The 1157 bulb is the sole turn signal bulb size specified for the Mack MH across all listed model years, covering both the front and rear turn signal positions from 1983-1994.
The 1157 is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope, allowing it to serve more than one lighting function at the same wattage rating.
The 1157 operates on a 12-volt electrical system and carries a dual-wattage rating of 21W and 5W, corresponding to its two filaments respectively.
The 1157 uses a BAY15d bayonet base, which is an offset twin-pin base. The offset pin placement is a deliberate design feature that prevents incorrect insertion orientation during installation.
The 1157 conforms to ANSI standards and holds recognition in both the United States and Canada, placing it within a standardized North American bulb classification system.
Because the same 1157 bulb size is specified for both the front and rear turn signal positions on the Mack MH, a single bulb type serves both locations across the entire 1983-1994 model range.
The BAY15d base of the 1157 is not interchangeable with single-filament bayonet bases such as the BA15s, which lacks the offset twin-pin configuration. Substituting an incompatible base type would result in a fitment failure.
The dual-filament nature of the 1157 means the two wattage ratings are not interchangeable in function. The 21W filament and the 5W filament serve distinct electrical roles within the circuit.