How firm should the spark plugs be

Torque wrench

Is a torque wrench required to change the spark plug? This question can be answered with a clear “Yes, absolutely!”, Because the spark plugs must not be tightened “by feeling”, but rather with the correct torque according to the manufacturer's instructions.

Too much torque can damage the spark plug and possibly the cylinder head. On the other hand, too low a torque causes poor sealing and heat build-up in the spark plug.

Precise release torque wrenches have been developed to ensure controlled screw tightening. The Remscheid-based manufacturer Hazet recommends a torque wrench in the Newton meter range from 5 to 60 Nm, such as the Hazet 6110-1 CT.

When changing spark plugs, the tightening torque is the quantity that indicates how firmly a spark plug must be tightened. The corresponding tightening torque is defined in such a way that it is not possible to loosen the connection under all operating states. Well-known spark plug manufacturers therefore point out the correct tightening torques in their documentation when changing spark plugs. Past experience has shown that, even if it rarely occurs, temperature and vibration damage are often caused by an incorrect tightening torque, i.e. by assembly errors. Modern engines react extremely sensitively to such errors.

If the tightening torque is too low, there is a risk of compression loss or loosening of the center electrode as well as temperature damage due to reduced heat dissipation. In the worst case, the spark plug can even come loose by itself. If the tightening torques are too high, there is a risk that the cylinder head can be damaged. Forces that are too strong on the spark plug can even tear the thread.

Can you still see afterwards whether a spark plug has been tightened with the correct tightening torque?

Yes, the size of the height (thickness) of the spark plug sealing ring provides information here. A spark plug sealing ring that is not compressed is an indication of tightening with too low a tightening torque. A heavily squashed sealing ring, on the other hand, indicates that the tightening torque is too high.


- The professional and proper assembly of the spark plug makes it imperative to adhere to the prescribed tightening torques.
- It is essential to observe the information provided by the manufacturer in their assembly instructions.
- The height of the spark plug sealing ring provides information about the tightening with the correct tightening torque.
- Modern motors are extremely sensitive to installation with the wrong tightening torque.

Installation tips for installing spark plugs from Beru

Tightening torques for spark plugs from NKG

For Bosch spark plugs, the tightening torques are specified on the packaging.
The following apply:

- 10 Nm for the U version

- 20 Nm for versions D, H, X, Y, Z, V, WS

- 28 Nm for versions F, M, W