Mercury Cyclone Map Light Bulb Size

Mercury Cyclone map light bulb sizes include 1816, 631, and 57 specifications. The replacement chart covers vehicle production years spanning 1964 to 1971.

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
1970-1971	1816
1969	631
1968	1816
1967	57
1966	1816
1964-1965	257

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Miniature wedge bulb (T3.25). 12V, 4W.	1816
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. Variant of 1156. 12V, 21W.	631
ANSI standard (US & Canada). Single-filament bulb with BA9s base. 12V, 1W.	57
ANSI standard (US & Canada). Festoon or BA9s base instrument bulb. 12V, 2W.	257

Other Locations

Additional Notes

The 1816 is the most frequently specified map light bulb across the Mercury Cyclone's production run, appearing in the 1966, 1968, 1970, and 1971 model years.
The 1816 is an ANSI-standard miniature wedge bulb rated at 12V and 4W, classified under the T3.25 designation, which refers to its tubular envelope size measured in eighths of an inch.
The 257 bulb was specified for the 1964-1965 Mercury Cyclone and is an ANSI-standard instrument bulb rated at 12V and 2W, available in either a festoon or BA9s base configuration.
The 1969 Mercury Cyclone uses the 631 bulb, which carries a BA15s bayonet base and is rated at 12V and 21W, making it substantially higher in wattage than any other bulb specified across this model's map light history.
The 631 is noted as a variant of the 1156 bulb. Despite the family relationship, the two are not always interchangeable due to potential differences in base indexing and socket fit.
The 1967 Mercury Cyclone uses the 57 bulb, an ANSI-standard single-filament type with a BA9s base rated at 12V and 1W, which is the lowest wattage among all bulbs listed in these specifications.
Four distinct bulb types are specified across the eight model years covered, and none of them share the same base type, meaning cross-year substitution is not supported by these specifications.
The BA9s base format is shared between the 57 and 257 bulbs, though their wattage ratings differ at 1W and 2W respectively, and the 257 also offers a festoon base variant as an alternative fitment.