1968 TRIUMPH BONNEVILLE


1968 Triumph T120 Bonneville 650, Triumph motorcycles, Triumph TR6



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 Show) front brake. But 1968 also marked the beginning of a very brief window when the Triumph Bonneville was in it's 'sweet spot', widely considered to be from 1969 through 1970. 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 650, Triumph motorcycles, Triumph TR6

The 1968 Triumph Bonneville got a new 8-inch Twin Leading Shoe (TLS) front brake.

1968 Triumph T120 Bonneville 650 gauges, Triumph motorcycles, Triumph TR6

1968 Triumph Bonneville T120R engine with new twin Amal Concentric carburetors.

1968 Triumph Bonneville gauges, Triumph Bonneville, Triumph motorcycles, Triumph TR6

1968 Triumph Bonneville gauges (clocks): Smiths tachometer (rev counter) & 150mph speedometer, plus ammeter on new 7-inch headlight bucket.


1968 Triumph Bonneville SPECIFICATIONS:



MODEL DESIGNATIONS:
1968 Triumph Bonneville T120R..................... Roadster

ENGINE:
Engine type....................................... OHV vertical twin
Horsepower at RPM............................ 49 BHP @ 6,200 rpm
Bore................................................ 71mm / 2.79"
Stroke............................................. 82mm / 3.23"
Displacement.................................... 649cc / 40 cu. in.
Compression Ratio............................. 9.0:1
Valve Clearance (cold):
Inlet................................................ 0.05mm / 0.002"
Exhaust............................................ 0.10mm / 0.004"
Valve Timing:
Inlet Valve Opens.............................. 34 degrees BTDC
Inlet Valve Closes.............................. 55 degrees BTDC
Exhaust Valve Opens.......................... 55 degrees BTDC
Exhaust Valve Closes.......................... 27 degrees BTDC

IGNITION:
Ignition Breaker Gap............................. 0.4mm / 0.015"
Spark Plug Gap.................................. 0.50mm / 0.020"
Timing (static).................................14 degrees BTDC
Timing (fully advanced):........................ 38 degrees BTDC

CARBURETORS:
Type................................................ Amal Concentric
Throat Size....................................... 30mm
Main Jet........................................... 210
Needle Jet........................................ .107
Needle Position.................................. 2
Needle Type.......................................D
Throttle Valve.................................... 2
Pilot Jet............................................ 25

TRANSMISSION:
Speeds.............................................. 4
Gear Ratios:
4th - Top........................................... 5.84
3rd - Third......................................... 6.76
2nd - Second...................................... 8.17
1st - Bottom...................................... 11.8

CLUTCH:
Type................................................ Multi-plate, wet
Number of Plates:
Drive Plates...................................... 6
Driven Plates.................................... 6

SPROCKETS:
Engine............................................. 29 teeth
Clutch............................................. 58 teeth
Gearbox.......................................... 19 teeth
Rear Wheel...................................... 47 teeth

CHAIN:
Primary, pitch.................................. 3/8" triplex
Primary, length................................. 84 links
Final Drive, pitch............................... 5/8" X .400" X 3/8"
Final Drive, length............................. 106 links

CAPACITIES:
Fuel (US versions)............................. 2.5 Imp. gal.
Fuel (UK & export versions)................. 4 Imp. gal.
Oil Tank........................................... 5 pints / 3 L
Gearbox........................................... 1 pt / 500cc
Primary Chaincase............................. 1/4 pt / 150cc
Front Forks....................................... 6.4 oz. / 190cc

TIRES:
Front............................................... 3.25 X 19"
Rear................................................ 4.00 X 18"

SUSPENSION:
Front............................................... Telescopic Forks
Rear................................................ Swing Arm

BRAKES:
Front............................................... 8" / 20.32cm TLS
Rear................................................ 7" / 17.78cm SLS

DIMENSIONS:
Seat Height...................................... 32.5" / 77.5cm
Wheelbase........................................ 54.5" / 140.3cm
Length.............................................. 85.5" / 202cm
Width............................................... 28.5" / 72cm
Ground Clearance.............................. 5" / 12.7cm
Weight, unladen................................ 363 lbs / 165kg


TRIUMPH MOTORCYCLE BOOKS


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

Triumph Motorcycles Triumph Bonneville, Pre-Unit Construction, 1959-1962
Triumph Bonneville, Unit Construction, 1963-1970
Triumph Bonneville, Oil-in-Frame, 1971-1983

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