The GMC FM350 front and rear turn signal bulbs utilize 1073 and 1129 size bulbs as specified in the replacement chart. The chart covers vehicle production spanning 1955 to 1957.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1073 bulb follows the ANSI standard recognized in both the US and Canada. It is a single-filament bulb rated at 12V and 13W, fitted with a BA15s bayonet base. It is a lower wattage variant of the more widely known 1156 bulb.
The 1129 bulb also carries ANSI standardization for the US and Canadian markets. Unlike the 1073, it is a dual-filament bulb rated at 12V with a 21/5W output, and it uses a BAY15d bayonet base. It is a variant of the 1157 bulb.
The BA15s base of the 1073 and the BAY15d base of the 1129 are not interchangeable. The BA15s is a single-contact base, while the BAY15d is a dual-contact base with offset pins to prevent incorrect installation.
For the GMC FM350, the front and rear turn signals share the same bulb size in each model year, meaning both positions require identical bulbs throughout the 1955-1957 production span.
The 1955 FM350 uses the 1129 bulb in both the front and rear turn signal positions. Its dual-filament design suggests the turn signal circuit in that year shared the bulb with another lighting function, such as the brake or parking light.
The 1956-1957 FM350 uses the 1073 bulb in all turn signal positions. As a single-filament bulb, it serves a dedicated function rather than a shared one, which represents a different circuit arrangement from the 1955 model year.
Substituting a 1073 for a 1129, or vice versa, would be incompatible due to the differing base types and filament configurations. A dual-filament socket cannot properly seat a single-filament BA15s base, and the reverse is equally true.