GMC 2500 Series front and rear turn signal bulbs utilize 1073, 1034, or 1156 bulb sizes depending on production specifications. The replacement chart covers 1960 through 1965 model 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 three bulb types specified across the GMC 2500 Series turn signal positions for 1960-1965 are the 1073, 1034, and 1156, all of which conform to ANSI standards recognized in both the United States and Canada.
The 1156 is a single-filament bulb rated at 12V and 21W, fitted with a BA15s bayonet base, and it appears in both front and rear turn signal positions across multiple model years of the GMC 2500 Series.
The 1073 shares the same BA15s bayonet base as the 1156 but operates at a lower wattage of 13W at 12V, making it a lower-output single-filament variant of the 1156.
Because the 1073 and 1156 share the BA15s base, the physical base is compatible between the two, but their wattage ratings differ, which affects light output and circuit load.
The 1034 is a dual-filament bulb rated at 12V with a 21/5W output, using a BAY15d bayonet base. The dual-filament design allows a single bulb to serve two lighting functions at different brightness levels.
The BAY15d base of the 1034 is not interchangeable with the BA15s base used by the 1073 and 1156, as the offset pin arrangement of the BAY15d is mechanically distinct from the single-contact BA15s.
The 1034 is noted as a variant of the 1157, meaning it shares the dual-filament, BAY15d configuration of that more widely referenced bulb type while differing in its specific electrical ratings.
For the GMC 2500 Series, the front and rear turn signal positions do not consistently use the same bulb type within a given model year, as seen in 1963, where the front position calls for a 1034 and the rear position calls for a 1156.
The 1073 is specified for the front turn signal only in 1965, while the same bulb type was used in the rear turn signal position for 1960, 1961, and 1965.
The 1034 is specified for the front turn signal in 1960-1961 and 1963-1964, while its rear turn signal use is limited to 1964 alone.