Spark plugs — description and marking (Mercedes-Benz S-Class W126)

The spark plug consists of a middle electrode, an insulator with a housing and a ground electrode. The ground electrode is hermetically secured in the insulator, the insulator is firmly connected to the housing. An ignition spark jumps between the middle and ground electrodes, which should ignite the working mixture. The engine starting, idling, acceleration and maximum speed depend on the spark plugs. In this regard, the type of spark plugs installed at the factory, which is determined by the heat rating, should not be changed without reason. The heat rating indicates the degree of heat output of the spark plug in the engine under certain operating conditions. Spark plugs for the engine are selected in such a way as to be able to achieve the self-cleaning temperature under all driving conditions. The lower the heat rating of the spark plug, the higher its resistance to pre-ignition and the lower its resistance to contamination. The higher the heat rating of the spark plug, the lower the resistance to pre-ignition and the higher the resistance to contamination.

The heat rating is contained in the spark plug designation. The designation is deciphered as follows.

Spark plugs from Bosch

• 1 W - thread M14x1.25 with flat sealing ring, key size 21 mm.
• 2 F - thread M14x1.25 with flat sealing ring, key size 16 mm.
• 3 M - thread M18x1.5 with flat sealing ring, key size 25 mm.
• 4 H - thread M14x1.25 with conical sealing ring, key size 16 mm.
• 5 D — thread M18x1.5 with conical sealing ring, key size 21 mm.

(2)R — with resistance for protection against interference. The operation of the ignition system does not change with the presence of resistance.


(3) — heat rating. The scale of numbers is designated from 05 ("cold") to 13 ("hot"). In this case, the designation 7 corresponds to the old heat rating of 175 (early designation), 6 - 200, 5 - 225, etc.
(4)A - thread length 12.7 mm, normal spark position.

B - thread length 12.7 mm, closed spark position.

C - thread length 19 mm, normal spark position.

D — thread length 19 mm, closed spark position.

(5) — central electrode material: Cr-Ni alloying, C — central electrode made of NiCu alloy, S — silver central electrode, P — platinum central electrode, O — standard spark plug with reinforced electrode.

The copper core in the central electrode and even more so the silver central electrode increases the thermal conductivity and thus the heat transfer. The advantage of a spark plug with a platinum central electrode is its high flammability, as well as low wear and a larger heat transfer area.

Beru spark plugs

• 1 — thread diameter in mm, here M14x1.25.
• 2 — design designation, eg K for conical fit, R for resistance to interference.
• 3 — heat rating (like BOSCH).
• 4 — thread length (like BOSCH).
• 5 — electrode material, eg V - for copper.

Deviation from the factory-recommended heat ratings should only be made when operating conditions deviate significantly from normal and production problems occur. If the spark plugs are constantly covered with soot, then the self-cleaning temperature is not reached (only when driving short distances), then spark plugs with the next higher heat rating are recommended. If the engine is used exclusively at full throttle, then spark plugs with the next lower heat rating can be used.