Ford LTD II headlight bulbs utilize H4651 size for high beam and H4656 size for low beam applications. The replacement chart covers 1977 to 1979 model years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Ford LTD II uses the H4651 sealed beam unit for high beam headlights across its 1977-1979 model years.
The H4656 sealed beam unit fills the low beam headlight position across the same 1977-1979 model years.
Both the H4651 and H4656 are 4x6-inch rectangular sealed beam units built to ANSI standards recognized in the United States and Canada.
A sealed beam unit integrates the filament, reflector, and lens into a single fused glass assembly, meaning the entire unit requires replacement rather than an internal bulb.
Both units operate on a 12-volt electrical system, which is standard for passenger vehicles of this type.
The H4651 and H4656 share the same physical form factor, a 4x6-inch rectangular housing, but they are not interchangeable because each is calibrated for a distinct beam pattern, high or low.
The ANSI designation on both units indicates conformance to a standardized specification, which governs the physical and photometric properties of the sealed beam and allows for cross-brand replacement compatibility within that standard.