Triumph Thunderbird ABS Brake Bulb Size

The Triumph Thunderbird ABS brake bulb uses a P21/5W size designation for replacement operations. The chart contains bulb size specifications across 2010 to 2013 vehicle production years.

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
2010-2013	P21/5W

Bulb Specifications

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

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Additional Notes

The P21/5W is the brake bulb type specified for the Triumph Thunderbird ABS across the 2010-2013 model years.
The P21/5W is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope. The higher-rated filament operates at 21 watts for the brake or turn signal function, while the lower-rated filament operates at 5 watts for a secondary function such as a tail or parking light.
The P21/5W uses a BAY15d bayonet base. The BAY15d designation indicates an offset dual-contact base, where the two pins are positioned at different heights to prevent incorrect installation of the bulb in the socket.
The bulb operates on a 12-volt electrical system, which is standard for most production motorcycles.
The P21/5W conforms to ECE (Economic Commission for Europe) standards, a regulatory framework recognized across Europe and numerous other markets worldwide.
Because the BAY15d base is offset, it is not interchangeable with the BA15d base, which shares the same bayonet style but features pins at equal heights. Substituting a BA15d bulb in a BAY15d socket is not possible without physical modification.