The Lincoln Mark V 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 1977 to 1979 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 V uses two distinct sealed beam headlight units across its 1977-1979 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. It conforms 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 applicable in the United States and Canada.
Both the H5001 and H5006 are sealed beam units, meaning the lens, reflector, and filament are fused into a single assembly. When the filament fails, the entire unit requires replacement rather than an internal bulb swap.
The H5001 and H5006 are not interchangeable with each other. Their differing shapes and sizes correspond to separate functions, with the square unit assigned to high beam and the round unit assigned to low beam.
The square format of the H5001 and the round format of the H5006 reflect a common ANSI-era practice of using shape to distinguish beam function, which also served as a physical safeguard against incorrect installation.