Triumph Rocket III Tail Light Bulb Size

The Triumph Rocket III tail light uses a P21/5W bulb size for illumination. The replacement chart covers 2004 to 2010 model 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.

Bulb Size by Year Range

Years	US Type	EU Type	Shop
2004-2010	1157	P21/5W	Check Price

Bulb Specifications

Description	Type	Shop
ECE standard (Europe & worldwide). Dual-filament signalling bulb with BAY15d bayonet base. 12V, 21W/5W.	P21/5W	Check Price
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. 12V, 21W/5W.	1157	Check Price

Other Locations

Additional Notes

The P21/5W is the tail light bulb used across all listed model years of the Triumph Rocket III, spanning 2004-2010.
The P21/5W is a dual-filament bulb, meaning it contains two separate filaments within a single glass envelope. The higher-output filament operates at 21 watts and serves the brake light function, while the lower-output filament operates at 5 watts and serves the running light function.
The P21/5W operates on a 12-volt electrical system, which is standard for motorcycles of this type.
This bulb follows the ECE (Economic Commission for Europe) standard, giving it broad recognition and availability across European and worldwide markets.
The P21/5W uses a BAY15d bayonet base. The BAY15d designation indicates an offset double-contact base, where the two pins are positioned at different heights on the base. This offset prevents the bulb from being inserted in the wrong orientation, ensuring each filament connects to its correct circuit.
Because the BAY15d base uses asymmetric pin placement, it is not interchangeable with the BA15d base, which shares the same physical size but has pins at equal heights. Substituting a BA15d bulb in a BAY15d socket would result in incorrect filament-to-circuit alignment.