Triumph TR8 Back Up Light Bulb Size

The Triumph TR8 back up light bulb uses a P21W size. The replacement chart covers 1980 to 1981 vehicle production years and specifies bulb sizes for this application.

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
1980-1981	P21W

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W

Other Locations

Additional Notes

The P21W is the bulb type specified for the Triumph TR8 backup light across both available model years, 1980-1981.
The P21W operates on 12 volts with a 21-watt output, placing it in the single-filament category designed for signalling applications such as backup lights.
This bulb uses a BA15s bayonet base, which is a single-contact bayonet fitting. The "s" in BA15s denotes a single contact pin, distinguishing it from the dual-contact BA15d variant, which is not interchangeable.
The P21W is governed by ECE (Economic Commission for Europe) standards, meaning it conforms to a regulated specification used across European and worldwide markets rather than SAE standards common in North American applications.
Because the BA15s base is keyed for single-contact use only, substituting a dual-contact bayonet bulb would result in an incompatible fit regardless of the bulb's wattage or voltage rating.