The GMC PM251 front and rear turn signal bulbs utilize 1073 and 1034 size bulbs for proper illumination. The replacement chart covers 1956 to 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 PM251 uses two distinct bulb types across its 1956 and 1957 model years: the 1073 and the 1034, both of which are ANSI-standardized bulbs recognized in the United States and Canada.
The 1073 bulb is a single-filament type operating at 12 volts and 13 watts, mounted on a BA15s bayonet base, which features a single locating pin on the base collar.
The 1073 is a lower-wattage variant of the more common 1156 bulb, meaning the two share the same BA15s base design but differ in wattage output, making them non-interchangeable in applications where wattage tolerance is strict.
The 1034 bulb is a dual-filament type rated at 12 volts with a 21/5-watt output, meaning it carries separate wattage levels for its two filaments, typically serving distinct functions such as running and signaling.
The 1034 uses a BAY15d bayonet base, which has two offset locating pins to prevent incorrect installation, distinguishing it physically from the single-pin BA15s base used by the 1073.
The 1034 is a variant of the 1157 bulb family, and while the two share a similar dual-filament design, wattage differences between variants within that family affect direct substitution.
The GMC PM251 (1956) uses the 1034 bulb for both front and rear turn signals, with the dual-filament design suggesting the bulb serves more than one lighting function at each position.
The 1957 GMC PM251 uses the 1073 bulb for both front and rear turn signals, representing a shift to a single-filament configuration compared to the prior model year.
Because the 1073 and 1034 use different bayonet base types, BA15s and BAY15d respectively, the two bulbs are physically incompatible with each other's sockets and cannot be substituted across the 1956 and 1957 model years.