| MODEL | BSA Q21 BLUE STAR 500 1936 |
| Year | 1936 |
| Engine Type | Single-cylinder four-stroke overhead-valve, enclosed valve gear |
| Displacement / Capacity | — |
| Bore X Stroke | 82 mm x 94 mm |
| Compression Ratio | High-compression piston (ratio not given) |
| Cooling System | — |
| Max Power | 4.96 hp (rating) |
| Max Torque | — |
| Ignition | — |
| Induction | — |
| Start System | — |
| TRANSMISSION SPECS | |
| Lubrication | Sump type, 4 pints, pressure feed to big end |
| Gear Box | BSA four-speed, foot change |
| Clutch | Dry plate with cush drive |
| Final Drive | Primary 1/2 x 0.305 in enclosed chain, rear 5/8 x 1/4 in |
| CHASSIS SPECS | |
| Frame | Duplex cradle with forged top member, sidecar lugs |
| Front Suspension | Constant wheelbase fork |
| Rear Suspension | — |
| Front Brakes | 7 x 1 3/8 in, ratchet front brake |
| Rear Brakes | 7 x 1 3/8 in |
| DIMENSION SPECS | |
| Overall Length | — |
| Overall Width | — |
| Overall Height | — |
| Seat Height | 29.25 in |
| Wheelbase | 54 in |
| Ground Clearance | 4 7/8 in |
| Dry Weight | — |
| Wet Weight | — |
| WHEEL SPECS | |
| Front Tyre | Dunlop 3.25-19 |
| Rear Tyre | Dunlop 3.25-19 |
| FUEL SYSTEM | |
| Fuel Capacity | 3.25 Imperial gal |
| Fuel Consumption | — |
| Top Speed | — |
The Q21 Blue Star was the sports 500 of the 1936 range, with special cams, a high-compression piston, heavy valve springs and upswept pipes. It sat on a duplex cradle frame with a tank instrument panel and a steering damper.
