The Mack RWS front and rear side marker bulb size is 1891 for this vehicle model. The replacement chart contains specifications covering 1981 to 1985 production years.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RWS uses the 1891 bulb for both front and rear side marker positions across the 1981-1985 model years.
The 1891 is an ANSI-standardized bulb, meaning it conforms to a defined set of electrical and physical specifications recognized across the United States and Canada.
The 1891 bulb is a T5 miniature wedge type, where "T5" refers to a tubular glass envelope with a width of 5/8 inch, and the wedge base replaces a traditional metal screw or bayonet fitting by relying on the glass pinch itself to make electrical contact.
The 1891 operates at 12 volts and draws 2 watts, placing it among the lower-wattage bulbs used in automotive marker lighting applications.
The identical bulb specification across both the front and rear side marker positions means a single bulb type services all four side marker sockets on the vehicle.
The wedge base design of the 1891 is not interchangeable with bayonet-base or screw-base sockets, as the socket contact configuration differs fundamentally between those base types.