The Mercedes-Benz CLK320 front side marker bulb uses 12961 or 12821 size, while the rear side marker bulb uses 12961 or P21/4W size. The replacement chart covers 1998 to 2005 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 two most common bulb types across the Mercedes-Benz CLK320 side marker specifications are the 12961, the 12821, and the P21/4W, each serving distinct positions and model years.
Both the 12961 and 12821 are Philips part numbers for miniature wedge bulbs of the T10 type, operating at 12V and 5W with a W2.1x9.5d base, making them electrically and physically equivalent to one another.
Despite their shared T10 form factor and identical electrical ratings, the 12961 and 12821 carry different Philips part numbers, which may reflect differences in construction, filament design, or product line classification within the Philips catalog.
The front side marker of the Mercedes-Benz CLK320 uses the 12821 for the 2000-2002 model years and the 12961 for the 1998-1999 and 2003-2005 model years, meaning the two T10 bulbs are not interchangeable across all years without confirming the specific model year first.
The P21/4W is a dual-filament bulb rated at 12V with a 21W primary filament and a 4W secondary filament, following the ECE standard used in European and worldwide automotive lighting applications.
The P21/4W uses a BAZ15d bayonet base, which is an offset-pin design that prevents incorrect installation by ensuring the bulb can only be seated in one orientation, a feature relevant to dual-filament applications where filament assignment is position-dependent.
The rear side marker of the CLK320 uses the P21/4W for the 1998-2002 model years, while the 2003-2005 model years use the 12961, representing a shift from a BAZ15d bayonet-based dual-filament bulb to a W2.1x9.5d wedge-based single-filament bulb.
Because the P21/4W and the 12961 use entirely different base types, a BAZ15d bayonet socket and a W2.1x9.5d wedge socket respectively, the two bulbs are physically incompatible and cannot be substituted for one another without a socket modification.
The W2.1x9.5d designation on the T10 wedge bulbs refers to the base standard, where the numeric values describe the contact wire width and the overall base length, both of which determine socket fit.