Marlborough Muscle Cars & Bikes Museum - 1970 Triumph Bonneville

The 1970 Triumph Bonneville T120R is arguably the best Triumph Bonneville ever built, and one of the most sought-after today, by collectors and enthusiasts alike. Not only are they drop-dead gorgeous machines with loads of history, they're fast and they're excellent handlers, even by today's standards. 1970 was the last year before the first of the dreaded oil-in-frame bikes arrived, and the problems with cash-strapped owner BSA began to surface.

'70 Bonne model year changes started with crankcase ventilation, always a problem with vertical twins where both pistons rise and fall together. The timed breather was eliminated allowing the engine to breathe through the primary case via three holes, into a vent tube behind the clutch, then out a through a rubber hose that ran out the back and along the left edge of the guard. The new system yielded a second benefit by keeping the primary always well oiled, while allowing excess oil to run back into the engine through the holes.

Minor changes were made to cams, timing gears, tacho drive and alternator cable. Plastic plugs were added to the Amal concentric carburettors to allow easy draining of the float bowls. The transmission was extensively modified for 1970. Changes to the inner cover, selector rod, mainshaft bearing circlip, aluminium-bronze shift forks with rollers (replacing steel forks), new cam plate and leaf spring were all aimed at delivering quicker and smoother shifting.

With a new frame schedules for 1971, changes to the 1970 frame were minimal. A new front engine mount with removable triangular mounting plates that bolted to the front frame rails and nose of the engine made it easier to install and remove the engine.

The '70 Bonne was powered by a twin carb OHV vertical twin 649cc engine that developed 50bhp. Power was delivered to the road by means of a 4-speed manual transmission, cable operated multi-plate wet clutch and chain final drive.