The GMC G1000 Series front turn signal bulb size measures 1034 or 1073, while the rear turn signal bulb size measures 1073 or 1156. The replacement chart covers 1963 through 1966 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 GMC G1000 Series turn signal specifications reference three distinct bulb types across the 1963-1966 model years: the 1034, 1073, and 1156.
The 1034 is a dual-filament bulb rated at 12V and 21/5W, fitted with a BAY15d bayonet base. Its dual-filament design means it serves two brightness levels within a single housing, a characteristic that distinguishes it from the single-filament types also present in these specifications.
The 1034 is classified as a variant of the 1157, sharing its BAY15d base and dual-filament construction while differing in wattage output.
The 1073 and 1156 are both single-filament bulbs operating at 12V and sharing the BA15s bayonet base, making their physical base type identical. The 1073 is rated at 13W, while the 1156 is rated at 21W, and the 1073 is formally classified as a lower wattage variant of the 1156.
Because the 1073 and 1156 share the same BA15s base, physical fitment between the two is possible, but their wattage difference means they are not electrically interchangeable without affecting turn signal brightness and potentially circuit timing in flasher-relay systems calibrated to a specific load.
The 1034 uses a BAY15d base, which is physically distinct from the BA15s base of the 1073 and 1156. The BAY15d base incorporates offset locating pins to prevent incorrect single-filament bulb insertion, so cross-fitment between the 1034 and the other two types is not physically possible.
For the GMC G1000 Series, the front turn signal position alternated between the 1034 and 1073 across model years, with 1964 and 1966 using the 1034 and 1963 and 1965 using the 1073.
The rear turn signal position used the 1156 in 1963-1964 and transitioned to the 1073 in 1965-1966, representing a shift from a 21W single-filament bulb to a 13W single-filament bulb in that position.
All three bulb types carry ANSI standard designations applicable in both the United States and Canada, indicating standardized performance and dimensional specifications under North American automotive lighting norms.
In 1966, the G1000 Series used the 1034 at the front and the 1073 at the rear, meaning the front and rear turn signal positions required bulbs with different base types that year and are not interchangeable with each other.