TRIUMPH THUNDERBIRD
Pre-Unit Construction
1950 through 1962


Triumph Thunderbird, Triumph motorcycles, Triumph Bonneville



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.

Triumph Thunderbird, Triumph motorcycles, Triumph Bonneville, Triumph TR6

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.

Triumph motorcycle engine, Triumph motorcycles, Triumph Bonneville


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:


Triumph Thunderbird, Triumph motorcycles, Triumph TR6, Triumph Bonneville

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. Suitably strengthened with a new crank, hotter cams, stronger clutch & gearbox, the Triumph 650 legend is born in the T-Bird.


Triumph Thunderbird, Triumph motorcycles, Triumph Bonneville, Triumph TR6 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.


Triumph Thunderbird, Triumph motorcycles, triumph bonneville, triumph tr6 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.


Triumph Thunderbird, Triumph Motorcycles, Triumph Bonneville, Triumph TR6 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.


Triumph Thunderbird, triumph bonneville, triumph motorcycles, triumph tr6 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!)


Triumph Thunderbird, triumph motorcycles, triumph bonneville, triumph tr6 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.



Triumph Blackbird, Triumph Thunderbird, Triumph motorcycles 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.


Triumph thunderbird, triumph blackbird, triumph motorcycles 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.


Triumph Thunderbird, Triumph Blackbird, Triumph Motorcycles, Triumph Bonneville 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)


Triumph Thunderbird, Triumph Blackbird, Triumph Bonneville, Triumph motorcycles 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)

Triumph Tiger T110, Triumph Thunderbird, Triumph motorcycles, Triumph Bonneville 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!


Triumph Tiger T110, Triumph Thunderbird, Triumph motorcycles, Triumph Bonneville 1961 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

Duplex frame is strengthened + new alloy head. T-Bird & Tiger T110 had become sister-bikes, both now wearing full "bathtub bodies". This is the last year for the T110.
(NOTE: Photo is a T110, but similar to T-Bird)


Triumph Thunderbird, Triumph Tiger T110, Triumph motorcycles, Triumph Bonneville 1962 TRIUMPH 6T THUNDERBIRD
Engine: 649cc OHV vertical twin

Last year for Non-Unit Construction (Pre-Unit) & Duplex Frame. "Slickshift" clutchless shifter (unpopular) is discontinued. A new 2-into-1 exhaust system ran out the right. Full 'Bathtub' bodywork.


TALES OF TRIUMPH & MERIDEN

TRIUMPH MOTORCYCLES: TRIUMPH ENGINEERING CO.

TRIUMPH WORKSHOP MANUAL, 1935-1939

TRIUMPH WORKSHOP MANUAL, 1937-1951

TRIUMPH WORKSHOP MANUAL, 1945-1955

TRIUMPH MOTORCYCLES: FROM SPEED TWIN TO BONNEVILLE

TRIUMPH MOTORCYCLES - HINKLEY RENAISSANCE

SAVE THE TRIUMPH BONNEVILLE

THE INSIDE STORY OF THE MERIDEN WORKERS' CO-OP.
STORY OF THE TRIUMPH BONNEVILLE - MOVIE

TRIUMPH 650 & 750 TWINS

ESSENTIAL BUYERS GUIDE TO TRIUMPH BONNEVILLE

TR BONNEVILLE 2001-08

TR BONNEVILLE, PORTRAIT OF LEGEND

TR BONNEVILLE: T120/T140

TRIUMPH MC RESTORATION

HAYNES TR TRIPLES & FOURS

TRIUMPH 21 TO DAYTONA

TRIUMPH TIGER CUB BIBLE


For more Great Books, please visit our
MOTORCYCLE BOOK STORE




HOME PAGE
Triumph Motorcycles
BSA Motorcycles
Norton Motorcycles
Matchless-AJS Motorcycles
AMC-Associated Motor Cycles
Royal Enfield Motorcycles
Ariel Motorcycles
Vincent Motorcycles
Velocette Motorcycles
Rickman Motorcycles
Japanese Motorcycles
Cafe Racers
Choppers & Bobbers
Rat Bikes
Classic Racing Motorcycles
Classic Motorcycle Auctions
Classic Motorcycle Shows
Classic Motorcycle Rides
Classic Motorcycle Restoration
Classic Motorcycle Shops
Classic Motorcycle Clubs
Classic Motorcycle Photo Gallery
Classic Motorcycle Events Calendar
Send us Pictures of YOUR BIKE
YOUR BIKE Pictures
About Us
Site Map
CONTACT US
Brit Iron Back Issues
Visit our STORE
Subscribe to our FREE e-zine & get a FREE 11X17 Poster
BACK TO TOP - Triumph Thunderbird Pre-Unit

Triumph Thunderbird Pre-Unit
1/8/14




CLASSIC TRIUMPHS
2015 Wall Calendar

HISTORIC MOTORCYCLES
2015 Wall Calendar
ULTIMATE MOTORCYCLES
2015 Wall Calendar

Check out all our
MOTORCYCLE CALENDARS