🏷️ Basic Information

Make:

BMW

Model:

R51, R61, R66, R71

Engine Codes:

M51 (R51, R61), M66 (R66), M71 (R71)

👋 Introduction

Historical Context:

These BMW models represent a significant era for BMW motorcycles, bridging the gap between pre-war and post-war designs. They were known for their robust engineering and reliable performance, often used for touring and military applications.

Era Competition:

Competed against other European motorcycle manufacturers known for their engineering prowess, such as Triumph, Norton, and various Italian brands, establishing BMW's reputation for quality and durability.

Model Strengths:

Characterized by their flat-twin (boxer) engines, shaft drive, and robust chassis construction. The R71, in particular, gained notoriety for its use by the German Wehrmacht during WWII.

Manual Value Proposition:

This manual is indispensable for owners and restorers of these classic BMW motorcycles, providing the authoritative information needed to maintain, repair, and preserve these iconic machines.

📋 Manual Details

Systems Covered:

Engine, Transmission, Drivetrain, Chassis, Electrical System, Fuel System, Cooling System, Lubrication, Brakes, Suspension, Steering, Bodywork, Troubleshooting, Maintenance

Pages:

Varies by specific manual, typically 200-400 pages for a comprehensive factory service manual.

Type:

Factory Service Repair Manual

Language:

English (primary), potential for German or other languages depending on the original publication.

Format:

Likely a scanned reproduction of the original factory manual. May be available as a searchable PDF or physical reproduction.

📖 Manual Coverage

Engine Systems:

Detailed coverage of engine assembly, disassembly, inspection, and repair for all specified models. Includes cylinder head, pistons, crankshaft, camshafts, valve train, ignition system (timing and components), carburetor adjustment and overhaul, and cooling/lubrication systems.

Transmission Drivetrain:

Comprehensive instructions for gearbox overhaul, clutch service, drive shaft maintenance, and final drive inspection. Covers gear selection mechanisms, lubrication, and common wear points.

Electrical System:

Includes wiring diagrams for all models, troubleshooting guides for lighting, ignition, and charging systems. Details on generator, starter motor, battery maintenance, and component testing.

Suspension Steering:

Detailed procedures for front forks (telescopic or Earles fork depending on model variant), rear suspension (swingarm, shocks), steering head bearing adjustment, and wheel bearing maintenance. Includes specifications for lubrication and seal replacement.

Chassis Frame:

Inspection of frame welds, alignment checks, and procedures for replacing wear items like bushings and mounts. Guidance on handling potential frame damage or modifications.

Bodywork Trim:

Covers the disassembly and reassembly of fuel tank, fenders, seat, handlebars, and other body panels. Includes mounting hardware specifications and paint/finish touch-up advice.

Maintenance Procedures:

Scheduled maintenance intervals, oil change procedures, filter replacement, ignition point adjustment, carburetor synchronization, valve clearance checks, and general lubrication points.

Repair Procedures:

Step-by-step instructions for component removal, disassembly, cleaning, inspection, repair, reassembly, and installation. Covers engine rebuilds, transmission overhauls, brake system repairs, and electrical diagnostics.

Troubleshooting:

Diagnostic flowcharts and symptom-based troubleshooting guides for common problems related to engine performance, starting issues, electrical faults, and transmission operation.

Diagnostic Procedures:

Systematic diagnostic approaches for engine misfires, abnormal noises, electrical faults, and fuel system issues. Includes troubleshooting charts and expected component readings.

Special Tools:

Lists and descriptions of special tools required for specific tasks, such as valve spring compressors, flywheel pullers, and specialized socket sets. Manual may provide dimensions for fabricating homemade tools.

Depth:

Factory-level detail suitable for professional mechanics and experienced enthusiasts. Includes precise torque specifications, clearance measurements, and diagnostic procedures. Covers both routine maintenance and complete mechanical overhauls.

Electrical Diagrams:

Includes detailed schematic diagrams for all electrical circuits, color-coded for easy identification of wires and components. Essential for troubleshooting electrical issues.

📊 Manual Specifications

Digital Format Details:

Searchable PDF with optical character recognition (OCR) for easy text retrieval. High-resolution scans ensure legibility of diagrams and illustrations. May include bookmarking for quick navigation.

Digital File Size:

Typically ranges from 50MB to 200MB depending on scan quality and inclusion of supplementary documents.

Print Quality:

If offered in print, expect high-quality reproduction on durable paper stock. Clear printing of diagrams, schematics, and text. Binding designed for frequent use in a workshop environment.

Binding Type:

For printed versions, often a comb binding, spiral binding, or lay-flat perfect binding for workshop convenience.

Paper Type:

For printed versions, typically a matte or semi-gloss paper suitable for writing notes and resistant to workshop grime.

⚙️ Engine Specifications

R51/R61 Engine:

M51, air-cooled, four-stroke, OHV flat-twin (boxer). Displacement: 494cc (R51), 594cc (R61). Bore x Stroke: 68mm x 68mm (R51), 66mm x 88mm (R61). Compression Ratio: Approximately 6.0:1.

R66 Engine:

M66, air-cooled, four-stroke, OHV flat-twin (boxer). Displacement: 594cc. Bore x Stroke: 66mm x 88mm. Compression Ratio: Approximately 6.0:1.

R71 Engine:

M71, air-cooled, four-stroke, OHV flat-twin (boxer). Displacement: 745cc. Bore x Stroke: 78mm x 78mm. Compression Ratio: Approximately 5.5:1.

R51 Power Torque:

R51: Approx. 24-26 hp @ 5800 rpm. R61: Approx. 26 hp @ 5000 rpm. R66: Approx. 30 hp @ 5000 rpm. R71: Approx. 22 hp @ 4000 rpm.

R66 Power Torque:

Approx. 30 hp @ 5000 rpm. Torque figures not typically published in the same detailed manner as modern vehicles but indicative of strong mid-range power.

R71 Power Torque:

Approx. 22 hp @ 4000 rpm. Designed for torque and reliability over outright speed.

Carburetion:

Typically equipped with dual Bing or Amal carburetors (size and model vary by specific model and year). Manual details overhaul, adjustment, and synchronization procedures.

🔧 Transmission

Gearbox Type:

4-speed manual constant mesh gearbox.

Sidecar Drive:

Some models were available with provisions for or factory-fitted with sidecar drive shafts.

Shaft Drive:

Shaft final drive to the rear wheel, a hallmark of BMW motorcycles, requiring minimal maintenance compared to chain drives.

📐 Dimensions

Wheelbase:

Varies by model, typically around 1400-1450 mm (55-57 inches).

Overall Length:

Varies by model, approximately 2100-2200 mm (82-86 inches).

Fuel Tank Capacity:

Typically around 15-18 liters (4-4.7 US gallons).

Dry Weight:

Varies by model, typically in the range of 170-200 kg (375-440 lbs).

🛡️ Safety Features

Crash Safety Era:

No formal crash ratings as per modern standards. Safety was primarily dependent on robust mechanical design and rider skill. Manual will cover brake system maintenance for optimal performance.

Braking System:

Typically featured drum brakes on both front and rear wheels. Manual provides detailed procedures for brake shoe replacement, drum inspection, and hydraulic system (if applicable on later models or conversions) maintenance.

Frame Construction:

Robust steel tube frame providing structural integrity. Manual will include frame inspection and potential repair guidelines.

💡 Technology Features

Infotainment System:

These models predate modern infotainment systems. Features would be limited to basic instrumentation (speedometer, ammeter, oil pressure light).

Driver Assistance Features:

None. Focus is on mechanical systems and rider input.

Connectivity Options:

No electronic connectivity features. Communication would be through the rider.

🔧 Maintenance

Oil Change Interval:

Typically every 2,000-3,000 km (1,200-1,800 miles) or annually. Manual will specify exact intervals based on operating conditions.

Valve Adjustment Interval:

Recommended every 5,000-10,000 km (3,000-6,000 miles). Manual provides detailed procedure and specifications.

Engine Oil Type:

Typically SAE 20W-50 or SAE 30 mineral oil. Manual will list exact BMW recommendations or acceptable SAE grades. Emphasis on suitable viscosity for air-cooled engines.

Gearbox Oil Type:

Gear oil with a viscosity rating of SAE 90 EP (Extreme Pressure) is generally recommended. Manual will specify capacity and exact type.

Final Drive Oil Type:

Similar to gearbox oil, SAE 90 EP is typical. Manual will detail capacity and specifications.

Brake Fluid Type:

For drum brakes, no hydraulic fluid is used in the primary brake system. If hydraulic clutch or other systems were retrofitted, manual may cover those.

Common Wear Points:

Valve guides, piston rings, gearbox synchros (on later models with enclosed shift forks), driveshaft U-joints (if equipped), and charging system components (generator brushes).

Ignition System Quirks:

Contact breaker points require regular cleaning and adjustment. Distributor cap and rotor inspection is crucial. Manual provides detailed troubleshooting for ignition timing.

Carburetor Issues:

Float levels, jetting, and synchronization can affect performance. Manual details the complete overhaul and tuning process.

📚 Vehicle History

Bmw Motorcycle Legacy:

BMW has a long and storied history in motorcycle manufacturing, dating back to the 1920s. These models are part of the formative years that built BMW's reputation for quality, innovation, and durability.

R Series Development:

The R51 (1938-1940) was an evolution of earlier models, followed by the R61 (1938-1941), R66 (1938-1941), and the R71 (1938-1941). These models shared many design principles but differed in displacement and specific features. Post-war models like the R51/2 and R51/3 built upon this foundation.

Technical Advancements Era:

These models showcased BMW's commitment to innovation with features like shaft drive and their robust boxer engine design, setting standards for durability and rider comfort.

Production Years:

R51: 1938-1940; R61: 1938-1941; R66: 1938-1941; R71: 1938-1941. Production was significantly impacted by World War II.

Wartime Significance:

The R71 was notably adapted for military use and served as a basis for the Soviet IMZ-Ural and KMZ-Dnepr motorcycles.

🏆 Awards & Recognition

Historical Performance Awards:

While specific awards for these models are rare in modern terms, their design and reliability earned them significant recognition in endurance events and military service, contributing to BMW's strong reputation.