On the 1948 BSA WM20, the underside of the original Lucas 4-position lighting switch shows quite a bit of corrosion and pitting on the brass contacts. When riding the bike previously, it took a bit of wiggling the switch to get good contact and make the lighting system work. With the rotor removed, it is easy to dress these posts with fine emory cloth. A line from the film "Pulp Fiction" keeps going through my head as I work on these small details: "its the little differences." The small details that mean the difference between lighting at night or not.