Historical Significance:
These models represent a pivotal era in BMW's motorcycle manufacturing, renowned for their robust engineering and off-road capabilities.
Predecessors Successors:
Built upon the success of earlier BMW flat-twin designs, these models set the standard for military and civilian use, influencing subsequent BMW R-series.
Engine Layout:
Air-cooled, horizontally opposed twin-cylinder (flat-twin or boxer) engine.
Drivetrain Type:
Shaft drive, eliminating the need for chain maintenance.
Suspension Design:
Leading-link telescopic fork (R51, R61, R66) or Earles fork (R71 and later) with rear plunger or swingarm suspension.
Engine:
Detailed overhaul, tuning, and repair procedures for the BMW flat-twin boxer engines.
Transmission:
Disassembly, inspection, and repair of the four-speed gearbox.
Drivetrain:
Shaft drive system, universal joint, and rear wheel hub maintenance and repair.
Suspension:
Front fork (leading link on most models) and rear plunger/swingarm suspension servicing.
Brakes:
Drum brake system inspection, adjustment, and component replacement.
Electrical:
Ignition system (magneto/battery-coil), lighting, charging system (dynamo), and wiring diagrams.
Fuel System:
Carburetor (e.g., Bing, Amal) disassembly, cleaning, tuning, and fuel tank/lines.
Frame Bodywork:
Frame inspection, seat, fenders, fuel tank, and handlebar assembly.
Routine Maintenance:
Oil changes, spark plug replacement, air filter cleaning, chain adjustment (if applicable), lubrication points.
Troubleshooting:
Diagnostic guides for common starting, running, and electrical issues.
Engine Overhaul:
Piston, ring, cylinder, crankshaft, bearing, valve train, and camshaft replacement/reconditioning.
Transmission Repair:
Gearset inspection, shaft replacement, and clutch assembly overhaul.
Electrical Diagnostics:
Testing ignition timing, dynamo output, and troubleshooting lighting circuits.
Level:
Comprehensive, covering from basic maintenance to complete engine and gearbox rebuilds.
Diagnostic Procedures:
Includes methods for diagnosing common performance issues, ignition timing problems, and carburetor malfunctions.
Engine Rebuilding:
Detailed instructions for crankshaft balancing, connecting rod replacement, piston ring gapping, and cylinder honing.
Torque Specifications:
Provides essential torque values for critical engine and chassis fasteners.
Special Tools:
Lists or implies the use of specialized BMW tools for certain repair procedures.
Digital Format:
Searchable PDF with clear, legible text and diagrams.
Diagram Clarity:
High-resolution schematics and exploded views.
Physical Format Typical:
Comb-bound or stapled (for printed versions, not applicable to this digital product).
Digital Storage:
Accessible on computers, tablets, and smartphones.
Engine Type:
BMW Flat-Twin Boxer, Four-stroke
Bore Stroke R51:
68 mm x 68 mm (R51)
Bore Stroke R61:
68 mm x 68 mm (R61)
Bore Stroke R66:
72 mm x 74 mm (R66)
Bore Stroke R71:
71 mm x 74 mm (R71)
Displacement R51:
494 cc (R51)
Displacement R61:
597 cc (R61)
Displacement R66:
676 cc (R66)
Displacement R71:
707 cc (R71)
Horsepower R51:
Approx. 18 HP @ 4500 rpm (R51)
Horsepower R61:
Approx. 20 HP @ 4500 rpm (R61)
Horsepower R66:
Approx. 26 HP @ 4700 rpm (R66)
Horsepower R71:
Approx. 22 HP @ 4200 rpm (R71)
Carburetion:
Single Bing or Amal carburetor (model dependent), manual choke.
Fuel Type:
Leaded gasoline (suitable octane rating as per era).
Oil Change Recommendation:
Every 1,000-1,500 km or annually, using a suitable mineral oil.
Valve Adjustment:
As needed, typically during major engine service or if excessive tappet noise is present.
Engine Oil Type:
SAE 30 or SAE 20W-50 mineral oil (as per era specifications and workshop manual guidance).
Gearbox Oil Type:
GL-4 gear oil or specific BMW gearbox oil (e.g., SAE 90).
Final Drive Oil Type:
GL-4 gear oil or specific BMW final drive oil (e.g., SAE 90).
Carburetor Tuning:
Periodic adjustment and cleaning of older Bing/Amal carburetors is essential for optimal running.
Electrical Connections:
Corrosion and wear on older wiring and connectors can lead to intermittent electrical faults.
Engine Seals:
Aging crank seals and valve stem seals can cause oil leaks or consumption.
Brake Adjustment:
Drum brakes require regular adjustment to maintain effective stopping power as linings wear.
Founding Era:
BMW Motorrad, established in 1916, has a long history of aviation and automotive engineering innovation.
R Series Development:
These models are descendants of the R32 (1923), BMW's first motorcycle, evolving the flat-twin boxer concept.
R51 Era:
Produced from 1938 to 1940, known for its refined design and performance for the time.
R61 Era:
Produced from 1938 to 1940, a slightly larger displacement touring variant.
R66 Era:
Produced from 1938 to 1941, the most powerful of this series, offering sportier performance.
R71 Era:
Produced from 1938 to 1941, this model was extensively used by the German military (Wehrmacht) and formed the basis for Soviet M-72 production.
Production Period:
Late 1930s to early 1940s, largely interrupted by World War II.
Manufacturing Location:
Berlin-Spandau, Germany.
