Let's go back to the year 1920. During the first world war, BMW had developed into a considerable factory for the production of aircraft engines, with even new works buildings and a qualified work-force. The ban placed by the allies after the war put an end to that. In their hour of need, Max Friz, who was actually an aviation engineer, with assistance from the younger, very talented Martin Stolle, converted the M2B15, his former Douglas-engine. At that time Great Britain was considered to be the leading motorcycle manufacturer.
It was a side-valve engine with the cam placed over the crankshaft. The valves were directly driven and the valve-drive was not encased. The adjustment of the valve play was particularly simple. An off-centre mounted carburettor supplied both cylinders with the fuel-air mixture. As can be seen in picture 3, the oil pump was driven by a worm-drive from the crankshaft.
Here you can see what the engine looked like when it was installed. The same as in the Douglas, the engine was transversely mounted, so that the rear wheel could be driven either by a belt or a chain. Because they never had their own motorcycle, the motor was supplied for Victoria and also other motorcycle manufacturers. Below you can see the engine mounted in a Bison.
Of course, supplying other manufacturers was not enough to keep the enormous factory going. This was the reason why BMW temporarily, made air-brakes for the railways. In the meantime, Stolle had left the company, because he wasn't able to convince Friz of his plan to convert the side-valve- into an OHC engine. His plans were then realised at Victoria, thus successfully becoming a competitor for BMW.
In 1922, BMW separated itself from the brake-manufacturing field and now needed a complete product of their own. Together with the new location, they acquired the bankruptcy assets for the light-motorcycle, the Flink and also for the stronger Helios. The latter was now fitted with the BMW-engine (see picture), this however, turned out to be virtually unsellable. The breakthrough was only achieved with the introduction of the R 32 shown below. In this case, the engine was straight-mounted and the rear wheel was driven by a shaft and bevel gear (final drive). 04/15