Coincidental to Lucas switching from mag/dyno to alternator and coil ignition systems, the British motorcycle industry changed its engines to unit-construction for 1963. Henceforth, a single casting would combine the gearbox and crankcase.
Triumph’s Bonneville led the way to the lucrative U.S. market, and in 1962 BSA introduced its unit construction A50 500 cc and A65 650 cc Star Twins. These were expanded in 1964, with the A50 offering the Royal Star touring and Cyclone Enduro models. Meanwhile, the A65 650 cc could be bought as the Thunderbolt tourer and sporty Lightning Rocket.
The twin carburetor 650 cc Spitfire arrived in 1965 and was capable of 108 mph. The first versions were fitted with the Clubman’s tank favored by British enthusiasts and Amal GP carburetors, which required constant attention. Later U.S. market Mark II and III Spitfires had Amal concentric carburetors, smaller tanks and higher bars, and BSA concentrated on making the engines more reliable and less leaky.
Close-ratio gearboxes were fitted to Lightings and Spitfires, but though the top speed was attainable, the engine’s vibration was enough to break the headlight bulb filament, and the bike tended to weave above 90 mph. Other problems were making themselves felt as well. The alloy oil pump could warp, reducing the oil pressure. Then in 1966, BSA changed the caged ball race on the left side of the crankshaft for a roller race, which allowed the crank to wander. The motion could wear out the shims, leading to a spun bearing, and catastrophic engine failure. By the time the problems were corrected, it was too late.
In 1968 the Honda CB 750 made its appearance, and its four-cylinder SOHC engine, front disc brake and electric start would revolutionize the industry. BSA and Triumph fought back, but the three-cylinder BSA Rocket and the Triumph Trident were heavier and $400 more expensive.
The A50 and A65 twins soldiered on, until the “oil-in-frame” of 1971. By using the frame as a sump, BSA and Triumph eliminated separate oil tanks, but the resulting increase in frame size raised the seat to 34.5 inches, which was too tall for some riders. More drastic was the result of an engine failure, which scattered pieces throughout the frame and meant – at the very least – that the bike had to be dismantled and the frame hot-tanked to clean it.
By 1972 the BSA story was winding up, and the last gasp was the 350 cc OHC twin Fury, and sibling Triumph Bandit that could rev to 9,000 rpm and even had a five-speed gearbox and electric start. They were advertized before production was finalized, though, and the failure to deliver finished off the BSA Group.