Looking towards the all-important American market, Triumph debuted a larger Speed Twin in 1950. Technically it was the T6, but Triumph named the new 650 cc tourer the Thunderbird, and did so five years before Ford used the name on its famous two-seater convertible. Marlon Brando rode a Triumph Thunderbird in the 1953 film “The Wild One”, and helped solidify the bike’s image in America.
The T6 engine was a bored and stroked version of designer Edward Turner’s 1938 500 cc vertical twin. The extra 150 cc of displacement made for 34 bhp (8 more than the old Speed Twin), and allowed the T6 Thunderbird to compete with the bigger Harley-Davidsons and Indian V-twins that dominated U.S. sales.
The Speed Twin and its T5 and T6 derivatives utilized a 360-degree crank so that a single carburetor and single magneto could be used. The bottom of the engine featured a vertically split crankshaft with a built-up crank bolted to a central flywheel. The crank ran in two big ball races and the connecting rods were made of light alloy.
The right side of the crank carried a gear meshed with another, which drove the two camshafts. The cylinder and the head were both cast iron. Three-ring pistons faced valves set at 90 degrees. Alloy boxes carried the rockers, with caps for adjustments. The oil system was dry sump with a tank below the seat, and the electrics were Lucas gear-driven mag-dyno. The four-speed gearbox was separate and located behind the engine on the right, ahead of the kick-start and behind the gearshift. The clutch, meanwhile, was on the left side.
The T6 650 cc Thunderbird tourer of 1950 was finished in a monotone paint scheme, with wheel rims in the same color, and the tank received side-stripes, new badges and a luggage rack. The four-speed gearbox was up-rated to cope with the extra horsepower and the speedometer drive integrated into the casting.
An increase of power over the Speed Twin, however, put a stress on the the sprung-hub rear suspension. A complicated set of springs were compressed inside the hub, which only allowed an inch of travel. The hub could cause the bike to weave at higher speeds and while riders became used to it, it could be a bit frustrating and it was finally replaced by a swing arm in 1955.
In 1954, the Thunderbird got an alternator, coil and distributor as well as larger engine bearings. The Thunderbird finally received an alloy cylinder head in 1961, and got a siamesed exhaust in 1962, which was the last year of the pre-unit engine.
Triumph also installed the 650 engine in the sporting T100 Tiger to create the T110. A flurry of development surrounded the twin-carbureted Bonneville of 1959, which would be Triumph’s biggest success. The performance image overshadowed the Thunderbird, which soldiered as a single-carb tourer until 1966, sometimes the with unloved “bathtub” rear fender , or the cut-down “bikini” version.
The Triumph T6 Thunderbird’s heyday was really the mid-1950s. Original survivors are no longer common, but are worth seeking out from older enthusiasts for their pleasant all-rounder characteristics – especially if they lack the sprung-hub rear suspension.