Triumph Thunderbird LT Brake Bulb Size

The Triumph Thunderbird LT brake bulb uses a P21/5W size for replacement applications across the specified model. The chart covers vehicle production years from 2014 to 2017 with corresponding bulb size data.

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
2016-2017	P21/5W
2014	P21/5W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Dual-filament signalling bulb with BAY15d bayonet base. 12V, 21W/5W.	P21/5W

Other Locations

Additional Notes

The P21/5W is the brake bulb type specified for the Triumph Thunderbird LT across the 2014-2017 model years covered in these specifications.
P21/5W is an ECE-standardized bulb, meaning it conforms to European and internationally recognized regulations for automotive lighting components.
The bulb operates on a 12-volt electrical system, which is standard for most production motorcycles and passenger vehicles.
As a dual-filament bulb, the P21/5W carries two separate filaments within a single housing, rated at 21 watts and 5 watts respectively, allowing it to serve both a brighter brake function and a lower-intensity running light function simultaneously.
The P21/5W uses a BAY15d bayonet base, which is an offset twin-pin fitting. The asymmetric pin placement of the BAY15d base is a deliberate design feature that prevents incorrect installation by ensuring the bulb can only be seated in one orientation.
The BAY15d base type is distinct from the single-filament BA15s base, and substituting one for the other is not possible due to the differing pin configurations and filament counts.