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1971 BSA B50SS – A Barn Find

 “Barn find” stories never grow dusty. But in this case the barn was right next door – a BSA B50SS.

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 favourite of the café racer crowd—by attaching it to an overgrown commuter bike? The original Gold Star was developed from the 1930’s 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 Goldie grew out of BSA’s prosaic 250cc Star, itself a development of the Triumph Tiger Cub.

But perhaps the older bike’s fans were being a little unfair. Jeff Smith won two world motocross championships (in 1964 and 65) with the B50’s immediate predecessor, the 441cc Victor; and John Banks narrowly missed giving BSA two more world motocross titles in 1968 and 69 with the works 500cc B50MX. So why was so much scorn heaped on the unit-construction bike? 

Though based on the BSA 441 Victor, the B50 engine was a significant upgrade with a new built-up crankshaft, larger crankpin diameter and heftier needle roller 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 crankshaft drove a wet multi-plate clutch and strengthened four-speed gearbox through a duplex chain.

The drivetrain was now capable of handling the 38 hp that a stock B50MX produced. (For comparison a good, stock DBD34 Gold Star Clubman made around 40 hp in 1961.) However, by 1970, two-strokes were totally dominant in the FIM Championship, and BSA —now running out of money—closed its motocross shop in 1971. But the development work was not for nothing, and a trio of 499cc B50-based bikes was announced for general sale in 1971. These were the off-road B50MX, the “dual-sport” B50T Victor Trail and the B50SS Gold Star street scrambler.

The BSA B50SS used an oil-bearing frame derived from the factory motocross bikes, featuring a tubular swingarm with a cam adjuster at the front needle-bearing pivot—ideas that BSA borrowed from the Rickman brothers. It got the new BSA Group alloy front fork and “conical hub” drum brakes with an eight-inch diameter TLS at the front. Frames and painted ancillaries got the unpopular 1971 dove grey paint scheme, though there’s some evidence this reverted to black during the model year. 

A completely new electrical system contained major components in an alloy case under the front of the fuel tank, and also featured a QD socket allowing the headlight to be removed. A switch on the right handlebar operated Lucas turn signals. Many cycle parts were common with other 1971 BSA-Triumph bikes, like the Thunderbolt, Lightning and Bonneville. Most distinctive, though, was the massive black-painted muffler covered with a perforated stainless heat shield. The styling was bold and aggressive, even if the bikes perhaps weren’t!

But was it any good? Well, yes and no. Mechanical reliability was certainly an improvement over the B44, thanks to the larger crankpin and three-bearing bottom end. And it was lighter than the last B44 Victor by 4.5 kg or so. Power was up from the Victor’s claimed 29 hp to 34. The engine was tunable, too. Simply fitting the B50MX cam and 32mm carburetor raised output to 38 hp, and more was available with cylinder head work. B50s were quite competitive in endurance events at the time, and continue to be raced on the track today in AHRMA events.

Author Steve Wilson owned a B50 back in the day, and describes his experience in BSA Motor Cycles since 1950. “For short haul, stop-and-go motoring, the engine’s punchy power characteristics made for an inspiring, pant-kicking, arm-wrenching ride … the B50’s engine clatter, plus the vibration from such a powerful engine in a comparatively light frame did make long-distance riding a bit of an ordeal …”

That said, Wilson found the BSA B50SS to be a “more satisfying compromise” than the later, similarly positioned Yamaha XT500. But it was tough to start, not helped by the 10:1 compression ratio and awkwardly placed decompressor lever. Accurate ignition timing was critical in this regard—a condition exacerbated by incorrect placement of the timing marks on some early bikes. 

The situation was compounded by the absence of a choke, and the tendency of the 30mm Amal concentric carburetor to flood the intake. However, owners have also noted that there are specific techniques to make starting easier—though they don’t always agree on what they are!

KEITH FOUND HIS BSA B50SS IN a neighbour’s barn. “I saw these two wheels sticking out from under a pile of hay,” he says. “They were rusty looking … so I asked him ‘what’s that?’ and he told me. I said, ’well, I’ve been looking for a project … is everything there?’” It turned out the only thing missing was one footpeg!

Dismantling the engine proved problematic. The piston was seized, and the piston rings, swollen with corrosion, cracked the cylinder liner when the barrel was removed. A direct replacement proved impossible to find, so Alec’s Automotive in Vancouver bored out the existing cylinder to accept a modern off-the-shelf sleeve, pinned it to the old cylinder, then bored it to suit the piston. The shop also sourced and installed new valves, guides, seats and springs. 

by Robert Smith Canadian Biker Issue #310


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