1966 Triumph Bonneville

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. 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!

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, 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. 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).

ABOVE: 1966 Triumph Bonneville T120R Roadster, with down pipes.

Inside the engine, things were heating up. Power output and vibration were at odds and it was only getting worse as more and more horsepower was demanded of the Triumph 650. An all new crankshaft shed 2-1/2 lbs of material from the edges of the flywheel, while retaining the 85% balance factor. A new 1-1/8″ shouldered roller main bearing (E2879) replaced the old ballrace 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 on non TT’s.

Starting with Engine #DU29738 all Triumph Bonnevilles got larger 1-3/16″ Amal Monobloc carburetors were fitted & 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 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.

ABOVE: 1966 Triumph Bonneville T120R from the right-rear timing-side.

A longer kick starter lever was installed 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.

Perhaps the biggest news for the 1966 Triumph Bonneville was the adoption of 12 volt electrics. 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. 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.

The 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.

While most 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 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.

1966 Triumph Bonneville TT SPECIAL

ABOVE & BELOW: 1966 Triumph Bonneville T120TT. This was the first year the “TT Special” had its own model designation. Until now, they were designated T120C’s just like the Street Scramblers.

BELOW: The US version of the 1966 Triumph Bonneville got a new ‘slimline’ tank without carrying rack. It held just 2-1/2 gallons. Note how slim that ‘slimline’ tank really is on this 1966 Triumph Bonneville TT Special.

1966 Triumph Bonneville SPECIFICATIONS

Bonneville T120R

Bonneville T120C

Bonneville TT Special

Engine type


Bore & Stroke




Engine output

Primary drive

Primary drive sprockets



Ratios, overall:

1st, bottom



4th, top

Final drive

Final drive sprockets

Frame type

Suspension, front

Suspension, rear

Brake, front

Brake, rear

Tire, front

Tire, rear

Fuel Capacity


Seat height

Ground clearance

Weight, unladen

Roadster, low pipes

Street Scrambler, high pipes

Competition, TT pipes

Air-cooled OHV vertical twin

649cc / 40.0 ci

71mm X 82mm / 2.79″ X 3.23″

8.5:1 (T120) / 11.0:1 (TT Special)

2- Amal Monobloc 1-1/16″ / 1-3/16″ (TT)

Battery & coil (T120) / Energy Transfer (TT)

47 bhp @ 6500 (T120) / 52 bhp (TT)

3/8″ triplex chain X 84 links

29T X 58T

Multi-plate, wet

4-speed constant-mesh, right-foot shift





5/8″ X .400″ X 3/8″ chain X 106 links

19T X 47T

Brazed lug, full-cradle, single downtube

Telescopic fork, hydraulic damping

Swing arm, 2 Girling dampers

8″ SLS drum, full width

7″ SLS drum

3.25″ X 19″ Dunlop

4.00″ X 18″ Dunlop

2.5 Imp gal (US) / 4 Imp gal (UK & export)

54.5″ / 140.3cm

32.5″ / 77.5cm

5.0″ / 12.7cm

363 lbs / 165 kg


  1. January 3, 2022

    What a beautiful motorcycle, beautifully engineered, I was very proud to own one!

    • William Bradford Utley
      January 6, 2022

      I owned a 65. Great bike. Loved the handling. A lot better than my current bike which is an 07 Harley Heritage Softail.

    • Matt Burke
      January 7, 2022

      Excellent article, well researched and accurate! Have a 67 TT Special and it always gathers a crowd of enthusiasts!

    • Tim
      January 13, 2022

      All those old Triumphs had Lucas electrical systems, Lucas made everything from washing machines to toasters, in fact there was a saying about Lucas, the only thing Lucas made that never sucked was there vacuumed cleaners! Everyone I knew that owned an old Triumph immediately ripped out and replaced the electrical system, they also leaked oil like they Exon Valdez, yet people bought them, I just bought a 2021 iron and will not apologize for that money pit or Harley dealers lies, was not told I needed front controls $1000, was told oil changes were free for 3 years a lie and did myself, had to buy a gascap with a lock and an oil resavor cap with lock $240 this is just plain cheapness on Harleys part and is ridiculous, not going to bother beefing up exhaust for a $1.000 to get 5 more hp on a 49hp Mc besides I don’t scrape pipes, need to put $300 13inc progressive rear shocks so won’t lose fillings when you hit a small bump, and that’s it except for mirror risers witch I ordered on Dec 3rd but Harley was unable to deliver them and don’t know were they are and refused to give a refund till I filed a dispute with cc company then there tune changed, I can’t see traffic bc Mc is so low this is extremely dangerous and Harley had the gall to apologize for my frustration, they say the Mc is a blank canvas, I say it’s a piece of crap and will not apologize for them, luckily I put a nice down payment will ride 1 more season sell it pay off note and buy a jap bike, and yes I do have buyers remorse bc a mc should be a joy to ride not a piece of crap.

  2. David
    January 6, 2022

    Never had a twin before. What a pleasant
    ride from my 500 Gold Star of 1951.
    Tri is so smooth and the added c. C. ‘s to
    865 O3 gets going right away. Love this
    Bike. David Evans lake Isabella Ca.

  3. Ken
    January 7, 2022

    I was lucky to own tt special . I applied headlight and taillight from triumph cub ran on mag. Great bike and fast at that time

  4. Jon minonno
    January 7, 2022

    Worked and raced for jack wilson Dallas triumph dealer. Lots of memories beautiful bikes.

    • Robert Dobbs
      January 20, 2022

      Jon, just to say hello. Hope you are doing well

  5. Jerry
    January 7, 2022

    Back in 1968 I was in 8th grade and used to watch for a Bonneville to drive by the school every day at about the same time. Sitting 2nd row in from the windows. There was a girl that told me to stop looking at her. She didn’t believe me when I told her I was watching for a motorcycle.

  6. Michael J Tennessen
    January 7, 2022

    I owned a 1965 Blue & SILVER
    Boni. My favorite bike ever. Still have a piçture of it in my parents yard, forever I hope!!

  7. Bob Zockoff
    January 7, 2022

    I still have my 1972 Bonnie. All original 12,000 miles except tires and battery. Now it has to go, due to a bad back.

  8. Brad
    January 8, 2022

    I had a 1968 650 Tiger. Best ever owned. So quick in the low gears. Ah, memories!

    • Gordy
      January 10, 2022

      Wow, so did I! Loved it, though it had been painted ugly as sin. Added a Mikuni carb (nice, cleaned up running), 750cc kit, cams (quick, fast, more vibration). I was young, should have left it more stock. Still miss it, though.

  9. Bob
    January 8, 2022

    1 owned a 1966 later model, April 1966. The later model had two very small lites on the top of the headline
    One for high beam the other ignition on. The early model only one lite. I think it was the ignition lite.
    I chromed the three cases and carbs plus all the nuts,bokts and washers.. Years later, bike got a chromemoly frame, alum swinging arm, aluminum forks, a Sommy Ruitt 800cc kit, a set of amal concentric carbs and a set of cams which were called 6/9’s. Head work by Big D in Texas.
    At a roll 40mph the bike would beat a 1000cc Kow to 135mph. I think the drive sprockets were 21/47, not sure.
    Problems: service clutch plates after each night of fun.

  10. January 8, 2022

    I bought a 1966 TT special in 1966. Wish I still had it. Best bike I ever owned

  11. January 8, 2022

    Lived in Doylestown Pa bought a 1966 Triumph TT Special from Riffs Cycle . Kept it till I left for the Army in 1967. Best twin I ever had. When I finished my duty in 1969 I bought a new Triumph Trident then had two more Tridents. And own a Custom built T160 today.

  12. Daniel J. Mohr
    January 9, 2022

    I have a 66 Trophy TR6C. Best road course bike. Have an 82 Super glide. It is only good on highways. Not good on the hilly curvy roads in PA. Haven’t run the Trumpet since 82. Am currently collecting missing parts to do a frame up reconstruction back to original.

  13. Larry J Williams
    January 10, 2022

    Had a 1967 Bonniville TT Special bought it from dealer in Thomasville NC. Was a Super Nice Motorcycle. Got drafted in September of 1968 & was gone for 2 years. Came home in 1970 & married my Sweetheart in June 27 the 1970. Wife didn’t like to ride so sold it to a friend. Always miss having it around.

  14. JW Reed
    January 10, 2022

    Very good article I always tell people the 66 triumph Bonneville was there high water mark. I still have four English bikes unfortunately not a 66. But I still keep my eye out for one. Anyhow great article confirms my opinion on the 66 Bonnie and TT. Thanks JW

  15. Tony Lentini
    January 13, 2022

    I built my 1966 Triump two years before the Happy Days show
    The engine was completely modified by The House of Triumph in the Bronx
    I had the very limited edition all aluminum 2 1/2 gallon Triumph tank ( only 50 made )
    Valve train was lightened and polished with 11 to 1 compression
    The sound of the machine was amazing with my 2 inch straight pipes with gutted mufflers ( very loud )
    When cornering sometimes had the bike at a 45 degree angle and blasting the throttle made her stand straight up sometimes crossing over into the on coming lane and running along that guard rail ( when in drought blast the throttle to take control )
    Went to watch the Trump Racing team at Bridge Hampton raceway on Long Island NY to find the team using the same single down tube frame with only the front forks changed
    ( Bisorini or something like that )
    Rode that bike for 28 years bought the bike from a good friend for $ 700 who moved into a Sportster and sold it for $ 2500 needing work
    The guy I bought the bike from so wanted it back at some point in time and but that never happened
    Love the machine until it was just time for me to stop riding ( growing the family ) and very responsible for me meeting my wife of 46 years now
    She loved riding and we did a lot until my son came along
    I did continue for a while but lost interest when she would no longer go for fear he would be an orphan if we both go killed
    The bike was one of the most amazing experiences I have ever had in my life being the gear head I have been all my life
    I held a track record in Super Stock with a 1963 Chevy super Sport 409 425 HP 4 speed 538 posi rear ( blue printed with a crane cam ) at NY’s Islip Speed way 1/8 mile raceway

  16. Tony Lentini
    January 13, 2022

    My bike had side car gearing and full race cams
    Went to a 750 kit and an oil cooler later on in the bikes development which just added to its pleasure
    John’s Cycle in Queens NY was the Truimph shop to know then
    John was an official at the Flat track races then also
    Just Loved wearing the T shirts with Triump on them which I still have a few of and do still wear
    The machine was just A TRUE BLAST TO OWN !!

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