Triumph Bonneville
Unit-Construction
1963-1970


TRIUMPH BONNEVILLE UNIT-CONSTRUCTION
For the 1963 model year, the Triumph Bonneville and the entire line of 650 Triumph motorcycles was completely revamped top-to-bottom, the most comprehensive in Triumph's history. The brand-new Triumph Bonneville unit-construction featured an all-new engine, a new frame and all new running gear. They had swapped frames recently in 1960, but the engine and most of the running gear stayed the same. In 1971, they changed frames and running gear again, but the engine remained relatively unchanged, at least at first.

A WHOLE NEW ENGINE
The changes for 1963 began with a whole new engine, the Triumph Bonneville Unit-Construction 650. Based on the basic architecture of the Pre-Unit engine, they shared very few parts in common. Unit construction was nothing new to Triumph. They had just gone through it with their 500 Twin line in 1959.

PRE-UNIT HISTORY
Prior to this switch to the Triumph Bonneville Unit-Construction, the basic pre-unit twin had changed very little since it's inception in 1937 in 500cc form as the 5T Speed Twin, the seminal game-changer created by Edward Turner. It was bored-and-stroked out to a 650 in 1950 with the Triumph 6T Thunderbird. In 1956, a new, much-improved alloy head dubbed the "Delta Head" replaced the old cast iron one. Soon the Meridian factory had reworked some Deltas to accommodate 2 carburetors, and by 1959 an entirely new model-line was built around it, the legendary Triumph T120 Bonneville. Of course there were constant changes and improvements year-to-year, but the basic engine didn't change much from its pre-war days as a 500. And its performance had been pushed about as far as it was going to pushed comfortably, with vibration a continuing problem. Clearly it was time for a whole new design.

ONE NEAT PACKAGE
For the new Triumph Bonneville Unit-Construcion bike, the engine enveloped the crankcase, primary case and gearbox, previous each separate components, into one neat package or "unit", hence the name Unit Construction. It was lighter, smaller, stronger, easier to keep clean, and easier and cheaper to assemble for the Triumph Motorcycles factory at Meriden. As before, and according to British custom, the cases were split vertically, with the left half of the crankcase carrying the inner primary case, and the right crankcase half carrying the gearbox housing. It was neat, and also very attractive, perhaps one of the best-looking motorcycle engines of all times.

NEW FRAME & RUNNING GEAR TOO
But the changes weren't limited to the engine. An entirely new frame was introduced for 1963, replacing the much-reviled 'duplex frame' of 1960-62, known for weakness in the steering head. The new frame had a single front downtube this time, but of larger (1-5/8") and thicker (12 gauge) tubing, heavy forged steel frame frame lugs at the steering head and swing arm pivot (also tied to the rear engine mounting plates for added stiffness) and reinforced with a second frame rail running under the main backbone, beneath the tank. All in all, it became known as a very stiff, rugged frame that gave very few problems and offered exceptional handling.

WHAT CAME AFTER UNIT-CONSTRUCTION?
Technically, the Triumph twins that were built after 1970 are still of unit-construction. However, the entire Triumph & BSA twin lines were totally revamped for the 1971 model year with all new frames and running gear. Only the engines remained unchanged (perhaps the one item that needed it most). The new bike held their oil in the enlarged backbone of the frame, rather than in a separate oil tank. Hence the name "Oil-in-frame". They weren't well received when they came out in '71, many purists feeling betrayed by the new look. But, over the next decade, they were sorted-out and became a well-developed machine. Still not enough to save the company, however.

A STORM IS BREWING
Triumph Motorcycles and the Triumph Bonneville were riding on a wave of popularity and success during this era. Unfortunately, trouble was brewing under the surface, invisible to all but a few insiders within the BSA organization (who had owned Triumph since 1951), that would seal the fate of Triumph Motorcycles, and the British motorcycle industry as a whole, forever! Triumph was the last of the British motorcycle companies still standing, but they finally closed their doors in 1983.


Triumph Bonneville Unit Construction
YEAR-BY-YEAR


1963 T120 BONNEVILLE

650cc: T120C (Scrambler)
650cc: T120R (Road)

First year for Triumph Bonneville Unit-Construction, new single downtube frame, coil ignition & 19" front wheel.


1964 T120 BONNEVILLE
650cc: T120C (Scrambler)
650cc: T120C (TT Special)
650cc: T120R (Road)

2nd-year Unit Construction, mostly carry-over from '63. TT Special makes its debut as a stripped-down, specially-tuned race bike.


1965 T120 BONNEVILLE
650cc: T120C (Scrambler)
650cc: T120C (TT Special)
650cc: T120R (Road)

Revised front forks, otherwise mostly carryover from '64. Gorgeous new Pacific Blue & Silver paintjob.


1966 T120 BONNEVILLE
650cc: T120C (Scrambler)
650cc: T120R (Road)
650cc: T120TT(TT Special)

62-degree steering angle (down from 65), slimmer tank w/new tank badges & w/o tank rack. New full-width front brake hub.


1967 T120 BONNEVILLE
650cc: T120C (Scrambler)
650cc: T120R (Road)
650cc: T120TT(TT Special)

Resin-encased alternator, new Smiths 150 MPH speedometer, gray quilted seat top w/black sides. Last year for TT Special.


1968 T120 BONNEVILLE
650cc: T120C (Scrambler)
650cc: T120R (Road)

Amal Concentric carbs replace Monoblocs, new 8" TLS front brake, new points & 12-point cylinder base nuts. Many consider the '68-70 Triumph Bonneville Unit-Construction was the peak of development.


1969 T120 BONNEVILLE
650cc: T120C (Scrambler)
650cc: T120R (Road)

Improvements to 8" TLS front brake, heavier crank flywheel, nitrided cams, engine numbers stamped on top of Triumph logos, exhaust cross-over is new.


1970 T120 BONNEVILLE
650cc: T120C (Scrambler)
650cc: T120R (Road)

Adjustable rear shocks, chromed grab rail behind seat. Last year of the Triumph Bonneville Unit-Construction, before the dreaded conversion to Oil-in-Frame.


NEW VIDEO:
"TRIUMPH BONNEVILLE THRU THE YEARS"

Take a video walk through every year of classic Triumph Bonneville, from 1959 to 1983. All are taken from the same camera angle to provide some continuity.


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