BSA unit twins

The BSA unit twins were a range of unit construction twin-cylinder motorcycles made by the Birmingham Small Arms Company (BSA) and aimed at the US market. A range of 500 cc (31 cu in), 650 cc (40 cu in) and 750 cc (46 cu in) twins were produced between 1962 and 1972,[1] but they were really developments of the older pre-unit A7/A10 model range with less weight.[2] The engines had a reputation for vibration, but acceleration was good for the time, to a top speed of 100 miles per hour (160 km/h).[1]

BSA unit twins
ManufacturerBirmingham Small Arms Company (BSA)
Production1962–1972
AssemblySmall Heath, Birmingham UK
PredecessorBSA A7/A10
Engine500 cc (31 cu in)
654 cc (39.9 cu in)
751 cc (45.8 cu in)
air-cooled OHV twin
Bore / stroke500: 65.5 mm × 74 mm (2.58 in × 2.91 in)
650: 75 mm × 74 mm (3.0 in × 2.9 in)
750: 75 mm × 85 mm (3.0 in × 3.3 in)
Ignition typeTwin coil
TransmissionTriplex primary chain, 4 speed gearbox
Frame typeHalf-duplex cradle
Brakes7"/8" drum front
7" drum rear
8" 2ls drum 1968 on
Wheelbase54 in (1,400 mm)
Turning radius14 ft (4.3 m)

Models for the US generally had smaller petrol tanks and higher wider handlebars than the UK models, and prior to 1966 different model names were used for the two markets.

The 500cc models were discontinued in 1971 when the single cylinder B50 model was introduced.

The BSA Group faced worsening financial difficulties and in 1973 merged with Norton-Villiers. As part of a rationalisation by the newly formed Norton Villiers Triumph, production of BSA motorcycles ceased.


Design

Background

Precipitated by Lucas's intent to discontinue magnetos and dynamos in favour of alternators, BSA's managing director and general manager, Bob Fearon, took the opportunity to update the engines of BSA's twins and give them a new look. He hoped the new models would succeed in the potentially lucrative, but competitive, US market. Working with chief development engineer Bert Perrigo, he developed the unit construction twins[3] using "The Power Egg" design concept.[4]

Engines
BSA unit construction engine

Pre-unit, known as "non-unit" before unit construction, engines had their roots in history when motorcycle manufacturers bought engines and gearboxes from separate specialists suppliers and mounted them in their own frames. By the late 1950s, most manufacturers made their own engines and gearboxes, and were therefore able to combine the engine and gearbox in an integrated casting, producing a more compact unit.

Perrigo's design team based the new engine on the old A7/A10 engines. To streamline production and keep costs down, as many parts as possible were common between the engine sizes. The 500 and 650 engines shared the same stroke (74 mm (2.9 in)) and cylinder spacing, so the bottom half of the engine could be common to both sizes. (The A7 and A10 models had used different strokes and cylinder spacing, so crankshaft and crankcases were not interchangeable between them). Among the improvements in the new engines were pushrod passages in the cylinder block casting, which reduced the risk of oil leaks,[3] and pillars holding the rocker spindles cast into the head, leading to better valve clearance control.[4][5] Primary drive was by a triplex chain rather than the single row chain of the previous models and rubber inserts were added to the clutch for a smoother transmission.[5]

Although the older engines used a roller bearing on the drive side of the crankshaft, the new engine used a caged ball bearing. On the timing side a plain bearing was retained but increased in size. This arrangement proved troublesome and the ball bearing was replaced with a roller bearing in 1966.[4][5]

Cycle parts

A new frame was designed for the more compact unit construction engine. The frame followed the lines of the frame for the older models; a half-duplex cradle with a single spine. The new frame was all welded. The rear swinging arm was controlled by Girling 3 position adjustable shock absorbers. BSA's own forks were used, which had compression damping only. The front brake was 7" drum on the 500 model and 8" on the 650, although later 500s were fitted with the 8" brake.[5]

The new model had a wheelbase of 54 in (1,400 mm), 2" shorter than its predecessor.[5]

Overview of models
Engine Size Type Market Year[6]
62 [7] 63 64 [8] 65 [9] 66 [10][11] 67 [12][13] 68 [14][15] 69 [16][17] 70 [18][19] 71 [20][21] 72 [22][23] 73
A50
500cc		TouringUKStar[24][25] [note 1]Royal Star
USStarRoyal Star[note 2]
SportsUKCyclone Road [28] [note 3]
USCyclone Road [28] [note 4]
Production racingAllCyclone Clubman [30]
Off roadUKCyclone Competition [28] [note 5]Wasp
USCyclone Competition [28] [note 6]
PoliceAllPolice
A65
650cc		TouringUKStar[24][25] [note 1]
USRoyal Star
Sports/ TouringUKRocket[25]Thunderbolt
USThunderbolt Rocket [note 2]
SportsUKLightning [28]Lightning
USLightning Rocket [note 7]
Production racingAllLightning Clubman [28]
Super SportsAllSpitfire
Off roadUSSpitfire HornetHornet
AllFirebird Scrambler
PoliceAllPolice
T65
650cc		TouringUSThunderbolt [note 8]
A70
750cc		SportsUSLightning 75[21][32]

Notes

  1. Also known as Star Twin[26]
  2. In 1965, this model was only listed in the US West Coast catalog[27][9]
  3. In the US, this model was only listed in the US West Coast catalog[27][9]
  4. Listed as "Cyclone" in 1964 Western US catalog[29]
  5. In the US, this model was listed as "Cyclone" except in the US West Coast catalog[27][9]
  6. Not listed in 1964 Western US catalog[29]
  7. In the US, this model was listed as "Lightning" except in the US West Coast catalog[27][9]
  8. Re-badged Triumph TR6 Trophy[31]

Timeline

1962
BSA A65 650 Star

500 and 650cc models launched with common cycle parts except the front brake, which was 7" on the 500 and 8" on the 650.[33] The 500cc model was designated "Star" and the 650 "Star" in the UK and "Royal Star" in the US.

1963

The 500 model's name was changed to "Royal Star" in the US.

1964
1964 File:BSA Lightning Rocket

Two new 500 models were introduced in the US. Both with twin carburettors and were designated the "Cyclone Road" and the "Cyclone Competition". The road version was fitted with a sports cam and the competition version was devoid of lights and had a higher compression ratio and a race cam.[8] Both had the 8" front brake from the 650.

A new, higher performance 650 model, the "Rocket" in the UK and "Thunderbird Rocket" in the US was introduced. A twin carb version, the "Lightning Rocket" was introduced in the US. These two models replaced the pre-unit "Golden Flash" and "Super Rocket" models that were discontinued this year.[26]

A 650 version of the Cyclone Competition was also introduced in the US and called the "Spitfire Hornet".[8] This was a replacement for the discontinued pre-unit "Spitfire Scrambler".[26]

Police equipped versions of both the 500 and 650 were also introduced.[6]

1965
BSA Lightning Clubman

The Cyclone models were made available in the UK as was the Lightning Rocket, although called "Lightning" in the UK. The Lightning Rocket/Lightning has a higher compression ratio than the previous year's model.[34]

A limited edition "Lightning Clubman" was introduced. This featured siamese exhaust, clubman handlebars, rearset footrests and a close-ratio gearbox. It was designed for production racing.[30] A 500cc version was also produced, the "Cyclone Clubman".[30]

Crankshaft main bearings had been unreliable, in an effort to resolve this problem roller bearing were introduced on the drive side and an improved bronze bush on the timing side.[5]

1966
1966 File:BSA Hornet

1966 saw the rationalisation of the range and also the end to calling the same model different names in different countries:

  • The 500cc Star (UK) and Royal Star (US) became the Royal Star in all areas, and was fitted with the 8" front brake of the rest of the range.[5]
  • The Cyclone models were renamed "Wasp"
  • The 650 Star (UK) and Royal Star (US) models were discontinued
  • The Rocket (UK) and Thunderbolt Rocket (US) became the Thunderbolt
  • The Lightning (UK) and Lightning Rocket (US) became the Lightning
  • The police models were discontinued although BSA continued to supply motorcycles to police forces but to that force's individual requirements.
  • The US only Spitfire Hornet was renamed "Hornet" to avoid confusion with the newly introduced high performance "Spitfire" model.

The high performance Spitfire model, designated Mk II, featured 10.5:1 compression ratio, a pair of Amal 1-5/32” GP2 carbs on open bell-mouths and a close-ratio gearbox.[35][36]

All models were upgraded to 12v electrics in this year.[5]

1967
1967 BSA Spitfire Mk III

The Wasp models were discontinued, leaving the Royal Star as the only 500 model.

The Spitfire evolved into the MK III model with the compression ratio reduced to 9:1 and the carbs changed to the new Amal Concentrics.[35] These changes gave improved starting and tractability at low speeds.

New forks with 2-way damping were introduced.

1968

The US only Hornet offroader was discontinued and a new model, the "Firebird Scrambler", was introduced in all sales areas.

All models were fitted with twin leading shoe front brakes and carburettors changed to Amal Concentrics. The Spitfire evolved into the Mk IV.

1969

With the introduction of the flagship Rocket 3 750cc triple, BSA decided that there was no longer a need for the top of the range 650 twin and the Spitfire was discontinued.[35]

1970
1970 BSA A65L Lightning

Apart from minor cosmetic changes, the range remained the same for 1970.

1971

With the introduction of the 500cc single cylinder B50 model, the Royal Star was discontinued.

The 650s were given new cycle parts, including the "oil-bearing frame".[37] Apart from cosmetic items, the cycle parts were shared with the Triumph twins. (Triumph was a subsidiary of BSA).

To produce a 750cc machine for AMA Class C racing, BSA lengthened the stroke of Lightning's crankshaft by 11mm to give a displacement of 751cc. 202 A70 Lightning machines were produced to meet the minimum production requirements. All were exported to the US, most to the East Coast distributor in Baltimore.[32]

1972

By 1972 BSA were in serious financial trouble and the Firebird Scrambler was discontinued in an attempt to reduce costs.

1973

With rival British motorcycle manufacturer, Norton-Villiers, also in financial trouble, the British government funded a rescue package that merged the two companies into a new company Norton Villiers Triumph (NVT). As part of the rationalisation of the products of the new company, NVT stopped production of BSA models.

Other models

E35 Fury
Main article: BSA Fury
BSA Fury 350

The 'BSA Fury was a prototype motorcycle made in 1970. Designed by Edward Turner but substantially redesigned by Bert Hopwood and Doug Hele,[38][39] the Fury never went into commercial production due to the collapse of the BSA Group.[40]

Launched in 1971, the 349 cc (21.3 cu in) double overhead cam twin BSA E35 was branded the BSA Fury. It was essentially the same motorcycle as the Triumph Bandit and represented the BSA factory's last attempt to compete against Japanese imports.[41] The frame used for the Fury was designed by Rob North,[42] designer of the frames used on the racing BSA triples. A 'Street Scrambler' E35SS and road version the E35R were developed during 1971. Featuring upswept twin silencers, the SS had an optional electric starter and indicators and 26 mm Amal concentric carburettors. The Fury delivered 34 bhp (25 kW) and could reach 110 mph (180 km/h).

T65 Thunderbolt

In order to fulfil existing export orders after production of the BSA twins had stopped, between 200 and 300 Triumph TR6 Trophys were rebadged as BSA T65 Thunderbolt and exported to Spain and Australia.[31][43]

Further reading
  • Bacon, Roy H. (1995). Bsa A50/A65 Twins: All Models. Haynes Manuals Inc. ISBN 9781856483094.
  • Bacon, Roy H. (2007). BSA Twins and Triples (First ed.). Mercian Manuals Ltd. ISBN 9781903088364. ASIN 1903088364.
  • Clarke, R. M. (1996). Bsa Twins A50 And A65 Gold Portfolio1962- 1973. Brooklands Books. ISBN 9781855203372.
  • Clymer, Floyd (2012). SECOND BOOK OF THE BSA TWINS 650cc & 500cc 1962-1969. Veloce Enterprises, Inc. ISBN 9781588501691.
  • Henshaw, Peter (2008). BSA 500 & 650 Twins: The Essential Buyer's Guide. Veloce Publishing. ISBN 9781845841362.
  • Kemp, Andrew (2004). Classic British Bikes. Abbeydale Press. ISBN 9781861471369.
  • Vale, Matthew (2005). BSA Unit Twins. The Crowood Press. ISBN 9781861266897.

References
  1. Kemp, Andrew; De Cet (2004). Classic British Bikes. Mirco. Bookmart Ltd. ISBN 978-1-86147-136-9.
  2. Bacon, Roy (1988). British Motorcycles of the 60's. Osprey Publishing. ISBN 978-1-85648-166-3.
  3. Clarke, R. M. (1996-05-26). BSA Twins – A50 & A65 Gold Portfolio. Brooklands Books Ltd. ISBN 978-1-85520-337-2.
  4. "BSA A65". Classic-British-Motorcycles.com. Retrieved 26 June 2018.
  5. Salvadori, Clement (August 2011). "1962 BSA A50 Royal Star". Motorcycle Classics. Retrieved 25 June 2018.
  6. Jones, Rob; Trigwell, Ray. "Machine Dating Information". BSA Owners Club - UK. Retrieved 24 June 2018.
  7. "1962 US catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  8. "1964 US catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  9. "1965 US West Coast catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  10. "1966 US Catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  11. "BSA 1966 Instruction Manual 499cc A50R A50W and 654cc A65L A65H A65T A65SP (mark 2) only". National Motorcycle Museum. Retrieved 24 June 2018.
  12. "1967 US catalog". BSA MCC of NSW Website. Retrieved 24 June 2018.
  13. "BSA 1967 Instruction Manual. 499cc A50R and 654cc A65L A65H A65T A65 Spitfire Mark 3 only". National Motorcycle Museum. Retrieved 24 June 2018.
  14. "1968 US catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  15. "BSA 1968 Instruction Manual. 499cc A50R&654cc A65L A65FS A65T A65 Spitfire Mark 4 unit Twins Not oil in frame". National Motorcycle Museum. Retrieved 24 June 2018.
  16. "1969 US catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  17. "BSA 1969 Instruction Manual. 499cc A50R and 654cc A65L A65FS A65 Spitfire M4 unit twins Not oil in frame". National Motorcycle Museum. Retrieved 24 June 2018.
  18. "1970 US catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  19. "BSA 1970 Instruction Manual for 650cc models A65T,A65L,A65FS. Manual in Spanish". National Motorcycle Museum. Retrieved 24 June 2018.
  20. "1971 US catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  21. "BSA 1971 to 1972 Instruction Manual 650cc models A65T Thunderbolt A65L Lightning and 750cc A75L Lightning 75". National Motorcycle Museum. Retrieved 24 June 2018.
  22. "1972 Home and General Export brochure". BSA MCC of NSW. Retrieved 24 June 2018.
  23. "1972 US catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  24. "BSA 1961 to 1965 Instruction Manual. OHV Models A50 Star & A65 Star". National Motorcycle Museum. Retrieved 24 June 2018.
  25. "BSA 1962 onwards Instruction Manual. 499cc A50 and 654cc A65 A65R Twins only". National Motorcycle Museum. Retrieved 24 June 2018.
  26. Jones, Rob; Trigwell, Ray. "BSAOC Year Listing". www.bsaownersclub.co.uk. Archived from the original on 7 October 2019. Retrieved 27 June 2018.
  27. "1965 US catalog". BSA MCC of NSW. Retrieved 24 June 2018.
  28. "BSA 1964 to 1965 Instruction Manual Cyclone models A50C A50cc and Lightning models A65L A65LC". National Motorcycle Museum. Retrieved 24 June 2018.
  29. "1964 US BSA catalog (Western edition)". BSA MCC of NSW. Retrieved 24 June 2018.
  30. "BSA Lightning 650 PR". Nolan Woodbury. Retrieved 25 June 2018.
  31. "BSA Motorcycles". Classic Motorcycles by Sheldon's Emu. Retrieved 25 June 2018.
  32. "1971 BSA A70 Lightning". Classic-British-Motorcycles.com. Retrieved 25 June 2018.
  33. "1962 BSA A65". Classic-British-Motorcycles.com. Retrieved 24 June 2018.
  34. American Motorcyclist. American Motorcyclist Assoc. 1965. Retrieved 25 June 2018.
  35. "Retrospectove: BSA A65S Spitfire Mark II/III/IV Special: 1966-1968". Rider Magazine. 2 October 2014. Retrieved 25 June 2018.
  36. Clayton, Graham (1 June 2013). "1966 BSA A65 Spitfire Speed Bike". www.motorcyclemojo.com. Retrieved 25 June 2018.
  37. "1971 BSA A65". Classic-British-Motorcycles.com. Retrieved 25 June 2018.
  38. Bandit In The Hills (The Classic Motorcycle 11/84)
  39. "1971 BSA Fury". Archived from the original on 31 January 2010. Retrieved 22 November 2009.
  40. Clew, Jeff (2007), Edward Turner: The Man Behind the Motorcycles, Veloce Publishing, p. 133, ISBN 978-1-84584-065-5
  41. "BSA 350 Fury SS Prototype DZ75". Archived from the original on 8 July 2011. Retrieved 22 November 2009.
  42. Ilminen, Gary (22 May 2017). "Sad Case of the Triumph Bandit/BSA Fury 350: What Might Have Been". Ultimate Motorcycling. Retrieved 24 June 2018.
  43. "BSA THUNDERBOLT T65 1972". imperiya.by (in Russian). Retrieved 25 June 2018.
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