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.


1956 TRIUMPH BLACKBIRD
Engine: 649cc vertical twin

Marlon Brando rode a black 6T Thunderbird in the 1953 movie "The Wild One", so Triumph started painting some T-birds black & the public nicknamed the "Blackbird"


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!


1962 TRIUMPH 6T THUNDERBIRD
Last year of pre-unit construction. "Bathtub"-styling makes them harder to sell in the USA.


Check out these TRIUMPH BOOKS




2018 Classic Triumph Calendar


Triumph Motorcycles: The art of the motorcycle


The Complete Book of Classic and Modern Triumph Motorcycles 1937-Today (Complete Book Series)


Triumph Motorcycles: From Speed-Twin to Bonneville


Triumph Bonneville and TR6 Motorcycle Restoration Guide: 1956-83


British Motorcycles Triumph (Little Books)


Triumph Motorcycles in America


McQueen's Motorcycles: Racing and Riding with the King of Cool


Triumph Motorcycle Restoration


Illustrated Triumph Motorcycles Buyer's Guide: From 1945 Through the Latest Models (Illustrated Buyer's Guide)


Tales of Triumph Motorcycles and the Meriden Factory


Hinckley Triumphs: The First Generation (Crowood Motoclassic)

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Last updated 12/7/17

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