Harley-Davidson FX Super Glide Brake Bulb Size

Harley-Davidson FX Super Glide brake bulb specifications indicate a 1157 bulb size for this motorcycle model. The replacement chart covers vehicle production years spanning 1971 to 1978.

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
1971-1978	1157

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. 12V, 21W/5W.	1157

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Additional Notes

The 1157 is the sole brake bulb type specified for the Harley-Davidson FX Super Glide across all listed model years, 1971-1978.
The 1157 is an ANSI-standardized bulb type recognized across the United States and Canada, meaning it conforms to a defined set of electrical and physical parameters shared by all compliant manufacturers.
As a dual-filament bulb, the 1157 operates two independent filaments within a single envelope, one rated at 21W for the brake function and one at 5W for the running light function.
The 1157 uses a BAY15d bayonet base, a two-pin offset base design in which the two pins are positioned at different heights to enforce a single correct insertion orientation, preventing incorrect filament alignment.
The bulb operates on a 12V electrical system, consistent with the standard automotive and motorcycle electrical architecture of that era.
The BAY15d base of the 1157 is not interchangeable with single-filament bayonet bases such as the BA15s, as the pin configuration and filament count differ between those two base types.