1966 Triumph Bonneville

1966 Triumph Bonneville Background & History

By the release of the new 1966 Triumph Bonneville T120 650, Meriden had had a few years to sort out the teething problems of the new frame and unit-construction engine, introduced in 1963. Most of the changes up to this point had been incremental, but now the real work was starting: how to get more power, more reliability, and less vibration out of the venerable 650 vertical twin. Things were really starting to heat up now, with the trickle of small displacement bikes from Japan starting to turn into a flood of larger, more powerful machines. Honda’s 450cc DOHC “Black Bomber” was the first Japanese bike big enough, and fast enough to challenge the big twins from Britain, but more were coming. For Triumph motorcycles, and the Triumph Bonneville in particular to maintain its position as one of the fastest production bikes in the world, serious work would need to be done, and fast!

This 1966 Triumph Bonneville T120R is the Roadster version, distinguished by its low pipes.  The T120C had high side pipes, one on each side.

1966 Triumph Bonneville Model Designations

Model designations for the 1966 Triumph Bonneville line were as before: T120R was the Road version, T120C was the Off-Road/Street Scrambler (like an enduro), and the TT Special was the Competition version, basically a stripped down T120C with model-specific pipes (TT pipes) and special race tuning. The TT Special had never had it’s own separate designation before, it was a T120C with an option package called the “TT Special”. This changed mid-year in 1966, when they became known as T120TTs, as they would until the model disappeared at the end of 1967.

Inside the engine, things were heating up. Power output and vibration were at odds and vibration was winning!  And it was only getting worse as more and more horsepower was demanded of the Triumph 650. For the 1966 Triumph Bonneville an all new crankshaft shed 2-1/2 lbs of material from the edges of the flywheel, while retaining the existing 85% balance factor. A new 1-1/8″ shouldered roller main bearing (E2879) replaced the old ball race on the drive side (left). Oil holes were drilled from the timing chest to the exhaust cam lobes to fight wear. The formerly-optional high-performance cams (E4819 inlet & E4855 exhaust) were now standard. Pushrod tubes and O-rings were revised to curb oil leakage (the pushrod tubes have always been a source of oil leaks in Triumphs, perhaps the worst source). New double valve springs, called “Red Spot” were introduced & the compression ratio was brought to 9:1 from 8.5:1 on non TT’s.  The TT remained at an ultra-high 11.0:1 compression ratio, being the full-race version.

Starting with Engine #DU29738 all 1966 Triumph Bonnevilles got larger 1-3/16″ Amal Monobloc carburetors. The safety wires formerly on the float bowl cover disappeared. Then, starting with Engine #DU34086, the carbs got a #4 slide cutaway with a needle position of 2. Some US East Coast models got pancake air filters for the first time.

1966 Triumph Bonneville goes American!

The 1966 Triumph Bonneville also marked the beginning of the process of replacing all the old British Standard (Whitworth) nuts & bolts with Unified (American) threads. Over the next several model years there would be an odd mix of British & American threads. To own a Classic British Motorcycle used to mean you had to own a set of Whitworth tools. Now, you needed both.  This move was so profoundly British.  While the rest of the world (with the exception of the US, but they were considering it) was converting over to the metric system, the Brits adopted the American system that even the Americans were considering abandoning in favor of metric.

1966 Triumph Bonneville TT Special engine.  Note the routing of those cool pipes.

1966 Triumph Bonneville Frame & Running Gear

A longer kick starter lever was installed on the 1966 Triumph Bonneville to help cope with the increased compression. The speedometer cable drive was moved off the layshaft (in the gearbox) to the rear wheel. Minor changes were made to the clutch & primary chain tensioner. A new right-angle tachometer drive cleaned up the routing of the tach cable from the drive side of the exhaust cam to the gauges.

Perhaps the biggest news for the 1966 Triumph Bonneville was the adoption of 12 volt electrics. This included a new 12 volt alternator, Lucas MA12 coils, a Zener Diode (to control overcharging), a new rectifier, and two Lucas MKZ9E 6-volt batteries hooked up in series to make 12 volts.

The 1966 Triumph Bonneville frame also received attention. Starting with Engine #DU25277, steering angle was brought from 65% to 62% to improve high-speed handling. The swing arm was widened by 1/4″ to make room for larger tires. Accommodation was made for a new battery box that came with the 12-volt electrics. The lower fork yoke (triple clamp) was changed to allow more steering lock, starting with Engine #DU27672. A new front brake (still an 8″ SLS as before) increased brake swept area by nearly 50% by increasing its width. The 46-tooth rear sprocket now became a bolt-on affair, rather than cast into the brake drum as before.

The 1966 Triumph Bonneville TT Special was a true race bike, and was quite competitive in the open class in desert racing and scrambles.

1966 Triumph Bonneville Paint & Trim

While most 1966 Triumph Bonneville seats were the familiar 2-tone gray & black, some US models got all black seats. Rubber handgrips started out a pale gray (although they were referred to as ‘white’), but reverted to the more-practical black later in the year. US-market T120Rs also got polished stainless steel fenders to replace the painted ones. The gas tanks (both UK & US) were completely redesigned. The UK version still held 4 Imp. gals, but now had a slimmer profile and still fitted the chrome parcel rack on top. The US models dropped from an already short 3 gals to 2-1/2 gals and lost the luggage rack. The sexy new ‘teardrop’ shape became the symbol of Triumph motorcycles and Triumph Bonnevilles in particular to the public at large. The tank emblem was also changed from the old-style “harmonica” grille, to the cleaner & more-modern “eyebrow” badge, which would remain in one form or another for years to come.

The paint scheme for 1966 differed for the first time from UK/Export and US models. UK and general export bikes were Grenadier Red with Alaskan White accents. US models were Alaskan White with Grenadier Red stripes (3 stripes running down the centerline of the tank, two thin stripes flanking one broad stripe, in the fashion of “racing stripes” on cars of the period).

1959 Triumph Bonneville TT Special

1966 Triumph Bonneville TT Special

Prior to the 1966 model year the TT Special was not a stand-alone model, it was instead an option package on the T120C.  But starting with the 1966 Triumph Bonneville, the TT Special broke out on its own.  The TT package consisted of some serious engine mods and those gorgeous 'TT pipes' that tuck under the engine.  They were larger in diameter than all the other pipes offered on the 650s because they didn't step down like the others.  The diameter they were as they exited the head was what they were all the way down. They also shot a little farther forward to clear the frame as they tucked under.  TT pipes don't really have a muffler, after all it's a race bike.  But some riders put them on their street bikes and several silencing methods were developed including one I remember growing up.  It was called the "Snuffernot" and looked like a big washer in the end of the pipe that you could rotate like a butterfly valve with small knobs on the sides of the pipes.  They worked...a little.

Inside the engine, compression was boosted from 9.0:1 to 11.0:1, which is why TT Specials didn't make good street bikes.  The high compression made them run hot.  The 1-1/6" Amal Monobloc carbs from the standard Bonnevilles were replaced by 1-3/16" units on the TT.  Hotter cams were used and the electrical system was converted to energy transfer which allowed the elimination of the battery, a heavy luxury on a race bike.  All of this combined to produce 52 hp to the standard Bonneville's 47 hp.

1966 Triumph Bonneville Specifications

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Triumph Bonneville Books

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More Triumph Bonneville Pages

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