The GMC 1000 Series front and rear turn signal bulb sizes include 1073, 1034, and 1156 options. The replacement chart covers 1960 to 1965 vehicle production years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1073 bulb is the most frequently specified type across the GMC 1000 Series turn signal data, appearing in both front and rear positions for the 1960, 1961, and 1965 model years.
The 1073 is a single-filament bulb rated at 12V and 13W, using a BA15s bayonet base. It is described as a lower wattage variant of the 1156 bulb.
The 1156 bulb is also a single-filament type with a BA15s bayonet base but is rated at a higher 21W output, making it a higher-wattage alternative within the same base family as the 1073.
Because the 1073 and 1156 share the same BA15s bayonet base, physical fitment between the two is possible, but their wattage difference of 8W means they are not direct electrical equivalents and are not interchangeable without consideration of circuit compatibility.
The 1034 bulb differs fundamentally from both the 1073 and 1156 in that it carries a dual-filament design with a BAY15d bayonet base, rated at 12V and 21/5W. The BAY15d base uses offset pins to prevent incorrect installation, making it physically incompatible with the BA15s sockets used by the 1073 and 1156.
The 1034 is described as a variant of the 1157 bulb. Its dual-filament construction means it serves two lighting functions from a single socket, a characteristic absent in the single-filament 1073 and 1156 types.
For the GMC 1000 Series, the 1964 model year is the only year in which both the front and rear turn signal positions share the same bulb type, with the 1034 specified for both locations.
The 1963 model year presents a split specification, with the 1073 assigned to the front turn signal and the higher-wattage 1156 assigned to the rear, meaning the two positions are not interchangeable despite both using single-filament bulbs.
The 1962 model year also carries a split specification, with the 1156 at the front and the 1034 at the rear. These two positions use bulbs with different base types, confirming they are not interchangeable.
All three bulb types specified across this data, the 1073, 1034, and 1156, conform to ANSI standards recognized in both the United States and Canada.