The Mack DM front and rear side marker bulb size is 1891, a specification documented for this truck model. The replacement chart covers vehicle production years spanning 1981 to 1997.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack DM uses the 1891 bulb in both the front and rear side marker positions across the full 1981-1997 model range covered by these specifications.
The 1891 is classified as a T5 miniature wedge bulb, a designation that refers to its tubular glass envelope measuring 5/8 of an inch in width.
The wedge base design means the bulb is held in place by the socket gripping the base of the glass 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 in the miniature automotive category.
The 1891 carries ANSI standardization recognized in both the United States and Canada, meaning the specification conforms to a defined, regulated standard rather than a proprietary sizing.
Because the same bulb number applies to both the front and rear side marker positions, a single bulb type serves all side marker sockets on the vehicle.
The T5 wedge form factor is a common platform that has several bulbs sharing the same physical base, so substitution with a different T5 wedge number is not automatically valid without confirming matching voltage and wattage ratings.
At 2 watts, the 1891 produces relatively low light output, consistent with its role as a marker lamp rather than a primary illumination source.