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The BMW R51/3 was BMW's second post-World War II 500 cc motorcycle, following the briefly produced R51/2. It featured a flat-twin engine and exposed drive shaft. In 1951, the R51/3 succeeded and modernized the 1950 R51/2, which was essentially a pre-war design that was produced after the war. The 600 cc R67, R67/2, and R67/3 series and the more sporting R68 model also followed the R51/2.
Following World War II, Germany was precluded from producing motorcycles of any sort by the Allies. When the ban was lifted, in Allied controlled Western Germany, BMW had to start from scratch. There were no plans, blueprints, or schematic drawings. Company engineers had to use surviving pre-war motorcycles to create new plans. In 1948, it introduced the 250 cc R24, which was essentially a pre-war R23, complete with rigid rear end.
When larger machines were permitted, BMW introduced its R51/2 in 1950, a model that was in production for only one year and that was essentially a pre-war BMW produced after the war. The R51/3 was then introduced in 1951 for a production run of four years.
The 600 cc R67, sister model to the R51/3 and almost identical visually, was also introduced in 1951, but it went through two revisions. The more powerful R67/2 came out one year later and was replaced in 1955 by the R67/3, which was in production through the 1956 model year.
In 1952, BMW introduced the 600 cc R68, which produced 35 hp (26 kW), had a compression ratio of 8.0:1.
Technical data 
|R51/3||R67 - R67/3|
|Design||Boxer Flat Twin|
|Bore||68 mm / 2.68 in||72 mm / 2.83 in|
|Stroke||68 mm / 2.68 in||73 mm / 2.87 in|
|Displacement||494 cc (30.1 cu in)||594 cc (36.2 cu in)|
|Power||24 hp (18 kW) @ 5800 rpm||26 hp (19 kW) / 28 hp (21 kW) @ 5500 rpm|
|Compression Ratio||6.3:1||5.6:1 / 6.5:1|
|Top Speed||87 mph (140 km/h)||93 mph (150 km/h)|
|Curb Weight||419 lb (190 kg)|
|Gross Vehicle Weight||783 lb (355 kg)|
|Fuel Tank Capacity||17 L (3.7 imp gal; 4.5 US gal)|
See also 
Media related to BMW R51 at Wikimedia Commons