BMW R 51/3 
| Make Model | BMW R 51/3 | | Year | 1951-55 | | Engine | Air cooled, four stroke, two cylinder horizontally opposed Boxer, pushrod operated 2 valves per cylinder. | | Capacity | 494 | | Bore x Stroke | 68 x 68 mm | | Compression Ratio | 6.3:1 | | Induction | 2x Bing 1/22/41 - 1/22/42 or 1/22/61 - 1/22/62 | | Ignition / Starting | Bosch coil with manual advance by the handlebar lever / Kick | | Clutch | Single plate, dry | | Max Power | 24 hp @ 5800 rpm | | Transmission / Drive | 4 Speed / Shaft | | Gear Ratio | 1st 4.0 2nd 2.28 3rd 1.7 4th 1.3 | | Frame | Double loop steel tubular frame | | Front Suspension | Telescopic front fork with hydraulic damping; plunger type | | Rear Suspension | Spring with coil compression springs and rubber deflection stops | | Front Brakes | 200mm drum duplex full hub | | Rear Brakes | 200mm drum simplex full hub | | Front Tyre | Metzeler 3.50 -19 | | Rear Tyre | Metzeler 3.50 -19 | | Wet-Weight | 190 kg / 418 lb | | Fuel Capacity | 17 Litres / 4.4 gal | 
At the Amsterdam Show in February 1951, BMW had two motorcycle models with new engines on display: the 500-cc R 51/3 and the 600-cc R 67.
In contrast to the first post-war flat twin with its two chain-driven camshafts, there was now a single central camshaft driven by gearwheels from the crankshaft. The engines were handsome designs with smooth surfaces and one-piece valve covers. Placing the magneto and the generator behind the front cover made the engines more compact. The new Noris magneto ignition with automatic advance and retard enabled the engine to run particularly smoothly.
The frame retained the familiar suspension designs dating from 1938, with a telescopic fork at the front and plungers at the rear. The half-width 200-mm diameter brake hubs also came from the earlier models, but were uprated.
In their first test reports, the motorcycling press praised the results achieved by BMW's engineers, particularly the smoothness of the engines and the bikes' excellent handling. When entered for the Six Day Trial in 1951, the bikes came through this first sporting challenge with flying colors.
In 1952 the leading and trailing shoe brake was replaced by a two leading-shoe pattern, and from 1953 on rubber gaiters were fitted to the telescopic forms in place of the previous metal protective sleeves. The most important innovations for the 1954 model year were full-width hub brakes and new light-alloy wheel rims instead of the previous steel ones with their two-colour paint finish. The fishtail silencers were also replaced by a less complex cigar-shaped pattern.
Whereas the R 67 was conceived with sidecar enthusiasts in mind, the R 51/3 was aimed primarily at the sporting solo rider, though it also proved its worth on more lengthy tours and with a sidecar. Source bmwdean.mapdv.com 
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