Mack R Back Up Light Bulb Size

The Mack R back up light bulb uses a 1156 size for this truck model. The replacement chart covers vehicle production from 1981 to 1991 with corresponding bulb specifications.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1981-1991	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

Other Locations

Additional Notes

The Mack R uses the 1156 bulb for its back up light across all listed model years from 1981 to 1991.
The 1156 is an ANSI-standardized bulb type, meaning its physical and electrical specifications conform to a recognized North American standard used in both the United States and Canada.
The 1156 operates on a 12-volt electrical system and draws 21 watts, which is consistent with standard automotive incandescent backup lighting.
The bulb uses a BA15s bayonet base, a single-contact fitting where the "s" designation indicates a single pin rather than the dual-pin BA15d variant, making the two types non-interchangeable despite sharing the same base width.
Because the 1156 is a single-filament bulb, it serves one lighting function only, distinguishing it from dual-filament types such as the 1157, which handles two functions from a single housing.
The BA15s bayonet mount is secured by twisting the bulb into the socket rather than threading it, a common fitment method in commercial vehicle lighting applications.