The 1157 is the sole bulb type specified for the Mack F across all listed turn signal positions and model years.
The 1157 follows the ANSI standard, meaning it conforms to a defined set of electrical and physical specifications recognized across the United States and Canada.
As a dual-filament bulb, the 1157 carries two separate tungsten filaments within a single envelope, allowing it to serve two distinct lighting functions from one unit.
The 1157 operates on a 12-volt electrical system, with the two filaments rated at 21 watts and 5 watts respectively.
The higher 21-watt filament corresponds to the active turn signal function, while the lower 5-watt filament serves as a running or parking light.
The 1157 uses a BAY15d bayonet base, which features two offset pins at different heights to ensure the bulb is seated in the correct rotational position, preventing the filaments from being assigned to the wrong circuit.
The Mack F (1981-1982) carries the same 1157 specification for both the front and rear turn signal positions, indicating a shared bulb type across those locations.