| MODEL | BMW R 51 1938 - 1940 |
| Year | 1938 - 1940 |
| Engine Type | Four stroke, two cylinder horizontally opposed boxer, pushrod operated OHV, 2 valves per cylinder |
| Displacement / Capacity | 494 cc / 30.1 cu in |
| Bore X Stroke | 68 x 68 mm |
| Compression Ratio | 6.7:1 |
| Cooling System | Air cooled |
| Max Power | 18 kW / 24 hp @ 5600 rpm |
| Max Torque | — |
| Ignition | Battery ignition |
| Induction | 2 x Amal 5/423 carburettors |
| Start System | Kick |
| TRANSMISSION SPECS | |
| Lubrication | — |
| Gear Box | 4 Speed, foot change with hand auxiliary lever |
| Clutch | Single plate, dry, cable operated |
| Final Drive | Shaft |
| CHASSIS SPECS | |
| Frame | Closed double loop steel tubular frame |
| Front Suspension | Telescopic fork |
| Rear Suspension | Plunger |
| Front Brakes | 200 mm drum |
| Rear Brakes | 200 mm drum |
| DIMENSION SPECS | |
| Overall Length | 2130 mm / 83.9 in |
| Overall Width | 815 mm / 32.1 in |
| Overall Height | 960 mm / 37.8 in |
| Seat Height | 690 mm / 27.2 in |
| Wheelbase | 1400 mm / 55.1 in |
| Ground Clearance | — |
| Dry Weight | — |
| Wet Weight | 182 kg / 400 lbs |
| WHEEL SPECS | |
| Front Tyre | 3.50 - 19 |
| Rear Tyre | 3.50 - 19 |
| FUEL SYSTEM | |
| Fuel Capacity | 14 Litres / 3.7 US gal |
| Fuel Consumption | 4.0 L/100 km / 59 mpg US |
| Top Speed | 140 km/h / 87 mph |
Introduced in 1938 alongside the R 61, R 66 and R 71, the R 51 succeeded the R 5 as BMW's 500 cc sports twin. Its big step was plunger rear suspension, proven in the 1937 International Six Days, added to the telescopic front fork. Civilian production ended with the war, and the design returned in 1950 as the R 51/2.
