Triumph Thunderbird LT Tail Light Bulb Size

The Triumph Thunderbird LT tail light requires a P21/5W bulb size for proper illumination. The replacement chart covers 2014 through 2017 vehicle production 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	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 tail light bulb type specified for the Triumph Thunderbird LT across all listed model years.
P21/5W is an ECE-standard bulb recognized across European and worldwide markets, operating on a 12-volt system with a dual-filament design rated at 21 watts and 5 watts respectively.
The dual-filament construction allows a single bulb to serve two lighting functions, with the higher-wattage filament typically handling brake or turn signal output and the lower-wattage filament handling the running or position light function.
The P21/5W uses a BAY15d bayonet base, which is an offset twin-pin fitting. The asymmetric pin placement of the BAY15d base ensures the bulb can only be inserted in one orientation, preventing incorrect filament alignment.
The 2014 and 2016-2017 Thunderbird LT model years share the same P21/5W specification, meaning the tail light bulb is interchangeable across those years without modification.