1973 TRIUMPH BONNEVILLE


1973 Triumph T140 Bonneville 750, Triumph motorcycles, Triumph TR7

1973 Triumph Bonneville T140RV, first year for 750cc, 5 speed (as standard) & front disk brake.




MODEL DESIGNATIONS
1973 was a big year for the new Oil-in-Frame Triumph Bonneville. 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, which started with Engine #JH15435.

PUNCHING OUT THE BONNEVILLE
The T140 got its boost in displacement in two stages. Early on, Meriden simply bored the old 650 barrels out to 75mm which, coupled with the same 82mm stroke yielded 724cc. However, starting with Engine #XH22019, a new cylinder block was cast which allowed the bore to be increased one more mm to 76mm for a displacement of 744cc. Then from Engine #CH29520 yet another new block was introduced that had threads for a 10th headbolt & the head was machined accordingly, henceforth known as a "10-bolt head". And as such, the Bonneville would remain for the rest of its life.

1973 Triumph Bonneville, Triumph motorcycles, Triumph TR6

This 1973 Triumph Bonneville is finished in Astral Red & Silver, a Canada-only color combo. The only other year they used Astral Red on a Bonneville was in 1970, and it was also trimmed in Silver.

1973 triumph bonneville, triumph bonneville, oil-in-frame triumphs, triumph motorcycles, triumph tr6

Note how the side covers on this 1973 Triumph Bonneville differ from the 1971-72 Bonnevilles in that these are smooth, devoid of the phony grilles incorporated into the earlier types. The foot pegs & rear engine mounting plates are chromed on this bike, which was not original. They would have been painted black from the factory.




1973 TRIUMPH BONNEVILLE - INCOMPARABLE INCOMPETENCE
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 & torqued down. It was messy, time-consuming, laborious & hence expensive business, all thanks to the bad planning at Umberslade Hall, parent-company BSA's opulent new technology center. So, shortening the height of the engine allowed them to be installed in one piece, greatly streamlining production. Can you imagine Honda, Yamaha, Kawasaki or Suzuki ever doing things this way?

1973 Triumph T140 Bonneville 750, Triumph motorcycles, Triumph TR7 1973 Triumph T140 Bonneville 750, Triumph motorcycles, Triumph TR7

1973 TRIUMPH BONNEVILLE ENGINE
The shorter barrels meant shorter connecting rods, shorter pushrods & pushrod tubes, changes to the exhaust, the head stay (top engine mount) & the cases. The crankcase mouth was relieved to make room for the bigger barrels, the main journal bosses 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 & 7.9:1 on UK & Exports.

1973 TRIUMPH BONNEVILLE TRANSMISSION
In the primary case a 3/8" triplex replaced the duplex, the clutch springs & clutch-hub rubber shock absorber were beefed up, & the primary cover widen slightly to accomodate. The 5-speed Quaife gear cluster was now standard equipment on every 1973 Triumph Bonneville, and Triumph TR7s. Early 5-speeds tended to jump out of gear, so a conversion kit (with new 1st, 2nd & 3rd gear layshaft pinions & 1st & 2nd mainshaft pinions, a layshaft shift fork & dog) was made available to dealers & the factory fitted them on the line. These trannies were in common with the Triumph T150 Trident Triple

1973 TRIUMPH BONNEVILLE RUNNING GEAR
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...

1973 TRIUMPH BONNEVILLE - FINALLY A DISK BRAKE
The new 10" cast iron disk brake rotor was chrome-plated (not a good idea, as it turns out) & 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 & it tended to peal off after some time. Tire sizes were still 3.25 X19" up front, but the rear was increased to a 4.25 X 18" Dunlop K70.

1973 TRIUMPH BONNEVILLE COLORS & TRIM
Colors for 1973 were Hi-Fi Vermilion (bright red) & Gold with White pinstriping on T140s, home-market (UK) T120s got Tiger Gold over Cold White. The fenders were now chromed & with the black plastic side covers, this left the only color on the bike being on the tank. It made it easier on. The top photo in this section shows a 1973 Triumph Bonneville with the so-called 'home market', 'British spec', or 'Euro' tank. It's larger in capacity at 4 Imperial gallons (more than 4 US gallons) & 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.

1973 TRIUMPH BONNEVILLE - MERIDEN MADNESS
1973 was a watershed year for the Triumph Bonneville not just because of the bump up to 750cc, the 5-speed & the disk brake, it was when the workers took over the factory at Meriden & 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, 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 starters yet, but they did now have 5-speeds and disk brakes plus good looks and performance to match. They looked like they were poised to turn the corner when the strike ensued & production stopped in October 1973, after they had already begun building 1974s. Very few 1974s were built, & 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!

1973 TRIUMPH BONNEVILLE FACTORY SHUT-DOWN
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 called Norton-Villiers-Triumph (NVT). Their plan was to shut down the Meriden factory (where Triumphs had been built for decades) & move production to the existing BSA & Norton factories. 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 & purchased the company. Things went from bad to worse. But, that story comes a little later...

1973 Triumph T140 Bonneville 750, Triumph motorcycles, Triumph TR6

1973 Triumph Bonneville T140RV with British-spec/Euro tank.


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1973 Triumph Bonneville SPECIFICATIONS:



MODEL DESIGNATIONS:
1973 Triumph Bonneville T120RV.................... 650cc
1973 Triumph Bonneville T140RV.................... 750cc

ENGINE:
Engine type....................................... OHV vertical twin
Horsepower at RPM............................ 49 BHP @ 6,200 rpm
Bore................................................ 76mm / 2.79"
Stroke............................................. 82mm / 3.23"
Displacement.................................... 744cc / 45 cu. in.
Compression Ratio............................. 8.6:1
Valve Clearance (cold):
Inlet................................................ 0.05mm / 0.002"
Exhaust............................................ 0.10mm / 0.004"
Valve Timing:
Inlet Valve Opens.............................. 34 degrees BTDC
Inlet Valve Closes.............................. 55 degrees BTDC
Exhaust Valve Opens.......................... 55 degrees BTDC
Exhaust Valve Closes.......................... 27 degrees BTDC

IGNITION:
Ignition Breaker Gap............................. 0.4mm / 0.015"
Spark Plug Gap.................................. 0.50mm / 0.020"
Timing (static).................................14 degrees BTDC
Timing (fully advanced):........................ 38 degrees BTDC

CARBURETORS:
Type................................................ Amal Concentric
Throat Size....................................... 30mm
Main Jet........................................... 210
Needle Jet........................................ .107
Needle Position.................................. 2
Needle Type.......................................D
Throttle Valve.................................... 2
Pilot Jet............................................ 25

TRANSMISSION:
Speeds.............................................. 4
Gear Ratios:
5th - Top........................................... 4.7
4th - Fourth........................................ 5.59
3rd - Third......................................... 6.58
2nd - Second...................................... 8.63
1st - Bottom...................................... 12.25

CLUTCH:
Type................................................ Multi-plate, wet
Number of Plates:
Drive Plates...................................... 6
Driven Plates.................................... 6

SPROCKETS:
Engine............................................. 29 teeth
Clutch............................................. 58 teeth
Gearbox.......................................... 20 teeth
Rear Wheel...................................... 47 teeth

CHAIN:
Primary, pitch.................................. 3/8" triplex
Primary, length................................. 84 links
Final Drive, pitch............................... 5/8" X .400" X 3/8"
Final Drive, length............................. 106 links

CAPACITIES:
Fuel (US versions)............................. 2.5 Imp. gal.
Fuel (UK & export versions)................. 4 Imp. gal.
Oil Tank........................................... 5 pints / 3 L
Gearbox........................................... 1 pt / 500cc
Primary Chaincase............................. 1/4 pt / 150cc
Front Forks....................................... 6.4 oz. / 190cc

TIRES:
Front............................................... 3.25 X 19"
Rear................................................ 4.00 X 18"

SUSPENSION:
Front............................................... Telescopic Forks
Rear................................................ Swing Arm

BRAKES:
Front............................................... 10" / 12.9cm disk
Rear................................................ 7" / 17.78cm SLS

DIMENSIONS:
Seat Height...................................... 32.5" / 77.5cm
Wheelbase........................................ 54.5" / 140.3cm
Length.............................................. 85.5" / 202cm
Width............................................... 28.5" / 72cm
Ground Clearance.............................. 5" / 12.7cm
Weight, unladen................................ 387 lbs / 176kg


SAVE THE BONNEVILLE!
Did you ever wonder what really happened to Triumph? This is the behind-the-scenes story of the Meriden Workers' Co-op, written by John Rosamond, the welder-turned-Company Chairman. From the factory takeover in '73, and the formation of the Co-op in '75, through their endless struggles to continue to produce Triumph Bonnevilles, despite constant setbacks & opposition, until its slow death in 1983. The book is loaded with amazing photos & information you won't find anywhere else. This is not hearsay either, this is the real deal right from the source. I lived through this stuff myself, working as an apprentice mechanic in a Triumph/BSA/Norton dealership in 1971 when the first Oil-in-Frame Triumphs arrived to a lukewarm response. There were lots of ups & unfortunately more downs for the cash-strapped Co-op, but the scrappy Brits soldiered on & even came out with some stunning new models (like the Diana watercooled DOHC 4-valve twin, the contra-rotating balance shaft Bonneville, the 8-valve TSS, Anti-Vibration frames with rubber engine mounts & more), all outlined in the book. It's a MUST READ for anyone who mourns the loss of the British Motorcycle Industry as a whole & of Triumph in particular.


You can order the book by clicking on the picture of the book cover, above.


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