1938 BMW R51
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.[1] 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.
History
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,[2] 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 | |
| Engine | Four-stroke OHV | |
| 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 (30.1 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 | 140 km/h (87 mph) | 150 km/h (93 mph) |
| Curb Weight | 190 kg (419 lb) | |
| Gross Vehicle Weight | 355 kg (783 lb) | |
| Fuel Tank Capacity | 17 L (3.7 imp gal; 4.5 US gal) |
Construction. The R5’s engine, a design that carried over to the R51 platform, was BMW’s first opposed-twin that didn’t use split engine cases; the R254-model 494cc overhead-valve engine utilized a single-piece tunnel-type case, with the crankshaft installed and removed from the front. Timing chains drove the two camshafts positioned slightly above and to each side of the crank, allowing the valve gear in each cylinder head to use shorter tappets and pushrods than previous BMW opposed twins. The valve gear was actuated by rockers moving on needle roller bearings and tensioned with hairpin springs.
Mounted on top of the unit engine case sat the Bosch generator; the distributor and coil were tucked with Teutonic efficiency neatly under the front engine cover. The R5’s 22mm Amal carburetors were positioned on either side of the engine, each feeding its own cylinder head. The 4-speed transmission had its foot-shift linkage on the left side; on the right was a hand-lever used primarily for quickly shifting the transmission into neutral for stops. The positive-stop foot gearshift design was based on a system developed by Englishman Harold Willis in 1928 for Velocette. This basic engine and transmission package remained a part of the R51 design that bowed in 1938.
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Parts for restorations
https://www.bmwclassicmotorcycles.com/parts/models/r51-2-r68/