Mack RM front and rear side marker bulbs utilize a 1891 bulb size across both positions. The replacement chart covers vehicle production spanning 1981 to 1993.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RM uses the 1891 bulb for both its front and rear side marker positions across the 1981-1993 model years.
The 1891 is an ANSI-standardized bulb, meaning its electrical and physical characteristics conform to a defined North American specification recognized in both the United States and Canada.
It belongs to the T5 bulb family, a designation that refers to a tubular glass envelope with a wedge-style base rather than a traditional pin or bayonet fitting.
The wedge base design means the bulb is held in its socket by the tension of the base itself, with no separate locking mechanism required.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lower-wattage bulbs used in commercial vehicle lighting applications.
Because the front and rear side marker positions share the same bulb type on the Mack RM, a single replacement part covers both locations on the vehicle.
The T5 wedge form factor is a widely produced bulb category, though fitment into a given socket depends on the specific wedge base size, and not all T5 wedge bulbs are interchangeable with one another.
The consistent use of the 1891 across the entire 1981-1993 production span of the Mack RM indicates no bulb specification change occurred during that period for either side marker position.