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.
1968 TRIUMPH BONNEVILLE 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.
1968 TRIUMPH BONNEVILLE 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.
1968 TRIUMPH BONNEVILLE GETS 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!

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1968 Triumph Bonneville
SPECIFICATIONS

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MORE TRIUMPH BONNEVILLES:
Triumph Bonneville
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Triumph Bonneville, Oil-in-Frame
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Last updated 12/12/20

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