BSA A50 Cyclone/Royal Star


Make Model

BSA A50 Cyclone/Royal Star


1962 - 70


Four stroke, twin cylinder


498 cc  / 30.4 cub in.
Bore x Stroke 65,5 x 74 mm
Cooling System Air cooled
Compression Ratio 9.0:1
Lubrication Dry sump
Oil Capacity 3 L / 6.3 US pints
Carburetors Dual, 1 1/16 in carburetors
Exhaust Twin, stainless steel


Spark Plug Champion N4, 0.025 in. gap
Starting Kick start

Max Power

24.3 kW / 33 hp @ 5800 rpm
Clutch Multi-plate


Final Drive Chain
Frame Tubular pipe, welded, double cradle with swingarm

Front Suspension

Telescopic forks

Rear Suspension

Double coil over

Front Brakes

Drum, 8 in.

Rear Brakes

Drum, 7 in.
Wheels Steel, wire spoke

Front Tyre

3.25 x 19 in., ribbed

Rear Tyre

4.00 x 18 in. K70


Length:  2100 mm / 81 in.

Height:  Western: 1117 mm / 44 in. / Standard: 1028 mm / 40.5 in

Wheelbase 1391 mm / 54.8 in.
Seat Height 840 mm / 33 in.

Dry Weight

141 kg / 311 lbs
Wet Weight 177 kg / 391 lbs (4 gal tank)

Fuel Capacity 

18.2 L / 4.0 Imp gal

7.57 L / 2 US gal

The decision by Lucas in the late 1950s to switch production of motorcycle electrical components (from magneto/dynamo systems to alternators/coil systems) forced British motorcycle manufacturers to completely redesign their engines. Triumph and BSA took the opportunity to move from pre-unit and semi-unit construction to full unit construction, that is, with the engine and gearbox together in an integrated casting. At the same time, Bob Fearon, Managing Director and General Manager of BSA recognised the need for an updated look that built on the best features of the A10 but which would succeed in the potentially lucrative but competitive United States market. Working with Chief Development Engineer Bert Perrigo they developed the unit construction Star twins. Launched in 1962 the as the 500 cc BSA Star and later the more sporty Royal Star The model was widely exported to the USA and Australia as well as becoming a top seller in the UK in the early 1960s.

The Royal Star had a relatively small 1" single Amal monobloc carburetor, and later Concentric carburetor, but in 1964 new 8.5:1 pistons and a new gearbox improved performance. The machine had a top speed of about 90 mph and was relatively free of vibration. Sharing common engine and cycle parts with the larger BSA A65 Star 650 twin, it was an over-engineered machine and proved very robust. The only major fault with the engine design was a plain crankshaft bearing on the timing side which, when worn, would cause a drop in oil pressure. In 1965, BSA addressed the bearing issues with a roller bearing on the drive side, as it had been on the pre-unit engines, with an improved bronze bushing on the timing side.

The Cyclone was the same basic machine as the Royal Star except fitted for high performance. The A50CC Cyclone Competition built for the US market model year 1964 had dual 1 1/16" carburetors, higher 10.5:1 engine compression, magneto ignition, larger front brake, 2 gallon fuel tank and upswept open exhaust.

Scramblers had no road lights. The A50C Cyclone Road built for the US market model years 1964 and 1965 had 1 1/16" dual carburetors, higher 9:1 compression engine and larger front brake. This model had battery ignition, full road lighting and the same 2 gallon fuel tank. Both models were available for the UK market with 9:1 engine compression, battery ignition and 4 gallon fuel tanks.