The GMC FM340 headlight bulb utilizes H6024 and H6006 sizes for both high and low beam applications. The replacement chart covers 1955 through 1957 production 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 two distinct sealed beam unit types across its 1955-1957 model years: the H6024 and the H6006, both operating at 12 volts and conforming to ANSI standards recognized in the United States and Canada.
Sealed beam units are self-contained assemblies in which the filament, reflector, and lens are fused into a single glass envelope. When a sealed beam unit fails, the entire assembly requires replacement rather than an internal bulb.
The H6024 is a 7-inch round sealed beam unit. It serves as both the high beam and low beam headlight on the 1956-1957 FM340, meaning a single unit type covers both lighting functions for those model years.
The H6006 is a 5x7-inch rectangular sealed beam unit. It serves as both the high beam and low beam headlight on the 1955 FM340.
The round H6024 and the rectangular H6006 are not interchangeable. Their differing form factors mean a housing designed for one will not accept the other, making correct year identification essential before sourcing a replacement unit.
Both the H6024 and H6006 share the same 12-volt operating requirement, which reflects the 12-volt electrical systems standard in automotive applications of that era.