Triumph Bonneville 650 T120R


Make Model

Triumph Bonneville 650 T120




Four stroke, parallel twin, OHV


649 cc / 39.6 cu in
Bore x Stroke 71 x 82 mm
Compression Ratio 9.0:1
Cooling System Air cooled


2 x 30 mm Amal concentric carburetors


Battery, coil 


Lighting system 12V alternator
Oil system. Double plunger pump, dry sump
Oil capacity 3.4 L / 7.2 US pt / 6 Imp pt
Clutch Multi-disc, wet

Max Power

37.3 kW / 50 hp @ 7000 rpm
Max Toque 52.1 Nm / 5.3 kgf-m / 38.5 lb-ft @ 6000 rpm


4 Speed

Final Drive

Gear Ratios 4th 4.95 / 3rd 6.14 / 2nd 8.36 / 1st 12.08

Front Suspension

Telescopic forks

Rear Suspension

Swinging arm, dual shocks

Front Brakes


Rear Brakes


Front Tyre

3.25 -19

Rear Tyre

4.00 -18
Fuel Capacity 16L / 4.2 US gal / 3.5 Imp gal

Wet Weight

181 kg / 399 lbs

The 1971 Triumph Bonneville was the problem child of a shotgun marriage. It was responsible for Triumph missing that year’s U.S. sales season, and it just about bankrupted the company.

The late Sixties Triumph Bonnevilles were, and still are, considered to be the best of the lot. But a major program of standardization was underway across the BSA Group, which also owned Triumph. For the 1971 season, BSA planned to use a new oil-bearing frame for both BSA and Triumph 650 twins.

However, the new Triumph frame had been designed around the BSA 650 engine, and when the first batches of frames were delivered to Triumph’s Meriden factory, assembly line workers found they couldn’t fit the Triumph engine in the frame without removing the rocker boxes from the cylinder head first.

The 1971 Bonnies were already behind schedule because of production delays caused by a shortage of parts - the result of teething troubles with a new computer system. The cumulative result was that very few Bonnevilles were at U.S. dealers for the critical April to June sales season. Other problems with the 1971 bike included a seat height only suitable for people over 6 feet tall, major frame failures caused by the center stand being mounted on the oil-bearing “sump,” and aesthetics only a short-sighted mother could love. The 1971 model is perhaps the least popular of all Bonnies.


AH, THE BONNEVILLE. It has long been a status symbol with the road rider. Not the same way as with other big-bore machines. A BMW, for example, connotes Engineering, with a capital E. An FLH connotes yet another feeling, that of bigness, luxuriousness, and is the most unabashed symbol of affluent arrival.

The Bonneville symbol is a more dynamic thing. If you are a sporting sort of rider, and deem yourself a bit of a jockey, you may buy yourself a stable of intermediate machines with good performance and handling, but you know you're waiting for the day you can buy your Bonnie.

Its image does not jibe with the purist. The Bonneville is more for the guy who would buy a Boss Mustang or a Z28. Performance-plus, flashiness in a moderate way, and a respectable handling. Like the Sportster, the Bonneville is a stud bike, although the two images aren't quite the same. But, in either case, purchase of one or the other indicates "arrival," the coming of the real motorcyclist, and real motorcycling.

It seems highly unlikely that the designers of the Triumph Big Twin had this image concept in mind when they created the basis for what was to become the Bonneville. That type of marketing acumen didn't exist in the late Thirties. One can only conclude that Triumph really lucked out, because the half-stud, half-racing "Bonneville image" is the very thing that has kept that firm alive and well to this date.

Triumph's 1938 Speed Twin 500 heralded what was to become the most popular design, and the most plentiful motorcycle of that design, to ever come out of England. The alternately firing, vertical twin-cylinder engine produced 28.5 bhp, and, with a machine weight of 365 lb., provided the sporting enthusiast with an ideal mount. In fact, the 1939 sports version, the Tiger 100, became a popular machine with motorcycle connoisseurs. It featured an individually built, dynamometer-tested engine producing 34 bhp, and had many other "racing-type" features. 1950 was the year of the first Triumph 650 Twin, the Thunderbird. It had a bore/stroke of 71 by 82mm, measurements which still hold today.

These husky machines rapidly gained popularity in America, and became favorites with sports minded riders. TT racing proved to be its forte and a look at the record books will probably reveal that more races of this type have been won by 650 Triumphs than any other single machine.

The first Thunderbirds had a sprung hub rear suspension system which consisted of a huge rear hub with springs inside that provided a couple of inches of travel. It rode better than a rigid frame, but the hubs were heavy, had no dampening characteristics, and were prone to bearing failures.

This form of rear suspension was employed on some of Triumph's models imtil 1954. when a swinging arm frame appeared. Along with the frame came the new T-110 model, a super-sports version of the Thunderbird. It featured an alloy cylinder head with large valves, high compression pistons and a sports camshaft.

At that time, they were about the fastest road bikes around. Several other 650 models appeared in the ensuing years, including the popular Trophy and TT versions. In fact, until the "Japanese Invasion," big Triumphs were the most popular machines around in most parts of the country.

For 1971, Triumph has revamped practically everything on the 650s except the engine/gearbox package, and even a new five-speed gearbox is optional. Still functionally British in appearance, Triumphs have sprouted many modern-day innovations and styling trends. Gone are the familiar front forks and rear wheel hub. Turn signals are now standard equipment, and gracefully tapered silencers with a gradual upsweep adorn the Bonneville. Large, conical wheel hubs housing improved brakes, and a wide dual seat improves rider comfort.

Perhaps the most interesting aspect of the new 650 is the frame. Twin tubes extend from the steering head below the engine, and then up to the top rear suspension mounts. A large diameter central tube holds 3.5 qt. of oil, which does away with the old, separate oil tank. The "spine's" larger size makes for a stronger frame. The engine is firmly bolted directly to the frame at the front and underneath, and steel plates support it at the rear.

A flat support anchors it on top. Gusseting at the front of the swinging arm, which pivots on bronze bushes, appears very beefy, but the tubes themselves look rather delicate. Welds are very smooth, and the paint is excellent. Steering geometry felt just right for normal road riding, with a bit of fast swervery thrown in. Gone is the tendency of earlier Bonnevilles towards understeering; in fact, the new Bonnie feels rather like its little brother, the Triumph T-100 500-cc Twin, which is several pounds lighter.

The installation of a 3.25-19 Dunlop K70 tire seems to be the trend these days, and it probably helps smooth out the bumps a little, too. Traction is very good, with no tendency towards side-slipping at low speeds.

Part of the Bonnie's hairline steering and good ride are attributable to the new front fork assembly, which rides in tapered roller bearings. The fork legs feature cast alloy sliders with rubber wipers and internal springs. Full two-way hydraulic dampening and almost 7 in. of fork travel help make them one of the best operating fork assemblies available. Even with two aboard, no bottoming occurs, and there is no annoying clank when they topped out going across railroad tracks or badly surfaced roads.

The rear suspension units are, of course, Girlings, and aside from seeming to have a slightly stiffer spring rate than earlier Bonnevilles, they added immeasurably to the handling and ride. The chrome plated springs are a nice touch, but the damper rods are left exposed, which could cause premature wear of the seals if the machine is ridden in wet or dusty surroundings. The wheel hubs are very racy in appearance and house an excellent set of brakes. Diameter of the front unit is 8 in. It has twin leading shoes. The pivot arms are pulled toward each other like a BMW and therefore no equalizing rod is used.

A functional airscoop funnels cool air into the drum and carries off excess heat and brake dust. Even after several hard, high speed applications, the front wheel continued to do its share of stopping the machine. The rear wheel is similar in appearance, but the brake drum is an inch smaller in diameter. Although less positive in feel than the front brake, the rear was smooth in operation.

In addition, the rear sprocket may be unbolted for easy gear ratio changes or replacement. One point we don't care for is the rear wheel speedometer drive. It necessitates a long cable, and is less accurate than a speedometer driven from the front wheel.

Basically unchanged from last year, the Triumph 650 engine remains one of the Western world's mechanical marvels. It starts easily, vibrates only mildly, and has good power available throughout a wide rpm range. Both ball and roller bearings support the ends of the crankshaft; plain insert rod bearings and wrist pin bearings provide quiet running and high load carrying capability, and 9:1 compression ratio three-ring pistons take advantage of today's high octane fuels.

A separate camshaft for the intake and exhaust valves makes it possible to have pushrods of equal length, and makes it easy to degree-in the camshafts for optimum performance. Both shafts are gear driven from the crankshaft and ride in sintered bronze bushings. The double-plunger-type oil pump is driven from the inlet shaft, as is the crankcase breather, and the exhaust shaft drives the contact breaker assembly.

A welcome change to the 1971 650s is the addition of four inspection. caps on the sides of the rocker boxes. When these caps are removed, a feeler gauge can be inserted to check the valve clearances. This makes it easier to obtain really accurate valve clearance settings. Twin 30-mm Amal concentric carburetors breathe through "tuned" intake tubes which terminate into gauze air filter elements. These are housed in metal boxes under the seat and are very clean looking and stylish.

Although the carburetors are fitted with choke slides, we didn't find it necessary to close the lever even on cold mornings. Just make sure the float bowls are liberally primed, the ignition switch is on, and most often the bike will come to life with, at most, two kicks. A twin-row primary chain is retained to drive the clutch. Adjustment is accomplished via a rubber-faced tension slipper blade under the lower run of the chain. Rubber inserts inside the clutch hub cushion the engine's power impulses and increase the life of the chains and gearbox components. As always, the Triumph's three-spring clutch was easy to operate and didn't slip or drag.

The Triumph gearbox shifts smoothly and is silent in operation. Shifting is accomplished by a spindle and plunger assembly which pivots a quadrant that in turn moves the cam plate. A spring-loaded index plunger fits into notches in the cam plate for each gear and for neutral. Robust is the keyword; essentially the same gearbox is used in the more powerful 750-cc three-cylinder Trident.

Announced as an optional extra is a five-speed gearbox with a slightly lower first gear, and the rest of the gear ratios closer together. If it shifts as well as the four-speed gearbox, it'll be a winner.

Due to the incorporation of the engine oil into the frame, with the resulting large main tube, the gas tank is now slightly bulbous in the front section in order to keep the area between the rider's knees down to a reasonable width and retain the 3.5 gal. capacity. The seat, although very comfortable, is quite high at 34.5 in. Two of our shorter staffers found it impossible to plant both feet squarely on the ground. Another point that vexed us slightly was the unsightly oil pressure indicator sender unit which protrudes from the right side of the crankcase and is covered with an ugly rubber cap.

The indicator light is located in a spot (on the back of the headlight shell) which makes it difficult to see when illuminated, except at night. Oil system failures on the large Triumphs are so rare these days, it seems that the indicator could have been left off.

Rubber is used to mount the handlebars to reduce vibration, and the new-style Smith tachometer and speedometer nestle in their own isolation mounts. Both are quite accurate, but it's beyond us why so many manufacturers (and the Japanese, too) insist on fitting 150-mph speedometers on machines that won't pass the 120-mph mark. Besides being superfluous, the numbers must be made smaller (or fewer must be used) to keep them from being too small to read comfortably.

Another sore point, which we recently found on another British machine, was the new, improved Lucas electrical control units/lever holders. These cast aluminum units are very dull in appearance and have sharp plastic blades which must be flicked up or down to operate the turn signals or dip the headlamp. These blades are sharp, of a peculiar shape, and look as though they'd be easy to break off. Two buttons, one above and one below the blade switch, operate the horn and high beam flasher on the left; one button on the right is an engine cutout and the other button is left free for some other function.

The remainder of the electrical system deserves praise. The headlight throws a wide, powerful beam, the horn is suitably loud in volume, and the wiring is very well done. Triumph has taken great pains to rubber-mount the coils, rectifier and battery, and the area under the seat is used to its best advantage to locate these items for easy access. A new four-position ignition and light switch allows the rider to leave his machine parked with the ignition locked off and the parking lights locked on at the same time.

Other nice touches are rubber-mounted front fender braces, the overall excellent quality of finish (both paint and chrome), and precise handling qualities. This machine is one of Triumph's best.

Source Cycle World magazine, copyright 1972.