The first part of this article covers the basics for this 12 volt VR unit, as used on BMW Airhead motorcycles before BMW changed to electronic ignition in 1981. This article applies ONLY to the mechanical voltage regulators. These were used on the Airheads from 1970 to ~1980. At the end of this article are some addendums. #1 is a description and schematic diagram.  #2 is for the BMW /2 (6 volt) motorcycle's VR. I suggest you DO read that section, even if you do not have a /2, as it has a slightly different way of explaining the servicing (included in this article). Voltage regulators, both mechanical & electronic, in metal cans, were installed as stock items on early BMW Airhead motorcycles. There is nothing wrong with the mechanical type for the early BMW Airhead Motorcycles, except that mechanical types DO wear, over time & mileage. In an emergency an old mechanical VR might do OK in any Airhead model, although the electronic ignition could possibly (not necessarily will) miss-fire occasionaly with a mechanical regulator. Mechanical regulators wear mostly from their constant points vibrating/buzzing/arcing. Electronic regulators will substitute for the mechanical regulator on any Airhead model, directly, with no problems. All the mechanical regulators were inside of tall metal cans. Later voltage regulators were electronic; but the early electronics versions ALSO were in metal cans, but they were shorter in height.  The next series were in smallish plastic cases, often with black bodies and red tops. There are aftermarket electronic regulators available, some are adjustable, & most 3 male spades type of voltage regulators from European cars (that the Airhead cable plug fits into) could also be substituted. The stock METAL can electronic regulators are also adjustable, there is an article on this Airheads website about how to go about that, since the adjustment is sealed. There is also a bit more of an article here:  http://bmwmotorcycletech.info/boschelreg.htm