The Mack RL front and rear side marker bulb size is 1891, a unique bulb specification for this truck model. The replacement chart covers vehicle production years from 1981 to 1987.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RL uses the 1891 bulb for both its front and rear side marker positions across the 1981-1987 model years.
The 1891 is classified as a miniature wedge bulb, designated T5 in the tubular bulb sizing system, where the number indicates a tube width of 5/8 of an inch.
The wedge base design means the 1891 bulb is held in place by the socket gripping the glass envelope directly, with no separate metal base or pins involved.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lower-wattage bulbs used in commercial vehicle marker lighting.
The 1891 conforms to ANSI standards recognized in both the United States and Canada, meaning it meets a shared North American specification for electrical and photometric performance.
The identical bulb fitment across both the front and rear side marker positions on the Mack RL means a single bulb type services all side marker sockets on the vehicle.
T5 wedge bulbs such as the 1891 are not interchangeable with bayonet-base or indexed-base bulb sockets, as the wedge format requires a socket specifically designed to accept that base style.