GMC 300 front turn signal bulbs utilize 1073 or 1129 size specifications, while rear turn signal bulbs use 1073, 1156, or 1129 sizes. The replacement chart covers 1951 through 1959 production 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 GMC 300 uses three distinct bulb types across its front and rear turn signal positions: the 1073, the 1129, and the 1156, all of which conform to ANSI standards recognized in both the United States and Canada.
The 1129 bulb is a dual-filament type rated at 12V with a 21/5W output, mounted on a BAY15d bayonet base. Its dual-filament design distinguishes it from the single-filament bulbs used in later model years of the GMC 300.
The 1073 and 1156 bulbs share the same BA15s single-contact bayonet base, which means the socket type is physically compatible between the two. However, their wattage ratings differ, with the 1073 rated at 13W and the 1156 rated at 21W.
Because the 1073 and 1156 share a BA15s base, substituting one for the other is physically possible, but the wattage difference may affect circuit behavior, particularly in turn signal systems that rely on load-sensitive flasher units.
The 1129 uses a BAY15d base, which is not interchangeable with the BA15s base of the 1073 or 1156. The offset pins on the BAY15d base are a keying feature that prevents incorrect single-filament bulb installation in a dual-filament socket.
The GMC 300 front turn signal positions used the 1129 bulb for model years 1951-1955 and transitioned to the 1073 for model years 1956-1959.
The rear turn signal of the GMC 300 followed the same 1129-to-1073 transition pattern as the front, with one exception: the 1958 model year rear position specifies the 1156 rather than the 1073 used in surrounding years.
The 1958 GMC 300 rear turn signal is the only position across all listed years and positions to specify the 1156 bulb, making it a unique fitment within this specification set.
The 1073 is described as a lower wattage variant of the 1156. Both are single-filament bulbs on BA15s bases, and the 1129 is described as a variant of the 1157, a common dual-filament automotive bulb.