1973 Triumph Bonneville

1973 Triumph Bonneville Background & History

The 1973 Triumph Bonneville was probably the one year with more improvements made to it than any other year of Bonneville. In just one year, the 5-speed gearbox became standard, a front disc brake finally made its appearance, and displacement was kicked up to 750cc. Starting the model year with Engine #JH15366, the T120 Bonneville 650 soldiered on for another year (it would finally be dropped in 1976), side-by-side with the new T140 Bonneville 750. Shock absorbers were 1/2″ shorter, lowering ride height. Of course the big news on the chassis front was the new front disk brake, long overdue, Honda had them on every model from the 450 on up since 1969. Well, it was finally here, and a 5-speed too! Happy days. Now if we could just have that electric starter…

The 1973 Triumph Bonneville is my personal favorite Bonneville, and in my opinion, the best Bonneville ever built.

1973 Triumph Bonneville Color Combos

Colors for 1973 Triumph Bonneviulle US-spec bikes were Hi-Fi Vermilion (bright red) and Gold with White pinstriping on T140s, while the home-market (UK) T120s got Tiger Gold over Cold White. The fenders were now chromed and with the black plastic side covers, this left the only color on the bike being on the tank. It made it easier on production. Of course, all US-spec 1973 Triumph Bonnevilles got the gorgeous teardrop tank, but the UK-spec bikes got the so-called ‘home market’, ‘British spec’, or ‘Euro’ tank (I’d always referred to them as ‘bulldog’ tanks, because of their chesty shape). It’s larger in capacity at 3.5 Imperial gallons (4 US gallons) and has a much chunkier, barrel-chested shape. US models now got a ‘slimline’ tank with a scant 2-1/2 US gals of capacity. At somewhere around 45 mpg, that isn’t much range. But, man, did those Triumph teardrop tanks look great! The headlight reverted back to a conventional shape, from the former ‘pancake’-style supported by new upper fork leg covers that incorporated mounting bosses for the headlight brackets, which were rubber-mounted. Gone were the slender, chromed wire supports of 1971 and 1972.

If you count the cooling fins on the barrels (black) on this 1973 Triumph Bonneville you should come up with 7.  A year earlier, it would have been 8.  This was done to shorten the height of the engine so that it would clear the frame upon assembly.

1973 Triumph Bonneville Engine Mods

With the boost in displacement to 750cc, which required new barrels (still cast iron, unfortunately), Triumph took the opportunity to solve another nagging issue. When the new oil-bearing frame was designed for the 1971 Triumph Bonneville and TR6, and BSA's 650 line, the geniuses at BSA’s lavish tech center, Umberslade Hall, never actually tested it to see if the Triumph engine would fit. It’s interesting though that BSA’s engineers made sure the BSA 650 engines fit. The only way the Triumph engine could be installed in the frame was to remove the rear rocker box from the already fully-assembled engine, stuff the engine into the frame, then reinstall the rocker box. All of this had to be done on the production line. By 1972 revisions had been made to the head, rocker boxes and head bolts to make it easier to and remove and reinstall the rocker box on the production line. So, when designing the new 750cc barrel kit, they simply shortened it by one fin. Pre-1973 bikes have 8 fins while the 1973-and-later bikes have only 7. This shortened the top end by over one-half inch which was just enough to solve the problem. From here on, the engines could be stuffed into the frames in one piece.

The shorter barrels meant shorter connecting rods, shorter pushrods and pushrod tubes, changes to the exhaust, the head stay (top engine mount) and the cases. The crankcase mouth was relieved to make room for the bigger barrels, the main journal bosses got beefed up also. A new crankshaft used the old flywheel but the balance factor was tweaked again to 74%. Compression ratio was 8.6:1 on US machines and 7.9:1 on UK and exports. Starting with engine # JH15435, early 1973 Triumph Bonnevilles actually displaced 724cc which was accomplished by boring the cylinders to 75mm, from the 650’s 72mm. But midyear, starting with engine #XH22019, the bore was taken out to 76mm, yielding a displacement of 744cc. And it remained as such for the rest of the Bonneville’s life. The shorter connecting rods meant that the angle of the rod would be greater than before at mid-stroke, and while it probably would have hurt the engine’s ultimate performance, such as in racing, few T140s were being pushed that hard by 1973. In normal operation, it wasn't really a problem. It did have a curious side effect, however. On worn engines, at an idle it sounded like someone was ringing a cowbell. Ding, ding, ding, ding…. I had a 1973 myself that did that when I bought it. But once I did the top end, the noise went away. It was from piston slap, and this caused worn pistons to wear even faster, and unevenly. But, in actuality, it was a fairly small problem, if a problem at all, for most owners of a 1973 Triumph Bonneville.

The 1973 Triumph Bonneville got a new triplex primary chain along with its 5-speed gearbox.

In the primary case a 3/8″ triplex replaced the duplex, the clutch springs and clutch-hub rubber shock absorbers were beefed up, and the primary cover widened slightly to accommodate. The 5-speed Quaife gear cluster was now standard equipment on every 1973 Triumph Bonneville, and TR6 . Early 5-speeds tended to jump out of gear, so a conversion kit (with new 1st, 2nd and 3rd gear layshaft pinions and 1st and 2nd mainshaft pinions, a layshaft shift fork and dog) was made available to dealers and the factory fitted them on the line. This same gearbox was also used on the Trident 750 triple. The new cylinder block for the 1973 Triumph Bonneville 750 was one fin shorter than the previous 650 cylinder, from 8 fins to 7. Ever since the adoption of the oil-bearing frame in 1971, the rocker boxes had to be removed from the assembled engines on the assembly line, the engines placed in the frames, then the rocker boxes reinstalled and torqued down. It was messy, time-consuming, laborious and hence expensive business, all thanks to the bad planning at Umberslade Hall, parent-company BSA’s opulent new technology center. So, the scrappy engineers at Triumph’s Meriden plant, practically on the production line themselves, came up with this brilliant, effective, and relatively cheap fix. Shortening the height of the engine in this way allowed them to be installed in one piece, greatly streamlining production. Can you imagine Honda, Yamaha, Kawasaki or Suzuki ever doing things like this?

The 1973 Triumph Bonneville got this cool new disc brake.  A new left fork slider was called for with a cast-in boss to mount the Lockheed caliper hiding under that pretty chrome cover.  Check out all the front fender bracing, new for 1973.  Note that the stock brake disc was not cross-drilled as the above bike is.

1973 Triumph Bonneville gets a Disc Brake at Last

The new 10″ cast iron disk brake rotor was chrome-plated (not a good idea, as it turns out) and bolted to a new 2-piece alloy hub. A new left fork slider had a cast-in lug for mounting the Lockheed single-piston caliper, hidden behind an attractive chrome cover with a small round ‘Triumph Hydraulic’ logo on it. Chrome is too smooth to make a good friction surface for braking and it tended to peal off after some time, so Triumph stopped chroming them part way through the 1973 model year. This, of course, led to rust, especially on those bike that sat a long time. The front tire size was still 3.25 X19″, but the rear was increased to a 4.25 X 18″ Dunlop K70.

This was a 2-page magazine ad from 1973 advertising the new 1973 Triumph Bonneville in all its glory.  It ran in all the major motorcycle magazines.

Worker Revolt Closes the Meriden Factory

The 1973 Triumph Bonneville was a watershed for the Bonneville line, not just because of the bump up to 750cc, the 5-speed and the disc brake, it was when the workers took over the factory at Meriden and refused to allow anything to come in or out for well over a year! The effect on Triumph motorcycle production was devastating. They weren’t exactly setting the world on fire anyway, but the new 1973 Triumph Bonneville, its sister bike the TR7, and the Triumph Trident 750 triple were all decent bikes by 1973. Granted they still didn’t have electric starters yet, but they did now have 5-speeds and disc brakes plus good looks, performance and handling to match. They looked like they were poised to turn the corner when the strike ensued and production stopped in October 1973, after they had already begun building 1974s. Very few 1974s were built, and virtually no 1975s. It wasn’t until the 1976 model year that Triumph began building Bonnevilles in earnest again, but by this time, they shifted on the other side!

The reasons for the shutdown are many, but once again it came down to backroom dealing on BSAs part. Norton-Villiers had merged with BSA to form a new conglomerate, but BSA went bust in 1972, right in the middle of the process. The merger continued, leaving Norton owning Triumph, with a new company called Norton-Villiers-Triumph (NVT). Their plan was to shut down the Meriden factory (where Triumphs had been built for decades) and move all Triumph production to Norton's factory, where they thought they could capitalize on economies of scale. When word of this reached the Meriden workers, they revolted. It destroyed Triumph’s last chance at survival, even though it would take a few more years to die. When the dust cleared in 1975, the workers had organized into the Meriden Co-operative and purchased the company from Norton. Things went from bad to worse. But, that story comes a little later…

This is a UK home market version of the 650cc 1973 Triumph Bonneville T120RV, which got the 5-speed, but not the disc brake.  It also has the earlier-style headlight and wire brackets.  The purpose of this photo was to show what a British-spec Bonnie looked like, including the T140, with its low bars and 'bulldog' tank, along with its unique color combo.

Is the 1973 Triumph Bonneville the Best One Ever?

There are a lot of reasons why I have a personal attachment to any 1973 Triumph Bonneville.  They're my personal favorite of all classic British motorcycles, believe it or not.  Not because they're collectable, because they're not.  You can still buy them really cheap in the auctions.  I was always a rider.  I wanted bikes I could ride.  And the 1973 Triumph Bonneville is really the best one for that and here are all my reasons why (again, this is just my opinion):

1.) First year for the front disc brake.

2.) First year for the 750.

3.) First year for the 5-speed (as standard equipment)

4.) Still had that gorgeous conical rear brake.

5.) Still shifted on the right, as God intended.

6.) Seat height was a comfortable 30 inches

7.) Triumph had had some time to sort out the oil-in-frame bikes.

8.) It's almost impossible to get a 1974, and almost that bad for 1975.

9.) By 1976, the Bonneville shifted on the left and had smog carburetors.

10.) I graduated high school in 1973.  That makes it special.

Talk about a narrow window, the 1973 Triumph Bonneville was the first to get all the good stuff, and since you could hardly find 74s or 75s, it was also the last, practically speaking.  By 1976, the Bonneville's teeth had been pulled, with its Amal Mark II (smog) carbs, lower compression, milder cams, a heavier crank, all done in the interest of emissions compliance.  But it turned them into underpowered dogs with poor drivability.  The factory still claimed it made 49 horsepower, but it didn't feel like that when compared to the 49 hp a 1970 Bonneville made.

1973 Triumph Bonneville Specifications

---

Triumph Bonneville Books

---

---

---

---

---

---

---

---

---

---

---

More Triumph Bonneville Pages

1959 T120 Bonneville

1960 T120 Bonneville

1961 T120 Bonneville

1962 T120 Bonneville

1963 T120 Bonneville

1964 T120 Bonneville

1965 T120 Bonneville

1966 T120 Bonneville

1967 T120 Bonneville

1968 T120 Bonneville

1969 T120 Bonneville

1970 T120 Bonneville

1971 T120 Bonneville

1972 T120 Bonneville

1973 T140 Bonneville

1974 T140 Bonneville

1975 T140 Bonneville

1976 T140 Bonneville

1977 T140 Bonneville

1977 T140J Silver Jubilee

1978 T140 Bonneville

1979 T140 Bonneville

1980 T140 Bonneville

1981 T140 Bonneville

1982 T140 Bonneville

1983 T140 Bonneville

1983 TSS Bonneville

1983 TSX Bonneville

Hope you're enjoying my website.  Glad you're here, take some time to look around.  I'm just getting started though, with lots of great content, pictures and specs on literally hundreds of classic British motorcycles left to go.  It is my goal to cover every year of every make, and every model of British bike.  You can join me in my quest.  If you have a classic Brit bike, take and/or send me some great photos of it along with a description.  Email coming soon.  More pages coming soon.  Please be patient, and hold on to your hat.  Thank you for visiting ClassicBritishMotorcycles.net.

Copyright 2024 by ClassicBritishMotorcycles.net.  All rights reserved.

---

---

Whitworth Tools for sale

---

---

2025 Classic Motorcycle Calendars are here!

---

---