| MODEL | BSA 4.5 HP 557 SINGLE 1920 - 1921 |
| Year | 1920 - 1921 |
| Engine Type | Single-cylinder four-stroke side-valve |
| Displacement / Capacity | 557 cc |
| Bore X Stroke | 85 mm x 98 mm |
| Compression Ratio | — |
| Cooling System | — |
| Max Power | 4.5 hp (rating) |
| Max Torque | — |
| Ignition | M-L chain-driven magneto |
| Induction | — |
| Start System | — |
| TRANSMISSION SPECS | |
| Lubrication | Drip feed |
| Gear Box | Three-speed constant mesh |
| Clutch | Mounted on primary shaft |
| Final Drive | Chain |
| CHASSIS SPECS | |
| Frame | — |
| Front Suspension | Improved fork |
| Rear Suspension | — |
| Front Brakes | — |
| Rear Brakes | — |
| DIMENSION SPECS | |
| Overall Length | — |
| Overall Width | — |
| Overall Height | — |
| Seat Height | — |
| Wheelbase | — |
| Ground Clearance | — |
| Dry Weight | — |
| Wet Weight | — |
| WHEEL SPECS | |
| Front Tyre | 26 x 2.5 in |
| Rear Tyre | 26 x 2.5 in |
| FUEL SYSTEM | |
| Fuel Capacity | — |
| Fuel Consumption | — |
| Top Speed | — |
The 4.5 hp 557 cc single was the staple big BSA single of 1920 and 1921, a three-speed machine with chain drive and a decompressor. It sat alongside the new 770 cc twin in the range.
