TRIUMPH THUNDERBIRD
Pre-Unit Construction
1950 through 1962

TRIUMPH THUNDERBIRD PRE-UNIT:
PUNCH A 500 OUT TO 650

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 Triumph Thunderbird was all new! This being the 1950's, that meant that it was the Triumph Thunderbird Pre-Unit Construction. 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.


WHAT IS PRE-UNIT?
we've been referring to the Triumph Thunderbird Pre-Unit. What is a Pre-Unit, anyway? 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.

SECOND STRING
The 'Golden Years' for the Triumph Thunderbird were definitely the 1950s, throughout which they were pre-unit (hence the Triumh Thunderbird 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 it as the "cooking version", meaning a workingman's commuter, meant as basic transportation, without sporting pretentions. But, during the 50's, until the arrival of the Triumph TR6 in 1956, the Triumph Thunderbird Pre-Unit 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. The Triumph Thunderbird Pre-Unit went 'unit' in 1963, along with all other 650 twins.


Triumph Thunderbird, Pre-Unit
YEAR-BY-YEAR


1950 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

First year for the 6T Triumph Thunderbird pre-unit line & the 650 engine. The quest for more power led Edward Turner to bore & stroke the 500cc Speed Twin into a fire-breathing 650. Strengthened with new crank, hotter cams, stronger clutch & gearbox, the Triumph 650 legend is born in the 1950 T-Bird.

1951 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

Few changes in the second year of the Triumph Thunderbird pre-unit. Most obvious was a handsome new paint color: "Thunderbird Metallic" (pictured left). Carburetor size increased from 1" to 1-1/16". 7-inch SLS front brake was improved.


1952 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

SU Type MC2 carburetor replaces the Amal & will stay unit 1959. Headlight nacelle is enlarged to accommodate new 7" headlight. This nacelle is a trademark of every Triumph Thunderbird pre-unit machine.


1953 TRIUMPH THUNDERBIRD
Engine: 649cc OHV vertical twin

The old Dynamo are replaced with an alternator for lights (across entire 1953 Triumph line). Last year for the rigid frame & notorious "sprung hub". A proper swing-arm rear suspension would come in 1954.


1954 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

New frame w/swing-arm rear suspension replaces rigid frame & dreaded "sprung hub". Ride improves, but the unsupported swing arm pivot flexes, causing erratic weaving & other bad handling traits.(NOTE: Photo is a 1956 Triumph Thunderbird pre-unit; PHOTO NEEDED!)


1955 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

Cooling problems prompted a new head casting, still in iron. New distributor. New alternator attached stator to outer primary cover, which now bore the Thunderbird's trademark "Paper Dart" logo.



1956 TRIUMPH 6T THUNDERBIRD
Engine: 649cc vertical twin

Triumph finally introduces an alloy head for it's 650 twins. The new "Delta Head" solves the cooling problems & establishes the trademark Triumph 'splayed exhaust'-look. TR6 is born & soon becomes the 'sport' model, relegating the T-Bird to more of a practical commuter role.


1957 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

High fashion hits the entire Triumph line. A new tank, new chrome grill tank badge, 2-tone paint & more flamboyant colors continue Triumph's tradition of building some of the world's most beautiful motorcycles.


1958 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

Evolutionary development continued & more power was always the goal. The 3-piece crank literally reached its breaking point & was replaced by a new forged one-piece crank.
(NOTE: Photo is a 1956 Thunderbird-PHOTO NEEDED)


1959 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

All the attention was on the launch of the new Triumph Bonneville. It was styled like the T-Bird, with full fenders & nacelle. US buyers wanted something more like the lean Triumph TR6. Last year for the 'whip iron' frame.
(NOTE: Photo is a Triumph Tiger T110, sister-bike to T-Bird)

1960 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

The new 'duplex frame' replaces the dreaded 'whip iron', so named for its tendency to weave & sway. The new frame has problems of it's own: the steering head is weak & must be reinforced. T-Bird & T110 get "bathtub" bodies. Yuk!



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


TIGER CUB BIBLE


TRIUMPH SINGLES: EARLY DAYS TO 1974 BY ROY BACON


BRITISH MOTORCYCLES: TRIUMPH


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


TRIUMPH MOTORCYCLES TWINS & TRIPLES


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


TRIUMPH TRIPLES


HOW TO RESTORE TRIUMPH TR5/250 & TR6


HOW TO RESTORE TRIUMPH TRIDENT T150/T160


TRIUMPH THRUXTON BONNEVILLE 1959-1969


TRIUMPH TIGER 800 SERVICE MANUAL 2010-2014, HAYNES


TRIUMPH TT600 MOTORCYCLE SERVICE MANUAL


TRIUMPH MOTORCYCLES


ORIGINAL TRIUMPH BONNEVILLE


TRIUMPH, BY ROY BACON


TRIUMPH ILLUSTRATED WORKSHOP MANUAL 1937-1951 BY ROY BACON


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


TRIUMPH T110R DAYTONA 1966-1969


TRIUMPH OWNERS MANUAL: UNIT CONSTRUCTION 350 & 500 TWINS

TRIUMPH INSTRUCTION MANUAL: TRW 499cc SV TWIN


TRIUMPH SPEED TWIN INSTRUCTION BOOK FOR 1939


TRIUMPH BONNEVILLE YEAR-BY-YEAR


CHILTON’S TRIUMPH MOTORCYCLE REPAIR & TUN-UP GUIDE


2010-14 TRIUMPH TIGER 800 HAYNES SERVICE MANUAL


TRIUMPH 650 & 750 TWINS: BONNEVILLE, TIGER, TROPHY & THUNDERBIRD


TRIUMPH MOTORCYCLE RESTORATION: PRE-UNIT, BY TIM REMUS


TRIUMPH MOTORCYCLE RESTORATION, BY TIM REMUS


TRIUMPH BONNEVILLE ROAD TEST PORTFOLIO, 2001-2008


TRIUMPH TRIDENT & BSA ROCKET 3 OWNERS WORKSHOP MANUAL, BY HAYNES


HINKLEY TRIUMPHS: THE FIRST GENERATION


TRIUMPH FUEL INJECTED TRIPLES 1997-2000


TRIUMPH SINGLES: EARLY DAYS TO 1974, BY ROY BACON


TRIUMPH PRE-UNIT TWINS WORKSHOP MANUAL, BY HAYNES


TRIUMPH DAYTONA, SPEED TRIPLE, SPRINT & TIGER 1995-2005 HAYNES SHOP MANUAL


TRIUMPH TIGER 100 & DAYTONA


McQUEEN’S MACHINES: THE CARS & BIKES OF A HOLLYWOOD ICON


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


TRIUMPH 750/900 TRIPLES & 1200 FOUR 1991-1999 HAYNES REPAIR MANUAL


TRIUMPH, THE LEGENDS SERIES


TRIUMPH MOTORCYCLES & CUSTOM BIKES


TRIUMPH MOTORCYCLES TWINS & TRIPLES, BY REMUS


TRIUMPH MOTORCYCLES: THEIR RENAISSANCE & THE HINKLEY FACTORY


TRIUMPH MOTORCYCLES FROM 1950-1988


TRIUMPH MOTORCYCLES: THE GOLDEN YEARS


THE SCRAPBOOK SERIES I - TRIUMPH


TRIUMPH DAYTONA 1991-2006 ROAD TEST PORTFOLIO


CYCLE WORLD ON TRIUMPH 1967-1972


TRIUMPH SPEED TWIN & THUNDERBIRD BIBLE


TRIUMPH BONNEVILLE: T120/T140, BY HAYNES


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


TRIUMPH: THE RACING STORY


TRIUMPH PRE-UNIT TWINS


LITTLE BOOK OF BRITISH MOTORCYCLES: TRIUMPH


CHILTON’S NEW REPAIR & TUNE-UP GUIDE FOR TRIUMPH MOTORCYCLES


THE TRIUMPH STORY: RACING & PRODUCTION MODELS 1902 TO PRESENT


TRIUMPH 1050 SPRINT, SPEED TRIPLE & TIGER, HAYNES REPAIR MANUAL


ILLUSTRATED TRIUMPH MOTORCYCLE BUYERS GUIDE POST-1945, ROY BACON


COMPLETE BOOK OF CLASSIC & MODERN TRIUMPHS


TRIUMPH BONNEVILLE


TIGER 100/DAYTONA: DEVEOPMENT HISTORY OF THE 500 TWIN


TOURING TRIUMPH TWINS, 3T, 5T, 6T, 3TA & 5TA, 1938-1966 BY ROY BACON


FACTORY TRIUMPH MOTORCYCLE SERVICE MANUAL


TRIUMPH CYCLES PATENT COLLECTION


TRIUMPH MOTORCYCLE OWNERS HANDBOOK, ALL MODELS 1937-1951


TRIUMPH MOTORCYCLES: A PRACTICAL GUIDE TO ALL MODELS FROM 1937




CHILTON’S REPAIR & TUNE-UP GUIDE, TRIUMPH THROUGH 1972


OVERHAUL MANUAL FOR TRIUMPH 3-CYLINDER ENGINE


TRIUMPH BONNEVILLE T120, 1959-1974, BY ROY BACON


TRIUMPH TIGER T100 & T110, 1939-1961, BY ROY BACON


TRIUMPH T90 & T100 UNIT TWINS, 1960-1974, BY ROY BACON


TRIUMPH REPLACEMENT PARTS CATALOGUE, 1972 TRIDENT T150


TRIUMPH TRIDENT SUPER PROFILE


TRIUMPH BONNEVILLE GOLD PORTFOLIO, 1959-1983


1969 TRIUMPH TROPHY 250 REPLACEMENT PARTS CATALOGUE


TRIUMPH 650 & 750 TWIN OWNERS WORKSHOP MANUAL


TRIUMPH: RED HOT BIKES


1966 TRIUMPH REPLACEMENT PARTS CATALOG #6: 3TA, 5TA, T90 & T100S/S


1961 TRIUMPH REPLACEMENT PARTS CATALOG: TWNETY-ONE, SPEED TWIN, TIGER 100


BONNEVILLE: THE WORLD’S FASTEST MOTORCYCLES


TRIUMPH WORKSHOP MANUAL FOR 750 BONNEVILLE & TIGER


TRIUMPH BONNEVILLE & TR6 BY TIM REMUS


TRIUMPH 650 & 750 UNIT TWINS 1963-1983, HAYNES REPAIR MANUAL


TRIUMPH OWNERS HANDBOOK FOR 1973 BONNEVILLE & TIGER


TRIUMPH PRE-UNIT TWINS WORKSHOP MANUAL


TRIUMPH 350/500 TWINS WORKSHOP MANUAL






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