The Ferrari 328 GTB uses a 1157 bulb size for the front turn signal and P21W bulb size for the rear turn signal. The replacement chart covers 1986 to 1989 production years with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Ferrari 328 GTB uses two distinct bulb standards across its turn signal positions: the 1157, which follows the ANSI standard common in the United States and Canada, and the P21W, which follows the ECE standard used in Europe and worldwide markets.
The 1157 is a dual-filament bulb operating at 12V with a 21W main filament and a 5W secondary filament, mounted on a BAY15d bayonet base. The dual-filament design allows a single bulb to serve both a brighter signalling function and a dimmer running light function simultaneously.
The P21W is a single-filament bulb operating at 12V at 21W, mounted on a BA15s bayonet base. Because it carries only one filament, it serves a single lighting function per socket.
The BAY15d base of the 1157 and the BA15s base of the P21W are physically incompatible. Although both are bayonet-style bases with a 15mm collar, the BAY15d has offset locking pins to accommodate its dual-filament design, while the BA15s has a single-pin configuration. These two bulb types are not interchangeable.
The front turn signal of the Ferrari 328 GTB (1987-1989) uses the 1157 bulb, while the 1986 model year uses the P21W for the same position.
The rear turn signal of the Ferrari 328 GTB uses the P21W across all four model years covered in the specifications, spanning 1986-1989.
The 1986 Ferrari 328 GTB is the only model year in these specifications where both the front and rear turn signals share the same bulb type, with the P21W fitted to both positions.
For the 1987-1989 model years, the front and rear turn signals of the 328 GTB use different bulb types, requiring separate bulb stocks for front and rear service on those vehicles.