The Fiat X-1/9 front side marker bulb uses 194 or 257 size, while the rear side marker bulb uses 194 or 57 size. The replacement chart covers 1974 through 1983 production years 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 Fiat X-1/9 side marker bulb specifications cover model years 1974 through 1983 for both front and rear positions.
The 194 bulb is the most widely used type across these specifications, appearing in both the front and rear side marker positions for nine of the ten model years listed.
The 194 is a T10 miniature wedge bulb rated at 12V and 3.78W, conforming to the ANSI standard recognized in the United States and Canada. Its wedge base design means it requires no separate metal base, as the glass envelope itself acts as the retention mechanism in the socket.
The 1980 model year is the sole exception to the otherwise consistent use of the 194 bulb, with both the front and rear side marker positions specifying different bulb types from the rest of the production run.
The Fiat X-1/9 (1980) front side marker calls for the 257 bulb, an ANSI-standard festoon or BA9s base instrument bulb rated at 12V and 2W. The BA9s base uses a bayonet-style single-pin fitting, which is physically incompatible with the wedge socket used by the 194.
The 1980 rear side marker specifies the 57 bulb, a single-filament BA9s base bulb rated at 12V and 1W. Although both the 57 and the 257 share the BA9s base type, their wattage ratings differ, and they are not interchangeable with each other or with the 194.
All three bulb types listed in these specifications conform to ANSI standards applicable in the United States and Canada, indicating standardized electrical and physical ratings across the range.
The wattage of the 194 bulb at 3.78W is notably higher than both the 257 at 2W and the 57 at 1W, reflecting differences in intended application and light output between the bulb types.
Because the 194 uses a wedge base and the 257 and 57 use a BA9s bayonet base, none of the three bulb types in these specifications are physically interchangeable with one another without socket modification.
Substituting a 194 in place of the 57 or 257 specified for the 1980 model year would involve both a base incompatibility and a significant wattage mismatch relative to the original socket and circuit design.