Triumph Rocket III Brake Bulb Size

Triumph Rocket III brake bulb specifications indicate a P21/5W bulb size for this motorcycle model. The replacement chart covers vehicle production spanning 2004 to 2010.

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
2004-2010	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 Triumph Rocket III uses the P21/5W bulb for the brake light function across all listed model years, spanning 2004-2010.
The P21/5W is a dual-filament bulb, meaning a single unit carries two separate filaments operating at different wattages: 21W for the brake/stop signal and 5W for the tail or running light function.
The bulb operates on a 12V electrical system, which is standard for motorcycles in this class.
The P21/5W follows the ECE (Economic Commission for Europe) standard, a regulatory framework adopted across Europe and numerous other markets worldwide.
The base type is BAY15d, a bayonet-style fitting with two offset pins. The offset pin placement is a deliberate design feature that prevents incorrect filament orientation during installation, as reversing the filaments would assign the wrong wattage to each function.
Because the BAY15d base uses asymmetric pin positioning, it is not interchangeable with the visually similar BA15d base, which has symmetrically placed pins.