// Workers AI · dad joke modeWhat did the BSA B31 say to its friend? "You're a wheel good buddy
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 1949 BSA B31 | |
| Manufacturer | BSA motorcycles |
|---|---|
| Parent company | Birmingham Small Arms Company |
| Production | 1945-1959 |
| Assembly | Small Heath, Birmingham, England |
| Predecessor | BSA B38 |
| Successor | BSA B40 |
| Engine | 348 cc (21.2 cu in) air-cooled single-cylinder OHV four-stroke |
| Bore / stroke | 71 mm × 88 mm (2.8 in × 3.5 in) |
| Compression ratio | 6.5:1 |
| Power | 17 bhp (13 kW) @ 5,500rpm |
| Ignition type | Magneto |
| Fuel delivery | 1 in (25 mm) Amal carburettor |
| Transmission | Wet clutch, 4 speed gearbox, chain drive |
| Suspension | Front: Telescopic forks |
| Brakes | Drums front & rear |
| Fuel capacity | 3 imp gal (14 L; 3.6 US gal) |
| Footnotes / references [1] | |
The BSA B31 is a 350 cc (21 cu in) pre-unit single-cylinder OHV four-stroke motorcycle that was made by Birmingham Small Arms Company (BSA) at its factory in Armoury Road, Small Heath, Birmingham. It was the first new motorcycle introduced by BSA after WW2 and produced from 1945 to 1959.
History
[edit]The B31 has its origins in the 1937 Silver Star, designed by Val Page. A revised version, the B29, which used an 'M series' with a cast iron OHV top end,[2] went into production in late 1939 but few were made before production of civilian motorcycle was halted and production of military motorcycles started.[3] A version of the B29 was offered to the army and designated the B30. Although the army placed an order for the B30, it was later changed to an order for the M20 side-valve. Development of the B30 continued during the war years with Joe Craig and Edward Turner working on it, the latter redesigning the cylinder head to use conventional coil valve spring rather than the original hairpin springs.[4]
At the end of hostilities civilian production resumed and the 350 single, now designated B31, introduced in August 1945.[5] The machines had a single downtube rigid frame and, new for BSA, telescopic forks.[a] The speedometer was mounted in the petrol tank.[4] This was moved to the top of the forks in 1948.[5]
Early in 1946 a competition version, the B32, was announced. It had lighter chrome mudguards, a high level exhaust and lower gearing. The following year a 500 cc (31 cu in) version, the B33, was introduced with the bore increased from 71 to 85 mm (2.8 to 3.3 in).[6]
The more substantial frame was introduced in 1949[6] and a plunger frame was offered as on option. In 1951 a dualseat became the standard fitment.[5]
In 1954 a swinging arm frame was offered for an extra £12. The all-welded frame was of the same type as used on the Gold Star and the A series twins. The rigid frame was discontinued in 1954 and the following year the swinging arm frame became the standard fitment along with a dual seat. The plunger frame was discontinued at the end of 1955.[4]
Full width Ariel[b] hubs were introduced in 1956[2] along with a quickly detachable rear wheel.[4] Brakes from Triumph[b] were fitted in 1958.[2]
The electrics were changed in 1958 with an alternator and coil ignition replacing the Lucas magdyno. This necessitated moving the transmission shock absorber from the end of the crankshaft to the centre of the clutch.[4]
Production of the B31 ended in September 1959.[4]
Notes
[edit]See also
[edit]References
[edit]- ↑ "BSA B33 | BSA B31". sumpmagazine.com. Retrieved 3 August 2026.
- 1 2 3 "Review: BSA B31". Classic Bike Hub. 5 January 2022. Retrieved 2 August 2026.
- ↑ Wood, Geoffrey (March 1965). "History of the Marque-- BSA". Cycle World. Retrieved 2 August 2026.
- 1 2 3 4 5 6 "Reference: When was it that? BSA B31". Old Bike Mart. 29 November 2011. Retrieved 2 August 2026.
- 1 2 3 4 Dumas, François-Marie. "BSA B 31 1945 - Moto Passion - Moto Collection". www.moto-collection.org (in French). Retrieved 2 August 2026.
- 1 2 Laverty, Peter (19 April 2024). "BSA B31 – Stout fellow". Old Bike Australasia. Retrieved 3 August 2026.
- ↑ "Reference: When was it that? Ariel". Old Bike Mart. 5 December 2011. Retrieved 2 August 2026.
External links
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