The GMC FM340 front turn signal bulb accommodates 1034, 1073, and 1129 sizes, while the rear turn signal bulb uses 1073 and 1129 sizes. The replacement chart covers 1955 to 1957 model 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 FM340 uses three distinct bulb types across its front and rear turn signal positions for the 1955-1957 model years: the 1034, 1073, and 1129.
The 1034 and 1129 are both dual-filament bulbs operating at 12V with a 21/5W output and share the BAY15d bayonet base, making them variants of the 1157 under the ANSI standard applicable in the US and Canada.
The 1073 is a single-filament bulb rated at 12V and 13W, fitted with a BA15s bayonet base, and is a lower wattage variant of the 1156 under the same ANSI standard.
Because the 1034 and 1129 share the same base type, wattage, and voltage rating, their physical and electrical characteristics are identical according to the provided specifications.
Despite the 1034 and 1129 sharing the same base and electrical ratings, they carry separate ANSI designations and are listed as distinct part numbers in the FM340 fitment data.
The 1073 uses a BA15s single-contact base, while the 1034 and 1129 use a BAY15d dual-contact base. These two base types are not interchangeable due to their differing contact configurations.
For the 1957 GMC FM340, the front turn signal takes a 1034 bulb while the rear takes a 1073, meaning the front and rear positions require different bulb types in that model year.
In the 1956 GMC FM340, both the front and rear turn signal positions use the 1073 bulb, making it the only model year in the specifications where a single bulb type covers both positions.
The 1955 GMC FM340 uses the 1129 bulb in both the front and rear turn signal positions.
The front turn signal position changes bulb type across each of the three listed model years, going from the 1129 in 1955 to the 1073 in 1956 and then to the 1034 in 1957.