BMW R 51/3
From BMW Tech
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The BMW R 51/3 was BMW's first modern postwar 500 cc motorcycle. It used BMW's time-tested Flat twin layout and exposed driveshaft. The R 51/3 succeeded and modernized the 1950 R 51/2, which was essentially a prewar design that was produced after the war.
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 Aktiengesellschaft had to start from scratch. There were no plans, blueprints, or schematic drawings. Company engineers had to use surviving prewar motorcycles to create new plans. In 1948, it introduced the 250 cc R 24, which was essentially a pre-war R 23, complete with rigid rear end.
When larger machines were permitted, BMW introduced its R 51/2  in 1950, a model that was in production for only one year and that was essentially a prewar BMW produced after the war. The R 51/3 was then introduced in 1951 for a production run of four years.
The 600 cc R 67, sister model to the R 51/3 and almost identical visually, was also introduced in 1951, but it went through two revisions. The R 67/2, with 2 hp (2 kW) more, came out one year later and was replaced in 1955 by the R 67/3, which was in production through the 1956 model year.
|R 51/3||R 67 - R 67/3|
|Engine||Four-stroke Overhead valve|
|Model Years||1951 - 1955||1951 - 1956|
|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 in³||594 cc / 36 in³|
|Power||24 hp||26 hp / 28 hp|
|Compression Ratio||1:6.3||1:5.6 / 1:6.5|
|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||4.5 US gallons (17 litres)|
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