Triumph Thunderbird
Unit-Construction
1963-1966


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.

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 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.

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.

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.


Triumph Thunderbird Unit-Construction
YEAR-BY-YEAR


1963 6T 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.


1965 6T 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.

Check out these TRIUMPH BOOKS


COMPLETE BOOK OF CLASSIC & MODERN TRIUMPHS


TRIUMPH MOTORCYCLES IN AMERICA


TRIUMPH MOTORCYCLE RESTORATION


TRIUMPH MOTORCYCLE RESTORATION, PRE-UNIT


TRIUMPH BONNEVILLE & TR6 RESTORATION GUIDE: 1956-1983


TALES OF TRIUMPH MOTORCYCLES & THE MERIDEN FACTORY


TRIUMPH 650 & 750 TWINS OWNERS WORKSHOP MANUAL, 1963-1983


TRIUMPH BONNEVILLE: ESSENTIAL BUYER’S GUIDE


TRIUMPH BONNEVILLE: PORTRAIT OF A LEGEND


ILLUSTRATED TRIUMPH BUYER’S GUIDE, by Roy Bacon


TRIUMPH BONNEVILLE, 2001-2015


TRIUMPH: PRODUCTION TESTERS TALES FROM MERIDEN


TRIUMPH MOTORCYCLE RESTORATION GUIDE


TRIUMPH OWNERS HANDBOOK, UNIT-CONSTRUCTION 350 & 500, Kindle


TRIUMPH ILLUSTRATED WORKSHOP MANUAL 1945-1955, by Clymer


TRIUMPH TWINS & TRIPLES


FACTORY-ORIGINAL TRIUMPH TWINS, 1938-1962


TRIUMPH BONNEVILLE & TR6, by Timothy Remus


TRIUMPH MOTORCYCLES: FROM SPEED TWIN TO BONNEVILLE


TRIUMPH MOTORCYCLES: THEIR RENAISSANCE & THE HINKLEY FACTORY


TRIUMPH PRE-UNIT TWINS OWNERS WORKSHOP MANUAL


TRIUMPH THRUXTON BONNEVILLE 1959-1969


TRIUMPH MOTORCYCLES ILLUSTRATED WORKSHOP MANUAL 1937-1951, by Clymer


TURNER’S TRIUMPHS: EDWARD TURNER & HIS TRIUMPHS


TRIUMPH TWIN RESTORATION, by Roy Bacon


TRIUMPH TIGER CUB & TERRIER 1952-1968, Haynes Repair Manual


TRIUMPH 675 DAYTONA & SPEED TRIPLE REPAIR MANUAL 2006-2010, Haynes


CLASSIC MOTORCYCLES: TRIUMPH


TRIUMPH BONNEVILLE YEAR BY YEAR


TRIUMPH TRIPLES


TRIUMPH RACING MOTORCYCLES IN AMERICA


TRIUMPH MOTORCYCLES


TRIUMPH, by Roy Bacon


TRIUMPH 750/900 TRIPLES & 1200 FOURS 1991-1999, Haynes Repair Manual


TRIUMPH TROPHY BIBLE


TRIUMPH DAYTONA, SPEED TRIPLE SPRINT & TIGER: 885/955 1997-2005, Haynes Repair Manual


BSA & TRIUMPH TRIPLES GOLD PORTFOLIO 1968-1976


TRIUMPH TRIDENT OWNERS WORKSHOP MANUAL, by Haynes


TRIUMPH: A CENTURY OF PASSION & POWER


TRIUMPH BONNEVILLE, by Osprey


TRIUMPH TIGER 100 & 110, 1939-1961, by Roy Bacon


TRIUMPH TIGER 100 & DAYTONA


TRIUMPH BONNEVILLE, by Crowood


TRIUMPH by Don Morley


TRIUMPH TRIPLES & FOURS 1991 & 2004


TRIUMPH 350 & 500 1957-1974 ESSENTIAL BUYERS GUIDE


BONNIE: THE DEVELOPMENT OF THE TRIUMPH BONNEVILLE


TRIUMPH SINGLES: EARLY DAYS TO 1974, by Roy Bacon


TRIUMPH: RETURN OF THE LEGEND


TRIUMPH FUEL INJECTED TRIPLES 1997-2000


BOOK OF TRIUMPH TWINS 1968-1969, by Floyd Clymer


THE TRIUMPH STORY: RACING & PRODUCTION MODELS 1902-PRESENT


TRIUMPH 350 & 500 TWINS 1958-1973, Haynes Repair Manual


TRIUMPH BONNEVILLE 2001-=2008 ROAD TEST PORTFOLIO


HINKLEY TRIPLES & FOURS


TRIUMPH PRE-UNIT TWINS


HINKLEY TRIUMPHS: THE FIRST GENERATION


TRIUMPH BONNEVILLE BIBLE: ALL MODELS 1959-1983


TRIUMPH TWENTY-ONE TO DAYTONA: C CLASS 350 & 500 TWINS


ORIGINAL TRIUMPH BONNEVILLE


TRIUMPH 650 & 750 TWINS


TRIUMPH MOTORCYCLES & CUSTOM BIKES


TRIUMPH MOTORCYCLES


TRIUMPH, THE LEGENDS SERIES


TRIUMPH THUNDERBIRD SUPER PROFILE


TRIUMPH SPEED TWIN & THUNDERBIRD BIBLE


TRIUMPH BONNEVILLE T120/T140


TRIUMPH MOTORCYCLES FROM 1950-1983


TRIUMPH & BSA TRIPLES: THE COMPLETE STORY


TRIUMPH THUNDERBIRD, TROPHY & TIGER ESSENTIAL BUYERS GUIDE


TRIUMPH TIGER CUB BIBLE


ROAD RACING HISTORY OF THE TRIUMPH 500 UNIT TWIN


EDWARD TURNER: THE MAN BEHIND THE MOTORCYCLES


SAVE THE BONNEVILLE: THE INSIDE STORY OF THE MERIDEN WORKERS CO-OP


TRIUMPH DAYTONA 1991-2006 ROAD TEST PORTFOLIO


TRIUMPH DAYTONA 2003-2009 ROAD TEST PORTFOLIO


TRIUMPH TIGER 100 / DAYTONA: DEVELOPMENT HISTORY


TRIUMPH, by Daniel Gilpin


BOOK OF TRIUMPH MOTORCYCLES ILLUSTRATED SHOP MANUAL 1935-1939


TRIUMPH: THE RACING STORY


TRIUMPH BONNEVILLE T120 1959-1974, by Roy Bacon


TRIUMPH BONNEVILLE GOLD PORTFOLIO 1959-1983


TRIUMPH TRIDENT SUPER PROFILE


ILLUSTRATED TRIUMPH BUYERS GUIDE, by Roy Bacon






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