Installing Spark Plugs: Torque and Gap Are Critical – UnderhoodService

Installing Spark Plugs: Torque and Gap Are Critical

Spark plug torquing
Fig. 1 This spark plug was overtorqued. The contact marks, plug wear and cracking on the ceramic insulator indicate that it was installed using an impact driver rather than a torque wrench.

When installing spark plugs, a technician must be aware of the torque specifications for that particular plug. Remember that either overtorquing or undertorquing a spark plug will affect the quality of the installation and the performance of the vehicle’s engine.

If a spark plug is undertorqued, it will be loose in the cylinder and, as a result, combustion gases will be allowed to escape from the engine instead of working to power it. Furthermore, a loose spark plug can be damaged by engine vibrations.

Many spark plugs that are submitted for warranty claims are damaged due to undertorquing, which may not be covered under warranty.

On the other hand, overtorquing a spark plug has equally damaging effects. If too much pressure is put on the seat of the spark plug, it will damage the insulator, the shell and other components of the plug. Such damage leads to premature failure of the spark plug and, again, loss of engine performance — and is most likely not covered under warranty.

Fig. 2 This spark plug shows evidence of damage due to undertorquing. Because it wasn’t tight enough in the cylinder, the spark plug vibrated itself loose and allowed combustion gas deposits to form around the plug. Additionally, the vibrations of the loose plug in the cylinder eventually separated the ceramic insulator from the shell and broke off the center and ground electrodes.

The only correct way to install a spark plug and ensure proper functioning is by using a torque wrench to tighten the plug to the proper ft.-lb. of pressure.

Spark Plug Gap

Another important factor in ensuring proper functioning of a spark plug is making sure that the gap is correct. Spark plug gap is the distance from the center electrode tip to the ground electrode. This gap is the area in which the electric spark is generated and the flame kernel forms to ignite the air-fuel mixture in the cylinder. If the gap is too small, the flame kernel will not grow and will leave the air-fuel mixture unburned, reducing engine performance and gas mileage.

On the other hand, if the gap is too large, there won’t be enough electricity provided to create a spark. This will lead to a misfire. In the past, technicians would have to manually check and adjust the gap to a precise measurement to ensure proper engine performance.

Courtesy of Robert Bosch LLC.

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