Harley-Davidson FXDF Fat Bob Turn Signal Bulb Size

Harley-Davidson FXDF Fat Bob rear turn signal bulb uses a P21W size. The replacement chart covers 2012 to 2017 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.

Rear Turn Signal

Years	Bulb Size
2012-2017	P21W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

Other Locations

Additional Notes

The P21W is the sole bulb type listed across all rear turn signal positions in these specifications, appearing in the Harley-Davidson FXDF Fat Bob for the 2012-2017 model years.
The P21W is an ECE-standard single-filament bulb rated at 12 volts and 21 watts, designed specifically for signalling functions such as turn signals rather than continuous illumination.
The P21W uses a BA15s bayonet base, which is a single-contact bayonet fitting. The "s" in BA15s denotes a single contact pin, distinguishing it from the dual-contact BA15d variant, which is not interchangeable with this application.
The ECE designation on the P21W indicates compliance with United Nations Economic Commission for Europe regulations, a standard recognized across European and many international markets.
Because the BA15s base is a single-contact design, substituting a dual-contact bayonet bulb such as the BA15d would result in an incorrect fit, even if the physical housing appears similar.
At 21 watts, the P21W produces a relatively high output for a signal bulb, which is a deliberate characteristic of ECE signalling standards to ensure adequate visibility in daylight conditions.