| BMW M2 B15 | |
| Manufacturer: | BMW |
| Aka: | Bayern-Kleinmotor, Bayern Motor |
| Production: | 1920 - 1923 |
| Configuration: | flat-twin |
| Displacement: | 494cc |
| Bore: | 68mm |
| Stroke: | 68mm |
| Block: | cast iron |
| Head: | cast iron, in unit with block |
| Valvetrain: | side-valve pushrod engine |
| Fuelsystem: | BMW carburettor |
| Fueltype: | petrol |
| Oilsystem: | wet sump[1] |
| Coolingsystem: | air-cooled |
| Power: | 6.5hp |
| Weight: | 31kg (68lb) |
The BMW M2B15 was BMW's first flat-twin engine.[2] Manufactured from 1920 to 1923, the M2B15 was intended to be a portable industrial engine, but it was used by several German motorcycle manufacturers to power their motorcycles.
In 1920, BMW engineer Max Friz reverse-engineered the engine of foreman Martin Stolle's 1914 Douglas motorcycle and developed a similar 500 cc side-valve flat engine from it.[3] This was referred to internally as the Type M2B15 and offered for sale officially as the "Bayern Motor".[4] The engine was tried out by various motorcycle manufacturers. Starting in 1920, Victoria of Nuremberg used the engine in their KR 1 motorcycle, and other manufacturers such as SMW and Bison also fitted it.[5]
Bayerische Flugzeugwerke used the M2B15 engine in their Helios motorcycle. BMW inherited the Helios when it was merged with BFW in 1922.