Mack FDM Headlight Bulb Size

Additional Notes

• The Mack FDM (1990-1992) uses sealed beam headlight units across all headlamp configurations. In sealed beam technology, the lens, reflector, and filament are fused into a single glass unit, meaning the entire assembly is replaced rather than an individual bulb.
• All six bulb types specified for the FDM (1990-1992) conform to ANSI standards recognized in both the United States and Canada, and all operate at 12 volts.
• The H6024 is a 7-inch round sealed beam unit used for both high and low beam functions in the two-round-headlamp configuration. Its dual-filament design allows a single unit to serve both beam functions.
• The four-round-headlamp configuration uses two distinct sealed beam types: the H5001 for high beam and the H5006 for low beam. The H5001 is a 4-inch square unit, while the H5006 is a 5.75-inch round unit, making these two types physically non-interchangeable despite serving the same vehicle configuration.
• The four-rectangular-headlamp configuration uses 4x6-inch sealed beam units across all three model years, though the specific low beam designation varies. The 1990 and 1992 model years specify the H4652 for low beam, while the 1991 model year specifies the H4656 instead.
• The H4651, H4652, and H4656 are all 4x6-inch rectangular sealed beam units within the same ANSI family. The H4652 is formally classified as a variant of the H4651, and substituting one 4x6-inch type for another without confirming the correct designation risks a beam pattern or wattage mismatch.