The R51RS motor

The engine for the R51RS is heavily based on the R51 motor from 1939. BMW produced basically the same engine from 1936 starting with the R5 to 1940 ending with the R51. In 1950, BMW's first twin cylinder motor was a copy of the R51, denoted as the R51/2. In 1951, the R51/3 was introduced with a completely revised motor. There is an interesting connection between the RS engine and the R51/3 engine which I will get to shortly.

What made the production R5/R51 engine interesting were the twin cams; one cam for the right cylinder and one for the left. The idea was to keep the pushrods short. This reduce the flexing of the pushrods at high RPM. The chain driven R5/R51 produced 24 hp at 5500 RPM.

Cutaway of a R51/2 engine

Photo by Bench Mark Works

For the R51RS engine, the BMW engineers changed how the cam was driven. They replaced the chain driven cams with gear driven cams. I assume the rational was to get more precise and consistent timing between the cams and the crank.

R51RS gear driven cams

The intermediate, floating gear in the middle of the cluster allows the cams to rotate in the same direction as the chain driven cams. Also the alternator on top of the motor is replaced with a magneto.

Note that BMW went to a gear driven cam on the R51/3 in 1951, although with only one cam. This was not the first time a production engine from BMW was gear driven, but all twins from 1951 to 1969 were gear driven. In 1970, with the introduction of the /5 series, BMW went back to chain driven cam.

Ok, so my project. Last year I saw an ad for a R51/2 engine block that was modified to accept a prewar Bosch Magneto, a Bosch Magneto, and a R51RS timing cover.

 Here are some photos that compare the modified R51/2 block with a R51 block. The R51 block is the more corroded block. The main difference is that block is flattened on top to accept the Bosch magneto. Also for this block, the vent holes used on the R51/2 generator has been welded over.

Here is a comparison of timing cover for the R51RS (left) and a R51 (right). The R51RS cover is made of a magnesium alloy, known as Elektron. This alloy was used by many race machines in the prewar era (and maybe after too).

Note that the production timing cover has a location for the points that is driven by the left cam (left as seen when sitting on the bike).

The points for the R51RS is part of the magneto that sits on top of the engine case. 

More on the engine in the next post.