BMW M2B15

The BMW M2B15 was BMW's first flat-twin engine.^[3] Manufactured from 1920 to 1923, the M2B15 was intended to be a portable industrial engine,^[2] but it was used by several German motorcycle manufacturers to power their motorcycles.^[5]

BMW M2 B15
BMW M2B15 at Deutsches Museum
Overview
Manufacturer	BMW
Also called	Bayern-Kleinmotor,[1] Bayern Motor[2]
Production	1920–1923[3]
Layout
Configuration	flat-twin[4]
Displacement	494 cc (30.1 cu in)
Cylinder bore	68 mm (2.7 in)[4]
Piston stroke	68 mm (2.7 in)[4]
Cylinder block material	cast iron[4]
Cylinder head material	cast iron, in unit with block[4]
Valvetrain	side-valve pushrod engine[4]
Combustion
Fuel system	BMW carburettor[1]
Fuel type	petrol
Oil system	wet sump[1]
Cooling system	air-cooled
Output
Power output	6.5 hp (4.8 kW)[2]
Dimensions
Dry weight	31 kg (68 lb)[1]

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.^[4] This was referred to internally as the Type M2B15 and offered for sale officially as the "Bayern Motor".^[2] The engine was tried out by various motorcycle manufacturers. Starting in 1920, Victoria of Nuremberg used the engine^[2] in their KR 1 motorcycle,^[3] 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.^[2][4]