The Mack RD front and rear turn signal bulb size is 1157 for this truck model. The replacement chart covers vehicle production years from 1981 to 1997 with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Mack RD uses the 1157 bulb for both its front and rear turn signals across all model years from 1981-1997.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope, each rated for a different wattage.
The two filaments in the 1157 are rated at 21W and 5W respectively, with the higher-wattage filament typically serving the turn signal or brake function and the lower-wattage filament serving as a running light.
The 1157 operates on a 12-volt electrical system, which is standard for automotive and commercial truck applications.
The 1157 uses a BAY15d bayonet base, a two-pin base where the two pins are offset at different heights to prevent incorrect installation of the bulb.
The 1157 conforms to ANSI standards and is recognized in both the United States and Canada, reflecting its widespread use in North American vehicle applications.
Because the same 1157 bulb is specified for both the front and rear turn signal positions, a single bulb type serves all four turn signal housings on the vehicle.
The dual-filament design of the 1157 makes it non-interchangeable with single-filament bulbs such as the 1156, which shares a similar bayonet base style but carries only one filament and lacks the offset pin configuration of the BAY15d.