BSA B31/BSA B33 buyers guide

1947 - 1960, 350cc & 500cc air-cooled, OHV singles


circa-1956 BSA B33 with swinging-arm frame


Bonhams sold this BSA B33 in Paris, France in February 2013. The frame and engine numbers don't match. So the bike was listed as circa 1956. The sale price was 2,255. Generally speaking, you can expect to pay around 2,500 to around 4,000 for a roadworthy B33 (2016 prices). BSA B31 prices are a little lower.


 BSA B33 front brake BSA B33 carburettor


In 1956, BSA was using Ariel full-width aluminium alloy hubs. At the best of times, these 7-inch stoppers are nothing more than "okay". But on a B33, there's a fair amount of engine braking to knock down your speed. The 1-1/6th Amal carburettor helps eke out the fuel. The touring economy is around 70-80mpg. A sorted B33 engine should return anything from 30,000 - 100,000 miles without needing a total strip, but only if you change the oil religiously and otherwise treat the engine with care (i.e. accurate ignition, tappet clearance, re-torquing nuts and bolts, and riding it appropriately, etc).




1947 BSA B33


1947 BSA B33. First year for this solid, dependable, OHV single. It's arguably the most handsome of the B-series pre-unit singles and represents what BSA did best, which was to provide fairly ordinary bikes for ordinary guys like us. The Gold Stars and Rocket Threes get the glory. But the B33, and its stablemate B31, kept Britain, and much of the world on the move during the post-war years. Buy reasonably sensibly, and you'll enjoy these motorcycle for many years and tens of thousands of miles.


1954 BSA B33 with headlight cowl


1954 BSA B33. This was the last year of these 499cc rigid-framed singles. Handsome, competent, and reliable, it's hard to see why anyone would buy a modern Royal Enfield when these classic British Beezas ride just as nicely, stop just as well (or very nearly), and appreciate in value. Suffolk classic bike dealer is offering this example for 6,350 (November 2016 price). Check the other side of the bike in the image below.




Specifications 1956 BSA B33 & B31

Engine: 499cc, OHV, air-cooled 4-stroke single (348cc, B31)
Bore & stroke: 85mm x 88mm (71mm x 88mm, B31)
Compression ratio: 6.8:1 (6.5:1, B31)

Carburettor: Amal, 1-1/16th (1-inch, B31)
Maximum power: 23hp @ 5,500rpm (17 @ 5,500rpm)

Transmission: 4-speed

Clutch: Wet, multi-plate

Primary drive: Single row chain, 1/2 x

Final drive: Chain
Front tyre: 3.25 x 19-inch
Rear tyre: 3.50 x 19-inch (3.25 x 19-inch B31)
Forks: Telescopic, external spring, one-way-hydraulically damped

Frame: Swinging arm, twin rear shocks/dampers

Fuel capacity: 3-gallons (13.5 litres)

Oil capacity: 4 imperial pints

Brakes: Ariel full width hub, 7-inch front and rear

Charging system: 60-watt E3L dynamo (later, 60-watt alternator)

Ignition: Lucas MO1/6 magneto (045160 or 045232)

Regulator: Lucas MCR2

Battery: 6-volt
Weight: 374lbs (170kg)
Top speed: 81mph (130kmh)




The 499cc "all iron" BSA B33 arrived in 1947. It was based upon the 1945 348cc B31 which was derived from the pre-WW2 OHV BSA pre-unit singles. Outwardly identical to the B31, the B33 featured a slightly larger carburettor, heavier flywheels and a larger rear tyre.

The first B33s were built around rigid frames. But in 1949, plunger suspension became an option. Five years later, in 1954, swinging arm frames were introduced.

Nobody bought the BSA B31 or B33 for sporting purposes; at least, not if they intended providing serious competition. Rather, these were road-going/commuting bikes, the B33 being especially suited to sidecar work where it acquitted itself well.

Reliability was the intention with these bikes, and that was exactly what BSA delivered. The engines are easily maintained and repaired by the average home mechanic. Parts availability was excellent in their days, and even now (2016) you won't have much trouble keeping one of these on the boil.




The crankshafts are built-up with two full flywheels spinning between a single roller bearing (timing-side), and a single roller and a single ball bearing (drive side). The big-end bearing is a caged double roller. The connecting rod is steel. The tapered crank pin is pressed into the flywheels and secured by rivets.

Lubrication is dry sump fed by a conventional BSA gear-type oil pump.

Starting the B33 (and B31) is simple. Retard. Tickle. Slight throttle. Bring it over TDC (with or without the valve lifter). And press through. A little choke will be needed in sold  weather, and the key part of the kickstart swing is, as ever, is the last third of travel. Just keep the pressure steady, rather than aggressive, and (if the ignition and carburetion is correctly set-up) it should start easily enough.

If not, a million angry kicks won't do it. But a little investigation probably will.

Performance of the B33 is about average for a workaday British single. You can hit 80mph if you're desperate. And you can cruise at around 65mph. But realistically, you won't want to hit anything above 70mph very often, and you'll cruise at 55mph.

The B31 has a similar cruising speed, but you can knock a little off the top end.

If you're regularly riding two-up or pulling a chair, the B33 is the one to go for.

Fuel consumption is as much as 70mpg. Or even 80mpg for the B31. But once again, you're probably looking at a more realistic 60-65mpg.

The handling is above average. The one-way damped forks are never more than adequate. Riding comfort overall isn't bad, and you can cruise all day without too much stress.

Gear changes are slowish, but dependable. The clutch is about medium. Brakes are merely okay (for a British classic), but are never great.

Overall, the BSA B33 is a good starter classic, and even a good finishing classic.

B33 engine parts are harder to come by, but you should be able to source everything pretty quickly. There's still a fair amount of tinware around too. Tip: always check the provenance of parts.

The last B33 were manufactured in 1960. The final bikes (1958-onward) were equipped with alternator electrics and coil ignition. The last B31s were built in 1959.




Copyright Sump Publishing 2016