Rolls-Royce Silver Shadow II Map Light Bulb Size

The Rolls-Royce Silver Shadow II map light requires bulb size 257 or 57 depending on the specific application. Data for the bulb size chart covers the range from 1977 to 1979.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1979	257
1978	57
1977	257

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Festoon or BA9s base instrument bulb. 12V, 2W.	257
ANSI standard (US & Canada). Single-filament bulb with BA9s base. 12V, 1W.	57

Other Locations

Additional Notes

The Rolls-Royce Silver Shadow II uses two distinct bulb types across its model years for the map light position: the 257 and the 57, both governed by ANSI standards applicable to the US and Canadian markets.
The 257 bulb operates at 12 volts and draws 2 watts, fitting either a festoon or BA9s base configuration, which classifies it as an instrument-grade bulb within the ANSI system.
The 57 bulb is a single-filament type with a BA9s base, also rated at 12 volts but drawing only 1 watt, making it the lower-output option of the two.
The BA9s base is shared between the 257 and the 57, meaning both bulbs use the same bayonet-style miniature base format. However, the two are not interchangeable due to their differing wattage and physical form factors.
The 1977 and 1979 Silver Shadow II both specify the 257 bulb for the map light, while the 1978 model year specifies the 57 bulb instead.
The shift to the 57 bulb in the 1978 model year represents a lower wattage draw compared to the 257 used in surrounding years, which reflects a difference in the electrical load placed on the map light circuit for that year.
A festoon base, as referenced in the 257 description, refers to a cylindrical bulb held at both ends by contact clips, whereas a BA9s base uses a small bayonet pin for socket retention. The 257 is listed as compatible with either configuration.