1968 Triumph Bonneville

OVERVIEW
The 1968 Triumph Bonneville had a bit of a watershed year. While the slow march of progress continued, 2 major improvement were made: Amal Concentric carburetors replaced the Amal Monoblocs; and a new 8" TLS (Twin Leading Shoe) front brake replaced the age-old SLS (single leading shoe) unit. But 1968 also marked the beginning of a very brief window when the Triumph Bonneville was in it's 'sweet spot', widely considered to run from 1968 through 1970. The 'Bonnie' had truly reached its zenith, and at a time when its performance was considered world-class. This was, of course, just before the arrival of the game-changing Honda 750, the bike that reset the world's opinion of what "performance" was. And in this new 4-cylinder OHC paradigm, the ancient pushrod twin just couldn't keep up. Model designations were as before, T120R was the road version & T120C was the off-road/street scrambler (ala enduro). Absent, however, was the race-ready TT Special, the model was dropped entirely for the 1968 model year.
ENGINE UPGRADES
Big changes were made to the engine. Hepolite pistons were back, but with reinforced crowns. Grey Spot outer valve springs replaced last years Red Spots, allowing higher valve lift. Starting with Engine #DU79965, the rocker arms were strengthened by eliminating the oil passages that ran from their pivots to the contact faces. The cylinder block got 12-point base nuts. Starting with Engine #DU66246, the threaded timing hole (used to locate TDC) that was behind the cylinders was moved to a location below the front engine mount. The flywheel now had 2 notches, one at TDC and one at 38-degrees BTDC (the standard advance at the time). But to make it more convenient to time the engine, an entirely new system was employed as of Engine #DU83021, which included a new primary cover with removable inspection plate over the alternator stator and corresponding timing marks on the stator & the primary cover. Ultimately, the threaded inspection hole in front was turned into a drain plug & the timing plug moved back to its original location, behind the cylinder block. The contact breakers were now independently adjustable (Lucas 6CA) allowing more precise timing. More Unified threads inside and out. Several detail changes were made to the transmission. The Zener diode was moved from behind the engine to the front of the bike under the front yoke, where it would remain from then on.
FRAME & RUNNING GEAR
An overall strengthening of the frame was undertaken, starting somewhere between Engine #s DU75430 & DU75449 (a 19-bike disparity), thicker-gauge tubing was used for the front frame section, and as of Engine #DU81196 for a stronger swing arm. The swing arm pivot lug was also strengthened. A new set of forks were fitted that looked practically identical to the units they replaced, but had internal differences such as 2-way shuttle valve damping & changed over to American threads.
NEW TLS FRONT BRAKE
The big news for 1968 however was the new 8" twin-leading-shoe (TLS) front brake to replace the old SLS unit. A modified version of the old hub was used with an entirely new guts & an alloy backing plate that had the coolest-looking air scoop in the front. It was 'all business' in more than appearance - it really stopped!

1968 Triumph T120 Bonneville
SPECIFICATIONS

Bonneville T120R

Bonneville T120C

Engine type

Displacement

Bore & Stroke

Compression

Carburetors

Ignition

Engine output

Primary drive

Primary drive sprockets

Clutch

Gearbox

Ratios, overall:

1st, bottom

2nd

3rd

4th, top

Final drive

Final drive sprockets

Frame type

Suspension, front

Suspension, rear

Brake, front

Brake, rear

Tire, front

Tire, rear

Fuel Capacity

Wheelbase

Seat height

Ground clearance

Weight, unladen

Roadster, low pipes

Street Scrambler, high pipes

Air-cooled OHV vertical twin

649cc / 40.0 ci

71mm X 82mm / 2.79" X 3.23"

9.0:1

2- Amal Concentric 30mm

Battery & coil, Lucas

49 bhp @ 6200

3/8" triplex chain X 84 links

29T X 58T

Multi-plate, wet

4-speed constant-mesh, right-foot shift


11.8:1

8.17

6.76

5.84

5/8" X .400" X 3/8" chain X 106 links

19T X 47T

Brazed lug, full-cradle, single downtube

Telescopic fork, hydraulic damping

Swing arm, 2 Girling dampers

8" TLS drum, full width

7" SLS drum

3.25" X 19" Dunlop 

4.00" X 18" Dunlop

2.5 Imp gal (US) / 4 Imp gal (UK & export)

54.5" / 140.3cm

32.5" / 77.5cm

5.0" / 12.7cm

363 lbs / 165 kg


MORE TRIUMPH BONNEVILLES:
Triumph Bonneville
Triumph Bonneville Pre-Unit (1959-1962)
.....1959
.....1960
.....1961
.....1962
Triumph Bonneville, Unit-Construction(1963-1970)
.....1963
.....1964
.....1965
.....1966
.....1967
.....1968
.....1969
.....1970
Triumph Bonneville, Oil-in-Frame
.....1971
.....1972
.....1973
.....1974
.....1975
.....1976
.....1977
.....1978
.....1979
.....1980
.....1981
.....1982
.....1983
Les Harris Bonnevilles(1983-1989)
OTHER TRIUMPHS:
Triumph TR6
Triumph TR7
Triumph Trophy
Triumph Tiger
Triumph Thunderbird
Triumph Trident
Triumph X-75 Hurricane


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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


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


TRIUMPH ILLUSTRATED WORKSHOP MANUAL 1945-1955, by Clymer


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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


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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


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