Mack R headlight bulbs utilize 3 distinct high beam sizes (H6024, H4651, H5001) and 3 distinct low beam sizes (H6024, H4656, H5006) across multiple configurations. The replacement chart covers 1981 through 1991 vehicle production years 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 Mack R series from 1981-1991 was offered with three distinct headlamp configurations: a two-round-lamp setup, a four-rectangular-lamp setup, and a four-round-lamp setup. Each configuration uses a different sealed beam unit, making headlamp configuration a critical factor in bulb fitment.
All six bulb types specified for the Mack R are sealed beam units conforming to ANSI standards recognized in both the United States and Canada. In a sealed beam design, the reflector, lens, and filament are fused into a single glass unit, so the entire assembly is replaced rather than an internal bulb.
The H6024 is a 7-inch round sealed beam unit rated at 12V. It serves as both the high beam and low beam on Mack R models equipped with the two-round-lamp configuration across all years in the specifications, meaning a single unit type handles both functions in that setup.
The four-round-lamp 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, so the two positions are not interchangeable despite both serving the same headlamp assembly style.
The four-rectangular-lamp configuration uses 4x6-inch sealed beam units across all positions, but the specific variant differs by function. The H4651 handles high beam duty, while low beam positions use either the H4652 or H4656 depending on the model year, making visual similarity between these units a potential source of incorrect fitment.
The H4652 is described as a variant of the H4651. Although both are 4x6-inch rectangular sealed beam units rated at 12V, they are designated for different beam functions and are not listed as interchangeable within these specifications.
In the four-rectangular-lamp low beam position, the 1991 and 1982 Mack R models are specified with the H4656, while the 1981 and 1983-1990 models use the H4652. Both are 4x6-inch rectangular sealed beam units, but they carry different ANSI designations and are listed as separate fitments.
All bulb types across both high beam and low beam positions in these specifications operate at 12V, consistent with standard 12-volt electrical systems used in commercial vehicles of this era.
Because the headlamp configuration determines which sealed beam unit is required, two Mack R trucks of the same model year may require entirely different bulb types if one is equipped with round lamps and the other with rectangular lamps.
The H4651, H4652, and H4656 all share the 4x6-inch rectangular sealed beam form factor but carry distinct ANSI designations corresponding to specific beam patterns. Substituting one for another based on physical size alone would not account for differences in beam function or optical design.