Main.TrailIndexPage | Electrical Systems | Ignition system | Electrical.SparkPlugs

Spark Plugs

It is a major component of the Ignition System.

Bosch spark plugs what do the letters and numbers mean?

The nomenclature "W7DC" breaks down as follows:

W means 14mm diameter, 7 is heat range (higher numbers 8 or 9 are hotter plugs), D is the length of the thread (how deep the plug goes), C means copper tip (an S plug has a Silver tip, P is Platinum.)

Bosch numbering

A hotter plug (9) reaches operating temperature faster, is more resistant to overheating, and does not foul as easily...

W7DC is the Bosch plug most commonly recommended for the 113 engines. W7 is a colder plug for warmer climates and high rpm or highway driving.

W8's and 9's are hotter running plugs and might indicate for colder climates and stop and go traffic. 8's and 9's are used by some drivers in "normal" driving conditions with satisfactory performance and plug life so the heat range does not seem critical in most applications.

Bosch plugs

An "R" (e.g., WR7DC) indicates a Resistor plug. The additional resistor adds about 5-6 kOhm to the total resistance of each wire plus connectors and may require a stronger coil.

R might help suppress Radio RF static and since Bosch does no longer sell non-resistor plugs both Bosch and Mercedes now recommend these plugs. However, some argue that Resistor plugs are not good for these engines.


Describe, in general terms the function of this component. Meaning what is it there for and what role it plays. Describe how it works, the inside mechanism. Use diagrams to explain.


Do not use anti-seize on NGK plugs! Read the NGK technical note here:


Design symbols used in NGK spark plugs:


Spark Plug Condition (Color)


< Electrical.Points | Main.TrailIndexPage | Electrical.SparkPlugWires >