Indexing spark plugs is to line up the diode on the plug so the engine functions as efficiently as possible. This is particularly important on direct injection engines, which are used in most modern vehicles.
In most cases on Genuine, OE, and OEM spark plugs, this is as simple as following the factory torque spec. That way, the plugs end up torqued within a 45-degree index window. Mercedes offers specific notes on specs for different plugs based on a particular vehicle.
Based on the spark plug and the specific car, there may be a torque spec or indexing procedure to ensure they are at the proper angle. This is due to there being minute tolerance differences between brands.
If you're interested in finding spark plugs that fit your vehicle, head over to fcpeuro.com and start by entering your vehicle into our vehicle selector.
Once your car is added to the selector, there are a couple of important ways that we advise correct fitment.
The first is the "fitment indicator" shown on the Product Page after selecting a part. This box is below the "add to cart button" and has two states. It will either say " Yes this part fits" or "Part may not fit, click to verify".
Sometimes the green box will be accompanied by extra fitment notes. These notes may be independent of the information gathered by the vehicle selector. IE: VIN-specific options, need to be taken into account when determining overall fitment.