TRIUMPH THUNDERBIRD Unit-Construction
1962 through 1966


Triumph Thunderbird, Triumph motorcycles, triumph tr6, triumph bonneville


THE TRIUMPH THUNDERBIRD UNIT CONSTRUCTION:
PUNCH IT OUT

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.
Triumph motorcycle engine, Triumph Thunderbird, Triumph Bonneville, Triumph motorcycles
PRE-UNIT VS. UNIT
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 & 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.

T-BIRD SECOND STRING
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.

Triumph motorcycle engine, Triumph Thunderbird, Triumph TR6, Triumph motorcycles


UNIT CONSTRUCTION
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.

NEW FRAME TOO
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.

HANDSOME ENGINE
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.


Unit Consruction Triumph Thunderbird, YEAR-BY-YEAR:


Triumph Thunderbird, Triumph motorcycles, Triumph Tr6, Triumph Bonneville

1963 TRIUMPH THUNDERBIRD
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.


Triumph Thunderbird, Triumph motorcycles, Triumph bonneville, Triumph TR6 1964 TRIUMPH THUNDERBIRD
Model: 6T
Engine: 649cc OHV vertical twin

Development continues with new external spring forks, which improve handling still further. T-Bird leads the way in adopting 12-volt electrics, using two 6-volt batteries & governed by a Zener Diode.

Triumph Thunderbird, Triumph motorcycles, Triumph Bonneville, Triumph TR6 1965 TRIUMPH THUNDERBIRD
Model: 6T
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.

Triumph Thunderbird, Triumph motorcycles, Triumph Bonneville, Triumph TR6 1966 TRIUMPH THUNDERBIRD
Model: 6T
Engine: 649cc OHV vertical twin

Last year for the 6T Thunderbird. Lots of performance upgrades across the entire Triumph 650 line made the T-Bird faster than ever. Once gone, the Triumph TR6 would replace the T-Bird as the entry-level 650.

Check out these TRIUMPH BOOKS


TALES OF TRIUMPH MOTORCYCLES & THE MERIDEN FACTORY


COMPLETE BOOK OF CLASSIC & MODERN TRIUMPHS


TRIUMPH BONNEVILLE & TR6


TRIUMPH MOTORCYCLES, THEIR RENAISSANCE & THE HINKLEY FACTORY


TRIUMPH SPEED TWIN & THUNDERBIRD BIBLE


HINKLEY TRIUMPH TRIPLES & FOURS ESSENTIAL BUYERS GUIDE


TRIUMPH PRODUCTION TESTERS’ TALES FROM THE MERIDEN FACTORY


TURNER’S TRIUMPHS: EDWARD TURNER & HIS TRIUMPH MOTORCYCLES


TRIUMPH TROPHY BIBLE


HINKLEY TRIUMPHS, THE FIRST GENERATION


BONNIE: THE DEVELOPMENT OF THE TRIUMPH BONNEVILLE


TRIUMPH: A CENTURY OF PASSION & POWER


TRIUMPH TWIN RESTORATION


CONENTRY’S BICYCLE HERITAGE


HOW TO RESTORE YOUR BSA ROCKET III/TRIUMPH TRIDENT


TRIUMPH & BSA TRIPLES: THE COMPLETE STORY OF THE TRIDENT & ROCKET 3


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TRIUMPH SINGLES: EARLY DAYS TO 1974 BY ROY BACON


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TRIUMPH MOTORCYCLE RESTORATION, PRE-UNIT


TRIUMPH PRE-UNIT TR5, 5T, 6T, T100 & T110 FACTORY SHOP MANUAL


TRIUMPH MOTORCYCLE RESTORATION


ILLUSTRATED TRIUMPH MOTORCYCLE BUYER’S GUIDE BY ROY BACON


FACTORY ORIGINAL TRIUMPH TWINS 1938-1962


TRIUMPH BONNEVILLE: PORTRAIT OF A LEGEND


TRIUMPH 650 & 750 TWIN REPAIR MANUAL 1963-1983, BY HAYNES


TRIUMPH MOTORCYCLES IN AMERICA


TRIUMPH BONNEVILLE 2001-2012 REPAIR MANUAL BY HAYNES


TRIUMPH BONNEVILLE: THE ESSENTIAL BUYERS GUIDE


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TRUIMPH MOTORCYCLES: FROM SPEED TWIN TO BONNEVILLE


BOOK OF TRIUMPH TWINS BY CLYMER


TRIUMPH BONNEVILLE BIBLE, ALL MODELS 1959-1983


TRIUMPH TIGER CUB & TERRIER 1952-1968 REPAIR MANUAL BY HAYNES


TRIUMPH RACING MOTORCYCLES IN AMERICA


TRIUMPH MOTORCYCLES ILLUSTRATED WORKSHOP MANUAL 1945-1955 BY CLYMER


TRIUMPH 675 DAYTONA & STREET TRIPLE REPAIR MANUAL, 2006-2010


TRIUMPH TRIPLES & FOURS 1991-2004, HAYNES REPAIR MANAUL


TRIUMPH 350 & 500 UNIT TWINS ESSENTIAL BUYERS GUIDE


TRIUMPH TWIN RESTORATION BY ROY BACON


TRIUMPH BONNEVILLE & TR6 MOTORCYCLE RESTORATION GUIDE


TRIUMPH: RETURN OF THE LEGEND


BSA & TRIUMPH TRIPLES: GOLD PORTFOLIO


TRIUMPH MOTORCYCLE RESTORATION GUIDE: BONNEVILLE & TR6


TRIUMPH THUNDERBIRD 6T ROAD TEST 1966


TRIUMPH 500-750 TWINS 1963-1979, SERVICE, REPAIR & PERFORMANCE MANUAL


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HOW TO RESTORE TRIUMPH TR5/250 & TR6


HOW TO RESTORE TRIUMPH TRIDENT T150/T160


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TRIUMPH TT600 MOTORCYCLE SERVICE MANUAL


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ORIGINAL TRIUMPH BONNEVILLE


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TRIUMPH ILLUSTRATED WORKSHOP MANUAL 1937-1951 BY ROY BACON


TRIUMPH 500 UNIT TWINS 1958-1973 REPAIR MANUAL, BY HAYNES


TRIUMPH T110R DAYTONA 1966-1969


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TRIUMPH SPEED TWIN INSTRUCTION BOOK FOR 1939


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Triumph Thunderbird
Triumph Thunderbird, Pre-Unit

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Triumph Thunderbird-Unit Construction
1/27/16