The Lincoln Mark IV high beam headlight uses an H5001 bulb size while the low beam headlight requires an H5006 bulb size. The replacement chart covers vehicle production spanning 1972 to 1976 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 Lincoln Mark IV used two distinct sealed beam units across its 1972-1976 model years: the H5001 for high beam and the H5006 for low beam.
The H5001 is a 4-inch square sealed beam unit operating at 12V, conforming to ANSI standards recognized in both the United States and Canada.
The H5006 is a 5.75-inch round sealed beam unit, also rated at 12V and built to the same ANSI standard as the H5001.
Both the H5001 and H5006 are sealed beam units, meaning the lens, reflector, and filament form a single assembly. When the filament fails, the entire unit requires replacement rather than an internal bulb swap.
The square profile of the H5001 and the round profile of the H5006 make the two units physically incompatible with each other's housings, and one cannot substitute for the other in the Mark IV's headlight system.
The H5001 and H5006 designations follow ANSI standardization, which means units from different manufacturers sharing the same designation are generally interchangeable, provided they meet the same standard.