Spark plug, copper grease?

NC 2.0

I have some NGK iridium plugs to install and just wondered if you need to apply copper grease to the threads?

ILTR6A-13G 3789

Hi - no please don’t use copper grease on the plug threads (or any other threaded joints for modern vehicles unless explicitly stated in the OEM manual).

Use of any lubricant on a threaded joint will put more stress through the threads and components than that are designed for when tightened to the recommended tightening torque. This is because tightening specs are set with a specific thread friction - reducing the friction for a given torque will increase the load generated by the threads (the clamp load), thus potentially damaging the components.

In this case, it would potentially lead to overloading of the spark plug threads, potentially failing the part either immediately or over time fatigue failure), leaving you with a broken spark plug in your head.

The other potential failure mode is that the plug may be more unacceptable to vibration unwinding due to the reduced thread friction, especially if under-torqued if trying to compensate for the copper grease.

Hope this helps


For further info, NGK released a technical bulletin regarding use of anti-seize compounds on their spark plugs - attached below.

For my sins, my career is as a threaded joint specialist for a uk oem… not glamorous but still interesting engineering!


That’s perfect thank you.

That figures well tbh.
Popped in a new set of NGK’s in the Mk1 last week.
A) Despite being 5 years old they were fine thread wise…still quite shiny.
B) The electrodes were fine as well…including gaps.
Booger it!

The old plugs were hard to remove. The first one was oily the other 3 dry (front of car to rear)

NGK went in nicely and it idles better now but the DSC is off. I unplugged the battery to do it. So maybe that will recalibrate itself when driving.

If you turn the steering wheel lock to lock and back again this will reset the light.


As TMW says definitely do not lubricate any fastener, just ensure threads are clean and dry. Any lubricant lowers the friction of the thread and when torque tightened results in higher loads in any bolt. As an engineer I have seen many a stretched and failed bolt through overtightening when the thread was lubricated. In the Doncaster railway works we ran a fasteners and torque tightening training courses because the consequences of failed bolts on a train could be disastrous.

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A slight tangent here. What about Threadlock?

I’ve noticed a lot of the MX5 bolts have a touch of blue Threadlock, and it helps keep the rust out of the thread as well for later release.

I normally use Loctite 248 (Medium) on things I don’t want to do up mega tight, but still worry about coming loose, eg on caliper bolts. But is this a good idea?

Don’t some plug manufacturers tell you to screw in until you feel the shoulder touch, then give another part turn of so many degrees (90?)

NGK plugs show hand tight and then 1/16 for tapered plugs which is what the MX5 uses.

Flat seat ones are 1/2 2/3 turn after hand tight.

There is no need on this engine due to it’s design, on some it is worth doing, if you know what you’re doing and why.

Thread lock is a difficult one - I know some OEMs use it, some (including the one I work for) don’t except for some very specific repair procedures.

The reason against it is whilst its possible to accurately meter the amount of thread lock used in production, in a service environment you have no control over who much is used short of issuing a guidance in a technical bulletin.

Too much and you can over-lubricate the thread and cause the issues discussed above (and then possibly have it fail when trying to remove it as the excess adhesive stops you undoing it!), too little and it wont have the efficacy indented by the designers.

Another issue is once the bolt has been removed again, there’s a fair chance you will have a load of set adhesive left over in the threads, so when you then come to re-assemble there will be more resistance to doing up the bolt. Thus if you are tightening to a set torque, you are using more of that torque to overcome that additional resistance and thus less goes into generating the clamp load. This is also true of rusty old bolts.

Ultimately if the OE guidance is to use it then use it in line with their specification, but if its not then I avoid it if at all possible.

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Someone I knew long ago seemed to work on the principle “tighten until they go soft then back off quarter of a turn” :grinning:


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