| Feature | Specification |
|---|---|
| Production Years | 1951 – 1954 |
| Engine Type | 494cc Opposed Twin (Boxer) |
| Oil Type | Non-Detergent Mineral SAE 40/20 (1.75 L) |
| Tire Size | F: 3.50 x 19 / R: 3.50 x 19 |
| Electrical System | 6V 7/8 Ah Battery |
| Spark Plug | Bosch W 240 T 1 (Modern: W4AC / NGK B8HS) |
While its predecessor, the R 51/2, was a transitional post-war stopgap using pre-war engine components, the BMW R 51/3 represents a pivotal moment in Bavarian engineering: it was the first BMW boxer to completely abandon the timing chain in favor of a gear-driven camshaft system. This change drastically improved timing precision and reduced mechanical noise, setting a new standard for internal combustion refinement in the early 1950s.
Technical Engineering Analysis
The R 51/3 (Internal code 0252/1) features a 500cc overhead-valve (OHV) engine that produces approximately 24 horsepower. Its design is characterized by the iconic plunger rear suspension and a telescopic front fork. The transition to a single, gear-driven camshaft located above the crankshaft allowed for shorter pushrods, which minimized reciprocating mass and enabled more reliable high-RPM performance compared to its predecessors.
Maintenance Guide & Fluid Requirements
Maintaining an R 51/3 requires an understanding of vintage lubrication needs. Because these engines lack a modern spin-on oil filter and rely on internal ‘oil slingers’ to clean the lubricant, the choice of oil is critical.
- Engine Oil: You must use Non-Detergent Mineral Oil. Detergent oils keep contaminants in suspension, which can clog the oil slingers. Use SAE 40 for summer riding and SAE 20 for winter. The total capacity is 1.75 liters.
- Tires and Pressure: The R 51/3 uses 3.50 x 19 tires on both the front and rear. For optimal handling and sidewall integrity, maintain 1.1 bar (16.0 psi) in the front and 1.3 bar (18.8 psi) in the rear.
Electrical and Ignition Systems
The 6-volt electrical system is powered by a Noris generator. The original 6V 7/8 Ah battery (DIN 007 14) is essential for maintaining the health of the ignition coil. For modern spark plug replacements, the original Bosch W 240 T 1 should be cross-referenced to a Bosch W4AC or an NGK B8HS. Ensure the gap is set to the factory specification for consistent combustion.
Collector’s Note
As a ‘Matching Numbers’ machine, the R 51/3 is highly sought after. Owners should pay close attention to the condition of the aluminum engine casings and the correct Noris ignition components, as these are increasingly difficult to source in original condition. Regular inspection of the oil slingers (requiring a partial engine teardown) is recommended every 30,000 to 50,000 kilometers to prevent catastrophic crankshaft failure.
Frequently Asked Questions (FAQ)
Q: Why can’t I use modern synthetic oil?
A: Modern synthetic oils contain detergents that will loosen decades of sediment and carry it through the oil galleries, potentially clogging the oil slingers and causing engine seizure.
Q: What is the correct spark plug gap?
A: For the NGK B8HS or Bosch W4AC, a gap of 0.6mm (0.024 inches) is generally recommended for the 6V ignition system.