The BMW 318i tail light utilizes bulb sizes 12821 and P21/5W and 5008 for various lighting applications. Data for the specified components covers the range from 1984 through 1998.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The BMW 318i inner tail light draws from four distinct bulb types across its model years: P21/4W, P21/5W, 12821, and 5008, each with different base types and filament configurations.
The P21/4W is an ECE-standard dual-filament bulb rated at 12V with outputs of 21W and 4W, mounted on a BAZ15d bayonet base. It serves the BMW 318i inner tail light position for the 1995-1998 model years.
The P21/5W is also an ECE-standard dual-filament bulb rated at 12V with outputs of 21W and 5W, but it uses a BAY15d bayonet base rather than the BAZ15d found on the P21/4W. Despite their similar appearance, these two bulbs are not interchangeable due to their different base configurations.
The BAZ15d and BAY15d bases differ in the offset positioning of their bayonet pins, a design feature that physically prevents incorrect installation between P21/4W and P21/5W bulbs.
The 12821 is a Philips part number designating a miniature wedge-type T10 bulb with a W2.1x9.5d base, rated at 12V and 5W. It is a single-filament bulb used in the inner tail light position across multiple 318i model years.
The 5008 is an ANSI-standard single-filament bulb with a BA15s bayonet base, rated at 12V. It is a lower-wattage variant of the 1156 bulb type and appears in the 1984-1985 BMW 318i inner tail light position.
The BA15s base of the 5008 is a single-contact bayonet fitting, which is distinct from the dual-contact bases used by the P21/4W and P21/5W bulbs, making cross-substitution between these types incompatible.
The 1996 BMW 318i inner tail light position lists the P21/5W, while the surrounding model years of 1995 and 1997-1998 list the P21/4W, indicating a base-type difference specific to that year that affects direct substitution.
The wedge base of the 12821 (W2.1x9.5d) is mechanically incompatible with the bayonet bases used by the P21/4W, P21/5W, and 5008 bulbs, as wedge and bayonet sockets require different housing designs.
The ECE-standard bulbs (P21/4W and P21/5W) are designated for use in Europe and worldwide markets, while the 5008 follows ANSI standards applicable to the US and Canadian markets, reflecting regional regulatory differences in automotive lighting.