Mercury Custom Dome Light Bulb Size

The Mercury Custom dome light requires bulb size 1003 or 63 depending on the specific application for the interior lighting fixture. Data for the bulb sizes covers the range from 1952 to 1956.

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
1956	1003
1952-1955	63

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1003
ANSI standard (US & Canada). Single-filament bulb with BA9s base. 12V, 0.63W.	63

Other Locations

Additional Notes

The Mercury Custom uses two distinct bulb types across its model years for the dome light: the 1003 and the 63.
The 1003 bulb is an ANSI-standardized single-filament type rated at 12V and 21W, fitted with a BA15s bayonet base.
The 63 bulb is also an ANSI-standardized single-filament type rated at 12V, but draws only 0.63W and uses a BA9s base.
The BA15s and BA9s are both bayonet-style bases, though they are physically incompatible with each other due to their differing sizes, making the 1003 and 63 non-interchangeable.
The Mercury Custom (1956) takes the 1003 bulb for its dome light.
The 63 bulb applies to the Mercury Custom across the 1952-1955 model years.
At 21W, the 1003 produces considerably more light output than the 63, which operates at a fraction of a watt, reflecting a significant difference in intended illumination intensity between the two bulb types.
Both bulb types conform to ANSI standards recognized in the United States and Canada, indicating standardized physical and electrical specifications within those markets.