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Bayonet cap light bulb fittings are commonly used on car and other vehicle lamps. On bayonet bulbs, such as BA9 (9mm diameter bayonet cap) and BA15 (15mm diameter bayonet cap), there are two pins that locate into the bulb holder. The pin locations vary according to the pin offset codes so that they are keyed to the bulb holder slots depending on the application. For example, 5/21 watt BAY offset pin bulbs are commonly used in stop and tail or side light and indicator applications, where the pin offset ensures the higher wattage element operates in case of brake/indication. The various codes and their differences are listed below.
NB The bulb may have a single or dual contact base and single or dual filaments. This guide references the bayonet locating pins only.
Same length pins (1mm) at 180o angle with no vertical pin offset.
Same length pins (1mm) at 180o angle with 3.2mm vertical pin offset. Popular fitment for tail/brake bulbs.
Same length pins (1mm) at +150o angle (plus 150) on the second pin - looking at the base) No vertical pin offset.
Same length pins (1mm) Second pin -150o angle (minus 150) with 3.2mm vertical pin offset
Different pin lengths (0.8mm and 2mm) at 180o angle with no vertical pin offset
Same length pins (1mm) with second pin +150o angle (plus 150) and 3.2mm vertical pin offset
