The outer sealing gasket of the spark plug is the only movable component on the spark plug, and it is also the "bridge"which connecting the spark plug and the engine.

Take the common M18 gasket as an example: the different states of the gasket can reflect the different working environments; these can be used to infer the usage; these different states include: color, deformation, completeness, use marks, etc.

Color: The initial color of the washer is galvanized

Initial state (front and back)

Color after normal use

After use, obvious signs of installation and use can be seen on the gasket. The outermost part (the rightmost gasket) does not touch the spark plug seat, so the coating is not damaged by friction. The black inner ring is the contact surface that fits tightly with the mounting seat.

The surface with four points facing upwards is the surface that fits with the engine cylinder head. You can see that the inner ring (including the four points) is black, while the outer ring is a coating color. This is because the outer ring fits tightly with the cylinder head mounting seat and is not exposed to the cylinder environment, while the inner black ring is exposed in the cylinder on the threaded side. The black attachment is carbon deposits.

The above picture shows a gasket that has changed color due to oxidation. The white block on the right picture is the residue of the coating that has not been completely ablated. The oxidation degree of the end faces on the two sides of the gasket is the same. This situation indicates that gas has leaked from this gasket during use. The high-temperature gas from the cylinder caused the gasket to change color.

Deformation: the thickness of the gasket after installation

After the spark plug is installed at the specified torque, the gasket thickness should be within a certain range;

The installation torque value of the M18 washer is 50N.M, and the deformation range of the washer is about 1.45-1.65 mm (initial washer thickness 2.0 mm).

Initial thickness state

Normal status after installation

It can be observed that after deformation, the gap in the middle disappears due to being squeezed by the applied force.

Example: It is obvious from the above picture that the gap in the middle still exists (the thickness can be measured to verify); therefore, the root cause of the gas leakage in this case is that it is not installed properly;

Integrity: Changes in the gasket appearance

Mainly refers to the integrity of the 4 "claws", especially when the gasket is not removed, observe the condition of the 4 "claws"

1.Whether the“claw”is broken;

2.Whether the "claw" is severely bent upward or downward;

If the above situation exists, it means that during the installation process, the "claw" was stuck on the thread, and under the torque installation, the "claw" deformed and forced the washer to "return to its position". In this case, after installation, the washer position and the concentricity of the spark plug body will be offset (check the installation traces at the same time). There is a certain probability that leakage will occur from the weak point.

Use marks: After installation, the indentation and friction marks left by the spark plug and cylinder head torque.

The "step" marks on the installation surface can be clearly seen in the picture. This is a normal mark. From this step mark, it can also be judged whether the installation is too loose, whether there is any deviation during the installation and use, and the degree of eccentricity, etc. These play an auxiliary role in analyzing and determining the cause of the fault.