Triumph 6T Thunderbird


Make Model

Triumph 6T Thunderbird


1950 - 53


Four stroke, vertical twin, OHV


649 cc / 40.0 cu in

Bore x Stroke

71 x 82 mm
Cooling System Air cooled
Compression Ratio 8.5:1 (Export), 7.0:1 (UK)
Exhaust Two-into-two


1 x Amal Monobloc, 25 mm (1953: SU type MC2)


Lucas magneto
Starting Kick

Max Power

25 kW / 34 hp @ 6500 rpm
Clutch Wet, multiplate


4-Speed constant mesh
Final Drive Chain, 101 links
Gear Ratios 1st 11.90:1 / 2nd 8.25:1 / 3rd 5.81:1 / 4th 4.88:1
Frame Brazed lug

Front Suspension

Telescopic fork, hydraulic damping

Rear Suspension

Rigid, spring loaded saddle

Front Brakes

SLS drum, 7 in

Rear Brakes

SLS drum, 7 in
Front Wheel 3.25 x 19
Rear Wheel 3.50 x 19
Wheelbase 1403 mm / 55.25 in
Seat Height 775 mm / 31 in
Ground Clearance 127 mm / 5.0 in

Dry Weight

168 kg / 370 lbs

Fuel Capacity 

13.6 L / 3.6 US gal / 3.0 Imp gal (US), 18 L / 4.8 US gal / 4.0 Imp gal (UK and export)


Classic British Motorcycles

The 1950 Triumph Thunderbird was not only the first Thunderbird, it was the first Triumph 650 & it began a long legacy that lasted until the demise of Triumph Motorcycles in 1983. It is said that Edward Turner got the idea for the name from a motel he stayed in while visiting the US on business.

He had another brilliant idea as well. As the quest for ever more power had pushed the 500cc Triumph Speed Twin to its limits, Turner simply expanded those limits. He bored it out to 71mm & stroked it out to 82mm & in doing so, created a 649cc fire-breather. The 6T Thunderbird was born & was instantly one of the fastest motorcycles on the market!

A huge advantage was that most of the major engine castings remained relatively unchanged from the 500cc units already in production. And of course this meant that it all fit into the same frame as well, saving even more development & tooling costs. Just as he had done back in 1937 with the seminal Triumph Speed Twin, Edward Turner had once again created a game-changing motorcycle out of components already on the shelf.

These included the brazed-lug rigid frame, the rest of the running gear & the infamous "sprung hub". This was another Edward Turner invention designed to bring some rear suspension travel (however slight) to the Triumph rigid frame without investing the money needed to replace it with a proper swing arm rear suspension. Essentially it placed springs radially between the rear axle & the hub/spokes/wheel. In theory, the springs would absorb the bumps while keeping everything in line. The problem should be obvious: any play in the system results in uncontrolled wheel movement & erratic handling. But, it remained in service until the swing arm came out in 1954.

To cope with the added power, the 1950 Triumph Thunderbird got a new stronger 3-piece pressed-together crankshaft & the engine cases were beefed up around the main bearings. The oil pump's capacity was increased by 20%. The gearbox was strengthened with larger pinions, a greater number of teeth (finer pitch) & improved shift forks.

Primary gearing was raised by replacing the Speed Twin's 22-tooth engine sprocket with a 24-tooth. Changing the engine sprocket like this is actually the way the factory preferred to change gearing, as the final-drive sprocket remained 18-teeth on all twins up to & beyond unit construction in 1963.

Another interesting feature was the crankshaft shock absorber. Introduced with the 1950 Triumph Thunderbird, it remained in service until the conversion to alternator electrics in 1953, which was now mounted in its place on the left end of the crank, inside the primary cover. This brilliant device decoupled the engine from the primary, linking them through a spring-loaded, cammed interface that allowed a controlled amount of 'give', smoothing out harmful vibrations. It is said to have made everything last longer: engines, primary chains, clutches, gearboxes & even final drive chains! When the alternator replaced it, Meriden attempted to do the same job with rubber shock absorbers inside the clutch hub, but it never worked nearly as well.

For the most part, the new 1950 Triumph Thunderbird 650 engine was the same as the 500cc Triumph Speed Twin it evolved from. The cast iron cylinder block & head were similar, but the 650 head was more generously finned & utilized a copper gasket instead of a 'spigot' arrangement, common for the day. The valve train was identical. Home market 650s had a 7:1 compression ratio to cope with the low octane British Pool fuels available at the time. However, export versions (read that US) had 8.5:1 which was enough to boost power to 34hp @ 6500 rpm, enough to push the 370lb 1950 Triumph Thunderbird past 100mph!

The look of the 1950 Triumph Thunderbird was established early on & followed normal convention for a machine of its type. It had generous fenders, made to cope with wet British riding conditions, & a headlight nacelle housing not only the headlight, but the gauges, top yoke, fork tube tops & the center section of the handlebars. All of this weather protection was also supposed to make the bike easier to clean after riding in the rain.

Source: Classic British Motorcycles