The GMC F370 front turn signal requires a 1073 or 1129 bulb size, while the rear turn signal uses a 1034 or 1129 bulb size. The replacement chart covers 1951 through 1957 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 F370 front turn signal uses two distinct bulb types across its model years: the 1129 for 1951-1955 and the 1073 for 1956-1957.
The 1129 bulb is a dual-filament type rated at 12V with a 21/5W output, mounted on a BAY15d bayonet base, and classified as an ANSI standard bulb for the US and Canada.
The BAY15d base found on the 1129 features offset pins, which physically prevents it from being seated in a single-filament socket, making it non-interchangeable with BA15s-based bulbs.
The 1073 bulb is a single-filament type rated at 12V and 13W, mounted on a BA15s bayonet base, and is described as a lower wattage variant of the 1156.
The 1129 is noted as a variant of the 1157, a widely referenced dual-filament automotive bulb, though the 1129 and 1157 are not directly interchangeable due to differing wattage ratings.
The GMC F370 rear turn signal also transitions between two bulb types: the 1129 for 1951-1955 and the 1034 for 1956-1957.
The 1034 bulb shares the same BAY15d bayonet base and 12V/21/5W dual-filament configuration as the 1129, and is likewise classified as a variant of the 1157.
Because the 1034 and 1129 share the same base type and electrical ratings, they occupy the same general bulb family, though they carry distinct ANSI designations.
The front and rear turn signals of the GMC F370 use the same bulb, the 1129, across 1951-1955, meaning both positions drew from a single bulb specification during those years.
In 1956-1957, the front and rear positions diverge, with the front using the single-filament 1073 and the rear using the dual-filament 1034, indicating the two positions are not interchangeable during those model years.