Triumph Thunderbird Brake Bulb Size

The Triumph Thunderbird brake bulb uses a P21/5W size for the specified model. The replacement chart covers 2010 through 2014 production years 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
2010-2014	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 across the 2010-2014 model years.
P21/5W is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope. The 21W filament serves the braking function, while the 5W filament handles a secondary role such as running or position lighting.
The bulb operates on a 12V electrical system, which is standard for automotive applications of this type.
The P21/5W uses a BAY15d bayonet base. The BAY15d designation indicates an offset pin configuration, where the two retaining pins are positioned at different heights on the base. This offset is a deliberate design feature that prevents incorrect insertion of a single-filament bulb into a dual-filament socket.
The P21/5W is classified under ECE (Economic Commission for Europe) standards, indicating it is a regulated type used across European and worldwide markets rather than a North American SAE-spec equivalent.
Because the BAY15d base uses asymmetric pin placement, substitution with a BAY15s base bulb, which has pins at equal heights, is not mechanically compatible and would result in incorrect fitment.