Triumph Thunderbird


The Triumph Thunderbird came when the quest for more power took the Triumph 500 as far as it would go & the decision was made to "punch it out" to a 650. The bore & stroke were increased, the same basic engine castings were used, along with the same rigid frame & running gear as the existing 500 line. But the 650 6T Thunderbird was all new! And it set the new standard by which all other motorcycles would be measured & soon virtually all other British motorcycle manufacturers were building 650 vertical twins.

Pre-Unit Construction refers to any 650 twin built prior to the 1963 model year (in other words, 1962 was the last model year for 'Pre-Unit Construction'), because in 1963, Triumph came out with a new "Unit-Construction" engine. Pre-Unit, or Non-Unit Construction means that the Engine Crankcase, the Primary Case and the Transmission are all separate components which are bolted together with brackets, in the frame. In the case of Pre-Unit Triumphs, and many other Classic British Motorcycles, the transmission was actually rotated in it's mounts as the only means of adjusting the primary chain. Once the primary was adjusted, the final drive chain then needed to be readjusted. Non-Unit Construction was adopted by nearly every British bike maker as well as Harley-Davidson and Indian Motorcycles. Most position the final drive sprocket between the transmission and the clutch, sandwiched behind the primary drive. This was done to keep the whole package rigid.

The 'Golden Years' for the Triumph Thunderbird were definitely the 1950s, throughout which they were pre-unit. By the time unit construction had arrived in 1963, the Thunderbird line was already relegated to 'entry level-status' behind the Triumph Bonneville & Triumph TR6. In England they referred to this as the "cooking version", meaning a workingman's commuter bike, meant as basic transportation, not a sporting bike. But, during the 50's, until the arrival of the Triumph TR6 in 1956, the T-Bird was the hottest bike on the market.

In 1963, Triumph introduced their 'Unit Construction' engine, which incorporated the Engine, Primary and Trans all in one unitized housing. The cast aluminum case split vertically into two halves, with the inner primary case going with the left crankcase half, and the transmission housing going with the right. The result was an engine that was lighter, stronger, more modern and cleaner than the Pre-Unit engine it replaced. And it made it possible to pull even more power out as the 1960s progressed. With all this, the age of the Pre-Unit Triumph was over, the 500 twin already gone unit-construction in 1959.

Unit construction brought with it many benefits, some for the public, some for the factory. They were certainly easier & less expensive to manufacture & assemble & were easier to maintain. Unit Construction saved 30lbs compared to the Pre-Unit Engine, Primary, Gearbox & Mounting Plates.

But the new engine wasn't the only big news. A whole new frame with a single front downtube replaced the 'flexy' duplex frame, virtually eliminating all the handling problems overnight. From this point on, Triumph Motorcycles would gain a reputation as some of the best-handling motorcycles in the world.

The Unit Construction Triumph 650 twin soldiered on until it was punched out to a 750 in 1973 & beyond, all the way to the end of Triumph & the last of the British Motorcycle Industry as we knew it. In fact, the 1983 Triumph Bonneville (a direct descendant of the unit construction engine introduced in 1963) was THE last Classic British Motorcycle to be built.

Throughout the nearly 4 decades since the release of the Unit Construction Triumph 650 twin, it remains one of the most attractive motorcycle engines in history. It's clean, well-defined lines are distinctive & instantly recognizable.

Triumph Thunderbird Unit-Construction

Model: 6T
Engine: 649cc OHV vertical twin

First year for Unit-Construction engine & new frame w/single front downtube. Practically everything is new or improved. Some problems pop up, but overall a huge leap forward for Triumph. Bathtub bodywork shrinks somewhat.

650cc OHV vertical twin

With each passing year, the T-Bird's "Bathtub"-styling steady shrank. By 1964, it's down to big side covers at meet in the middle.

Engine: 649cc OHV vertical twin

Engine improvements come to the entire Triumph 650 line, in quest for more power, less vibration & better reliability. The T-Bird finally loses the Bathtub bodywork, but retains its headlight nacelle. Front fender is trimmed down also.

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Last updated 12/17/20

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