Spark Plug Failure Analysis #3 Overtorque / Undertorque

Spark Plug Failure Analysis #3 Overtorque / Undertorque

Spark Plug Failure Analysis #3 Over-torque / Under-torque

The technique used for installing spark plugs is essential in avoiding premature plug failure and maintaining top engine performance. It’s a vital process that you may very well have performed yourself at home, and all was well. But after you’ve read this article, you may just reconsider how you go about installing them during your next service.

If done incorrectly, it can lead to ‘knocking’ and ‘pinging’, which is an engine condition that nobody wants to go through. It may even cause bent conrods or other engine damage in serious cases. However, with a bit of knowledge and armed with the correct tools, you can avoid this mishap. Now, let’s delve into why this is so important.

Why Do Spark Plugs Have to Be Correctly Torqued?

When a spark plug is installed to the recommended turn-angle or torque setting, optimal performance is ensured. The amount a plug is tightened is crucial because the torque affects the spark plug’s ability to effectively draw the correct amount of heat away from the combustion chamber and transfer it to the cylinder head.

This means we keep the combustion temperatures correct, resulting in better performance. For argument’s sake, if we were to install a spark plug with the incorrect heat range on purpose, the dissipation of heat would be completely incorrect, meaning poor performance and a huge risk of overheating.

As mentioned previously, incorrect tightening can cause various issues, such as pre-ignition, detonation and overheating.

Of course, the torque setting or turn angle is also set to prevent the spark plug from neither becoming too loose or too tight. During operation and over prolonged periods, the extreme heat cycles spark plugs undergo are quite significant.

spark plug failure analysis

What Causes Incorrect Tightening?

1 ) Out-of-calibration torque wrench – Please ensure the torque wrench is of good quality and calibrated correctly, if you don’t trust the tool, then we don’t recommend using it.

2 ) Excessive dirt/debris in the cylinder head spark plug threads – This can cause a considerable amount of under-tightening. The threads must be cleaned before installation, so spray some brake cleaner onto some kitchen/blue roll and it works wonders. If this step is neglected, your torque wrench may prematurely click before it should, and so the plugs won’t be tight enough. We understand that sometimes access is difficult, so it might be wise to figure out an ingenious contraption to clean them!

3 ) Oil on the cylinder head spark plug threads – Lubricant on the threads can cause over-torqued values by up to 25%! – This is the most common cause of spark plug breakage at the top section of the thread. It is imperative to blow out the spark plug chambers with compressed air before removing the plugs and then clean the threads until they are dry.

4 ) Installing the plugs into a hot engine – Apart from being common sense, it is essential to ensure that spark plugs are only removed and installed on a cool engine. The reason behind this is quite simple, as metal gets hot, it expands; meaning the spark plugs will screw in further and over-torque can occur. This is why you should always replace the spark plugs first, before warming the engine up to perform an oil change.

5 ) Not using the correct spark plug socket – It’s the elephant in the room, but if the plugs are in an awkward location or the engine designer had a serious afterthought, then replacing spark plugs might just become your largest nemesis. For example, consider yourself lucky if you don’t own a BMW N43 engine:

N43 sparkplug-600x481-optimized

Replacing the spark plugs on this engine is merely impossible without the correct type of swivel socket. Socket strike will likely occur in most cases where a normal socket is used instead of a spark plug socket. The number one rule is to only use spark plug-specific sockets with the correct rubber or sprung clamp fitting inside.

Overtightening Consequences

Overtightening causes undue pressure on the spark plug’s thread/hex section, this makes it swell and expand enough to cause the ceramic insulator to become squashed/crushed between the hex and terminal nut at the top. Of course, this leads to the insulator cracking = misfires (voltage can arc through cracks and ground to the cylinder head instead of travelling through the HT lead/coil). Also, the stretching of the metal shell can break the internal gas seals, further causing issues.

Under-tightening Consequences

On the other hand, insufficient tightening will for one; prevent the spark plug from being securely seated on the cylinder head, resulting in inadequate contact with the head which hinders heat transfer.

NGK Recommended Torque and Tightening Angles

With a Torque Wrench:

To find NGK’s recommended torque setting, please enter the spark plug number in the Buyers Guide section at

While viewing the part detail page, click on the “view” link to the right. If a recommendation has been made, it will appear in the specifications image.

Without a Torque Wrench:

If a torque wrench is unavailable, finger-tighten the spark plug, then use a plug socket and wrench to tighten. 1/2-2/3 turn on plugs with a gasket and 1/12-1/8 turn on plugs with a tapered seat.

NGK Recommend Torque Settings (NEW PLUG – DRY THREAD)


DENSO Recommended Torque and Tightening Angles

Thread SizeApplicable ModelsRecommended
Recommended Tightening Angle
New PlugPreviously Used
M 8All Types8-10N・mAbout 1/3 turnAbout 1/12 turn
M10Types other than the ones shown below10-15N・mAbout 1/3 turnAbout 1/12 turn
M10UFE, IUH, VUH, VNH Types10-15N・mAbout 2/3 turnAbout 1/12 turn
M10Stainless Gasket Type
10-15N・mAbout 3/4 turnAbout 1/12 turn
M12All Types15-20N・mAbout 1/3 turnAbout 1/12 turn
M14Types other than the ones shown below20-30N・mAbout 1/2 turnAbout 1/12 turn
M14Stainless Gasket Type

20-30N・mAbout 2/3 turnAbout 1/12 turn
M18All Types30-40N・mAbout 1/4 turnAbout 1/12 turn
Taper seat
All Types10-20N・mAbout 1/16 turnAbout 1/16 turn
If the cylinder head material is cast iron20N・m
If the cylinder head material is aluminum17.5N・m
If the cylinder head material is cast iron30N・m


It is vital to ensure the correct tightness when installing spark plugs, either scenario (under or overtightening) will ultimately lead to heightened temperatures within the combustion chamber, subsequently increasing the risk of pre-ignition and detonation.

Check out NGK’s video regarding Knocking and Pre-ignition due to poorly installed spark plugs.

Previous Spark Plug Failure Articles:

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