Mack U Headlight Bulb Size

Additional Notes

• The Mack U spans the 1981-1989 model years in these specifications and was offered with three distinct headlamp configurations: a two-lamp round setup, a four-lamp rectangular setup, and a four-lamp round setup. Each configuration requires a different set of sealed beam units for high and low beam functions.
• The H6024 is a 7-inch round, 12V sealed beam unit conforming to the ANSI standard used in the US and Canada. On the Mack U with 2 round headlamps, it serves as both the high beam and low beam unit across all 1981-1989 model years, meaning a single bulb type handles both functions in that configuration.
• All bulb types listed across these specifications are sealed beam units, meaning the lens, reflector, and filament are fused into a single assembly. When a filament fails, the entire unit is replaced rather than an individual bulb.
• The four-rectangular-headlamp configuration uses separate sealed beam types for high and low beams. The H4651 handles the high beam role across all 1981-1989 model years, while the low beam role is filled by either the H4652 or H4656 depending on the specific year.
• The H4652 and H4656 are both 4x6-inch rectangular, 12V ANSI-standard sealed beam units. Though they share the same physical format as the H4651, they are distinct part numbers designated for low beam use and are not interchangeable with the H4651 high beam unit.
• The low beam for the four-rectangular-headlamp configuration is specified as H4656 for the 1982, 1986, and 1988 model years, while H4652 is specified for 1981, 1983-1985, 1987, and 1989. These two units occupy the same physical position but carry different part numbers and should not be substituted for one another without confirming the correct designation for the specific year.
• The four-round-headlamp configuration uses the H5001 for high beam and the H5006 for low beam across all 1981-1989 model years. The H5001 is a 4-inch square sealed beam unit, while the H5006 is a 5.75-inch round sealed beam unit, making them physically incompatible with each other despite serving the same vehicle configuration.
• All six sealed beam types listed in these specifications operate at 12V and conform to ANSI standards recognized in both the US and Canada, placing them within the standardized sealed beam system that was common in North American commercial vehicle applications of that era.