Collection: BA21d Bulbs

The BA21d base: a specific format for vintage motorcycles

The BA21d is a bayonet base with 2 offset pins, 21 mm in diameter. Its particularity: it falls between the most common standard formats — larger than the BA20d (20 mm) motorcycle bulb and slightly larger than the BA15s (15 mm) car bulb.

This intermediate diameter does not correspond to any modern standard — it belongs to an era when each motorcycle manufacturer had its own dimensions. The BA21d is found on:

• Old European mopeds (before 1970)
• Pre-war French motorcycles and those from the 1950s (Motobécane, Terrot, Peugeot two-wheelers)
• Certain old Italian motorcycles (Gilera, Aermacchi)
• High-end 1960s mopeds

BA21d: not interchangeable with other BA bases

Important to know: the BA21d does not fit into a BA15s socket (too large), BA20d (different), or BA15d (different diameter). Its socket is specific — you need a BA21d bulb and nothing else.

If you have a vintage motorcycle with a medium-sized headlight and BA20D bulbs don't fit, measure the diameter of the housing — it is probably a BA21d.

Availability and alternatives

A very rare format outside of specialists. Pharos Boutique keeps this base in stock specifically for vintage motorcycle enthusiasts and restoration workshops. Note: on some vehicles, it is possible (with the agreement of a specialist) to install a new bulb holder in BA20d or BA15s — but this alters the authenticity of the restoration and is not recommended for valued collection motorcycles.