Bulb G18.5 (BA15d)

A pretty special type of dual-filament bulb you won’t find at your average gas station. It’s mainly used for older bikes like the Knucklehead or K and V models. Small round glass globe, about 18 mm in diameter, with two contacts for 12 V. One is dim with just 5 W for the tail light, the other bright with 21 W for the brake light. The base isn’t the usual BAY type with two offset pins that only fits one way into the socket — it’s a BA type, which means you can insert it 180° reversed into the bayonet socket. So don’t be too quick to curse after changing the bulb if the tail light suddenly ends up brighter than the brake light. Just flip the bulb around.