor (the frames were prone to cracking, and the works team lacked the equipment to repair them in the field) saw Smith finish third in the Championship, with a second place finish in 1967. New team member John Banks took over the mantle in 1968 and 1969, placing second both years. But the main title went to the new lightweight 2-stroke CZs in 1966-1968 and Husqvarna in 1969. It’s not clear exactly when, but during this time BSA recognized that the “stretched” Victor engine had reached the end of its development. They needed a full 500cc engine if they were to remain competitive.

The Victor clearly exceeded the design limits of the B40 bottom end (big end failures were common), so the engine was completely redesigned with a new built-up crankshaft, larger crankpin diameter and heftier needle roller connecting rod big end. The crank was supported on no fewer than three main bearings — a roller on the timing side, and roller and ball bearings on the drive side — in beefed-up cases. A new iron-lined alloy cylinder took the bore out to 84mm, which, combined with the 90mm stroke, gave 499cc. The distortion-prone alloy-bodied oil pump was ditched in favor of a steel-bodied item from the unit-construction twins. Engine breathing was completely revised, the crankcase now breathing into the primary case with a half-inch diameter vent to air. The crankshaft drove a wet multiplate clutch and strengthened 4-speed gearbox through a duplex chain.

Ride 'Em, Don't Hide 'Em Getaway

Classic Motorcycle Touring and Events.

The latest classic motorcycle events and tours.