The Fiat X-1/9 front turn signal bulb size is 1157, while rear turn signal bulbs measure 1073 and 1156 in size. The replacement chart covers vehicle production spanning 1974 to 1983.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Fiat X-1/9 front turn signal uses the 1157 bulb across all model years from 1974-1983, making it the only front turn signal bulb type in these specifications.
The 1157 is a dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base, which is an offset twin-pin base designed to prevent incorrect installation.
The dual-filament design of the 1157 allows a single bulb to serve two lighting functions simultaneously, such as turn signaling and parking light operation, within one housing.
The rear turn signal position uses two different bulb types across the 1974-1983 model years, specifically the 1156 and the 1073, and these two types are not directly interchangeable despite sharing the same BA15s bayonet base.
The 1156 appears in the Fiat X-1/9 rear turn signal for 1974-1978 and 1980-1982, while the 1073 is specified for 1979 and 1983.
Both the 1156 and 1073 are single-filament bulbs with the same BA15s single-contact bayonet base, meaning they are physically compatible with the same socket type.
The 1156 is rated at 12V and 21W, whereas the 1073 is rated at 12V and 13W, making the 1073 a lower-wattage variant that produces less light output and draws less current from the vehicle's electrical system.
Substituting a 1156 in a socket specified for a 1073 would result in higher current draw than the circuit was designed for in those model years, as the wattage difference between the two bulbs is approximately 8W.
All three bulb types listed in these specifications, the 1157, 1156, and 1073, conform to ANSI standards applicable in the United States and Canada.
The 1157 and the 1156 or 1073 are not interchangeable with each other, as the 1157 uses a BAY15d base with offset pins, while the 1156 and 1073 use a BA15s base with a single contact pin.