Finding the trade number of the bulb is sometimes easy - if we can find a NOS package, or by finding an existing original bulb on an original bike. But sometimes we have to try and match an existing, available, trade number to the Harley specs. To do this, we're gonna have to learn something about light bulbs.

Voltage is pretty easy - most bulbs are rated at a slighly higher value, like 6.4, than the nominal 6 or 12 volts. Amperage, the amount of current the bulb draws, is seldom given anywhere but in formal specifications. Wattage (volts x amps) is the usual specification. Candlepower is an ancient term that hasn't been used in 50 years, but our Harley books still use it. Base is a description of how the bulb mounts in its socket. Glass is the shape of the glass envelope. And Filament is a description of how the tungsten "heating element" is constructed.