GMC Truck front turn signal bulbs utilize 1073 or 1129 size specifications, while rear turn signal bulbs use 1073, 1034, or 1129 sizes. The replacement chart covers 1950 through 1957 production years with corresponding bulb size data.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1129 bulb is the most frequently specified type across the GMC Truck turn signal data, covering both front and rear positions for the 1950-1955 model years.
The 1129 is a dual-filament bulb rated at 12V with a 21/5W output, meaning it draws 21 watts on the primary filament and 5 watts on the secondary, and it uses a BAY15d bayonet base.
The BAY15d base features two offset pins that prevent incorrect insertion, a design intended to ensure the correct filament aligns with the intended circuit.
The 1129 is classified as a variant of the 1157, a widely recognized automotive bulb type under the ANSI standard used in both the United States and Canada.
For the 1957 GMC Truck, the 1073 bulb is specified for both the front and rear turn signal positions, making it the sole bulb type used across both positions that year.
The 1073 is a single-filament bulb rated at 12V and 13W, using a BA15s bayonet base, and is described as a lower wattage variant of the 1156.
Unlike the BAY15d base found on dual-filament bulbs, the BA15s base used by the 1073 has a single pin and no offset, reflecting its single-filament design.
The 1034 bulb shares the same BAY15d base and 12V, 21/5W dual-filament configuration as the 1129, and both are listed as variants of the 1157, though they carry distinct ANSI designations and are not listed as interchangeable within these specifications.
The 1034 appears exclusively in the 1956 GMC Truck rear turn signal position, while the front turn signal for that same year uses the 1073, meaning the two positions on the 1956 model require different bulb types.
The shift from the 1129 to the 1073 in the front turn signal position between 1955 and 1956 represents a change from a dual-filament to a single-filament bulb, which reflects a difference in circuit function rather than a simple substitution.