From BMW Tech
Airheads are the traditional BMW motorcycles with a Flat-twin Air cooling engine (including the BMW 247 engine) that were built from 1923 to 1995. The airhead engine was replaced by the oilhead, also a flat-twin, which includes a number of design changes including partial Oil cooling.
While the term Airheads provides description of the Type 247 engine produced by BMW until 1995, the term can also describe someone who rides these machines. Airheads are a collective group of people with a shared passion for these machines. There are significant airhead communities which share knowledge about the motorcycles and how to maintain them. As of 2006, there are approximately 8,000 current members of the international Airheads Beemer Club with about 3,000 active members.
Several shops and businesses have grown nationally and internationally to support Airhead BMWs.