The BMW 318i front and rear side marker bulb size specifications include P21/5W and P21/4W options, with alternative part numbers 12961 and 3893 available for this bulb size. The replacement chart covers vehicle production years spanning 1984 to 1998 with corresponding bulb size data.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The BMW 318i front side marker uses three distinct bulb types across its model years: P21/5W, 12961, and 3893. The rear side marker uses P21/5W, P21/4W, and 3893.
P21/5W is the most frequently specified bulb across both front and rear side marker positions on the BMW 318i, covering the 1993-1994 and 1997-1998 model years in both locations.
The P21/5W is an ECE-standard dual-filament bulb rated at 12V with a 21W main filament and a 5W secondary filament. It uses a BAY15d bayonet base.
The rear side marker for 1993-1994 uses the P21/4W rather than the P21/5W found in the front position for those same years. Although both are ECE-standard dual-filament bulbs with a 12V rating, the P21/4W uses a BAZ15d base, which is physically distinct from the BAY15d base of the P21/5W. These two bulbs are not interchangeable.
The 12961 is a Philips part number designation for a miniature wedge-style T10 bulb with a W2.1x9.5d base, rated at 12V and 5W. It is a single-filament wedge type, which is a fundamentally different construction from the dual-filament bayonet bulbs used in other model years.
The 12961 bulb is specified only for the front side marker position on the BMW 318i, covering the 1991, 1995, and 1996 model years. It does not appear in any rear side marker specification.
The 3893 is an ANSI-standard dual-filament bulb with a BAY15d bayonet base, rated at 12V. It is designated for specialty applications and is specified for both the front and rear side marker positions for the 1984-1985 model years.
Although the 3893 and P21/5W share the same BAY15d bayonet base type, they are governed by different standards, ANSI and ECE respectively, and carry different wattage ratings. Substitution between them is not supported by the specifications.
The BAY15d and BAZ15d bayonet bases differ in the offset positioning of their locating pins. This physical difference means P21/5W and P21/4W bulbs cannot be fitted into each other's sockets despite their otherwise similar appearance.
The front side marker position on the BMW 318i shows the widest range of bulb variation across model years, with three different bulb types specified depending on the year, compared to the rear position, which uses two ECE-standard types for later years and the 3893 for earlier ones.