• Under-torqued spark plugs do not make full contact with the cylinder head which reduces the plug’s ability to transfer heat out of the combustion chamber, this results in elevated combustion temperatures and as such can cause pre-ignition and detonation. These two conditions can lead to engine damage. Evidence of under-torqueing can be seen via the crush washer gasket not being as thin as expected (unless it's a tapered type plug).

• Over-torqued spark plugs can cause stress to the metal shell, leading to thread breakage (sheer) as shown in the image below. Over tightening can also compromise the plug’s internal gas seal or even cause a hairline fracture to the insulator. Evidence of over torque is easy to spot, for example hex swelling is visible and usually the crush washer is thinner than expected. If the thread has sheered the breakage will look similar to the plug shown below.
• It's advisable to use a good quality calibration certified torque-wrench. Users should always wind down their torque-wrench to the minimum scale setting and never all the way down after each use. A fully loaded torque wrench left in storage for a long period of time can cause a 'set' in the spring and weaken it. The result can be an inaccurate torque reading.

Installation Notes

• Compressed air should be used to clean the area around the existing spark plugs before removal. Torqueing the new spark plugs up against any form of debris and build-up in the threads may prevent the plugs from making full contact with the cylinder head, even if a torque wrench indicates otherwise. This can cause poor heat dissipation which leads to higher cylinder temperatures, possibly causing pre-ignition or detonation.

• On vehicles where positioning a torque wrench is difficult or impossible, finger tighten the plugs first and then follow the rotational guide shown on the NGK packaging with a standard wrench.
• If the engine design means it's difficult to fit or remove spark plugs (or even see them), the plug wrench may tilt off-axis or slip. If the socket slips on the plugs hex and the inside wall of it strikes the ribbing on the plug's insulator, or its terminal, the insulator may break.

• Some new spark plugs feature a washer gasket that is designed to be crushed during the last stages of tightening. This will feel different to tightening any normal bolt or nut up.

• Only use a spark plug gapping tool which pulls on the ground electrodes neck to adjust the gap on precious metal spark plugs.

• The only time anti-seize compound should be used is on vintage spark plugs that do not feature a specialised coating.