Make Model



1971 - 73
Production 5700 units


Four stroke, single cylinder, OHV, 2 valve


499 cc / 30.5 cub in.
Bore x Stroke 84 x 90 mm


 Amal R930/62

Cooling System Air cooled
Compression Ratio 10.0:1
Lubrication Wet sump
Engine Oil SAE 20W/50
Oil Capacity 3.4 L / 6 pints / 0.9 US gal
Exhaust Single, chrome plated


Lucas RM21
Spark Plug Champion N4
Battery 12V
Starting Kick

Max Power

25 kW / 34 hp @ 6200 rpm

Max Torque

37.9 Nm / 3.86 kgf-m / 28 ft/lb @ 5000 rpm
Clutch Multi-plate, wet, cable operated


4 Speed
Final Drive Chain
Frame Welded frame with larger diameter top tube for strength

Front Suspension

Telescopic forks

Rear Suspension

Swing arm, 2 shocks wiith dampers

Front Brakes

8 in. drum, leading shoewith snail cam shoe adjustment

Rear Brakes

7 in. drum with floating cams
Wheels Steel, spokes

Front Tyre

3.00 x 20 in. Trials

Rear Tyre

4.00 x 19 in. Trials


Length: 2159 mm / 85.0 in.
Width: 737 mm / 29.0 in.
Height: 1105 mm / 43.5 in
Wheelbase 1372 mm / 54 in.

Ground Clearance

191 mm / 7.5 in.

Seat Height 813 mm / 32 in.

Dry Weight

 135.0 kg / 298 lbs

Wet Weight 184 kg / 406 lbs

Fuel Capacity 

UK: 18.2 L / 4.8 US gal

US:   9.1 L / 2.4 US gal

Standing 0 - 100 km/h / 62 mph 7.4 sec (B50SS model)

Top Speed

145 km/h / 90 mph



Of the 5,700 or so that were built, survival rates are low. Which is partly why today the B50 is as rare as a priest with a clean conscience. It’s not the easiest bike to live with. And it needs a good dose of retro-engineering to get the very best from it. But you can be sure that ownership of one will, at least, never be dull.

But was it ever worth the hallowed Gold Star name that was bestowed upon it? It’s a debate that still rages.

Overall, these are great little BSA singles. You wouldn’t want to ride two-up on one—unless most of your journeys were shorter rather than longer. But as a solo mount, it’s a stylish thumper with a hidden chunk of lead in its glove.

Remember to duck.

BSA B50 tips

1. The unit single crankcases were originally designed for a 250cc engine. Accordingly, the B50 pressures beneath the piston are considerable and put a huge load on gaskets and seals. A timed breather is vital to keep the crankcase at negative, or at least neutral, pressure. Well-fitting mating surfaces are essential if you want to keep the oil in rather than out.

2. Poor handling is often attributed to the swinging arm bearings. These pivot on needle rollers and need plenty of grease to avoid seizure. Raise the bike and check for play. Replacement is straightforward.

3. The extra torque of the B50 means that clutches are prone to slippage and so need need to be kept in tip-top shape. Solutions include an extra friction plate, an extra steel plate, stronger clutch springs, and an alloy pressure plate.

4. The bore can be opened up to 90mm giving 572cc for extra stomp. You’ll also need to gas flow and fit a larger (32mm) carb.

5. Ignition coils were always weak leading to poor starting, irregular tickover, and misfires. Fit the best quality replacements you can afford along with a new condenser and quality plug lead.

6. Whilst good enough for general use, high performance B50s will require an upgraded crankpin such as Alpha. A Carillo conrod is also highly recommended.

7. Rocker boxes are prone to leakage which can be sorted by careful linishing.

8. Poor crankcase scavenging seems to affect some bikes more than others, and is said to be due sometimes to mismatched sump covers blocking oil pick-up holes.