GMC B6000 front and rear turn signal bulbs both utilize the 1157 bulb size for proper operation. The replacement chart covers vehicle production spanning 1980 to 1991.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC B6000 uses the 1157 bulb for both the front and rear turn signals across all model years listed in the specifications, from 1980 through 1991.
The 1157 is an ANSI-standard bulb recognized in both the United States and Canada, operating at 12 volts with a dual-filament configuration rated at 21W and 5W.
The dual-filament design of the 1157 allows it to serve more than one lighting function within a single housing, with the higher-wattage filament typically handling the turn signal function and the lower-wattage filament handling a secondary function such as a running light.
The 1157 uses a BAY15d bayonet base, which is an offset twin-pin mount. The offset pin placement is a deliberate fitment feature that prevents the bulb from being inserted in the wrong orientation.
For the GMC B6000, the 1981-1991 model years carry a Medium Duty designation alongside the 1157 bulb size for both front and rear turn signals, while the 1980 model year lists the 1157 without that designation.
The Medium Duty designation in the specifications refers to the vehicle's weight class rather than a variation of the bulb itself. The 1157 bulb type remains consistent regardless of this classification.
Because the BAY15d base uses an offset pin pattern, it is not interchangeable with single-filament bayonet bulbs such as the 1156, which uses a BA15s base with a single centered pin. Substituting one for the other is not supported by the socket design.
The 1157 bulb's 21W filament draws a predictable current load that turn signal flasher units are calibrated to detect. Installing a bulb with a significantly different wattage rating can affect flasher timing behavior.