The GMC M340 high beam and low beam headlight bulbs utilize H6024 and H6006 sizes. The replacement chart covers 1955 to 1956 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 H6024 and H6006 are both sealed beam units, meaning the lens, reflector, and filament are fused into a single assembly. When the filament fails, the entire unit is replaced rather than an individual bulb.
The H6024 is a 7-inch round sealed beam unit operating at 12V, conforming to ANSI standards recognized in both the United States and Canada. The GMC M340 used this unit for both high and low beam functions in 1956.
The H6006 is a 5x7-inch rectangular sealed beam unit, also rated at 12V and built to the same ANSI standard. It was fitted to the 1955 GMC M340 for both high and low beam positions.
Because the H6024 is round and the H6006 is rectangular, the two units are not interchangeable despite sharing the same 12V rating and ANSI compliance. Physical housing shape determines fitment, and substituting one for the other is not possible without modifying the mounting structure.
The M340 used a dual-purpose sealed beam arrangement in both model years, with the same bulb size serving the high beam and low beam positions. This was achieved through a single unit containing separate filaments for each function rather than separate housings.
Both units conform to ANSI standards applicable in the United States and Canada, which defined fixed specifications for sealed beam size, shape, and electrical characteristics during this period of automotive lighting regulation.