The Mack R front and rear side marker bulb size is 1891, a specification applicable to this truck model. A replacement chart documents bulb specifications across the 1981 to 1991 production range.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack R series uses the 1891 bulb in both the front and rear side marker positions across the 1981-1991 model years.
The 1891 is an ANSI-standardized bulb type, meaning its specifications are governed by a recognized North American standard applicable in both the United States and Canada.
The 1891 bulb follows the T5 form factor, a miniature wedge-base design. The wedge base eliminates the need for a separate socket with locking pins, as the glass envelope itself acts as the retention mechanism.
At 12 volts and 2 watts, the 1891 operates at a very low wattage, consistent with its role as a marker lamp rather than a primary illumination source.
The uniform use of the 1891 across all eleven model years and both marker positions means a single bulb type serves the entire side marker system on the Mack R for that production span.
Because the front and rear side marker positions share the same bulb specification, a 1891 sourced for one position is directly interchangeable with the other on this vehicle.