1951 BMW R51/3
BMW started to offer a 500cc motorcycle after the war beginning in 1950. They dusted off their design from 1938 for the engine, frame and suspension and the pre-war R51 was reborn as the R51/2. BMW redesigned the engine and in 1951 an all new engine was put into the same pre-war plunger frame and the R51/3 was born (see a trend?) An example of this new engine in an old frame is on sale now here on eBay.
From the seller
It has been restored, but needed some parts to finish; proper controls, tail light, a few small parts. I drove it around the back roads of CT over the summer, putting on about 200 miles. Odometer now shows 700, clearly not original miles. The bike starts easily and runs/ idles as it should. Clutch, brakes , transmission are fine. the bike is a pleasure to ride, although not the fastest thing around ( maybe it was in 1951…) The restorer installed alternator/ electronic ignition unit, which is 12 volt. This makes for bright lights and eliminates the magneto.
In conjunction with the alternator, there is a pull-push switch on the bottom of the headlight ( which is otherwise correct), the lights are controlled by the key switch.
From 1951 until 1954, the R51/3 was offered as a sport/single as a companion to the 600cc R67 that was often attached to a sidecar for the family. Performance came from a squared bore and stroke of 68cm and would produce 24hp at 5800rpm and gives you a top speed of 83mph. 18,420 were produced in the four years of production with some changes being made over those years.
The bike offered here on eBay has the half hubs and metal shrouds over the front telescoping fork springs, each original 1951 items. They were replaced with full width hubs and rubber gators by 1954. The mufflers for 1951 would have originally been either the stylized fishtail, or swallow tail muffler, the change to the cigar style in later years mandated by noice restrictions. These later mufflers can be considered original as many early bikes were retrofitted back in the 1950’s when the factory had to comply with bikes still unsold. The rear plunger suspension on this bike are also topped off with a mushroom shaped cap, were later frames had a large threaded nut capping what was considered suspension in the day.
If you were to look at the R51 or R51/2 sitting next to the R51/3, visually the only difference would be the engine. Prior to the new design, the generator sat on the top of the engine, under the gas tank, the R51/3 engine moved this under the front engine cover. The old design also gave each cylinder its own cam shaft to operate the push-rods, and the new /3 engine only has one cam to operate all four valves. The R51/3 even kept the hand shift lever on the transmission that was a hold over from pre-war bikes.
This is a very nice example of the post war BMW and is very collectable when compared to the R50/2 (confused?) that replace this model in 1955 and offered until 1969. If you are already a BMW owner this is likely high on your wish list, but if you are looking for your first BMW, this would not be a bad starting place. BB