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An easy way to destroy a spark plug if your not extra careful, is to use a feeler gauge in between the centre electrode and the earth electrode to adjust the gap wider. You will not even know you have done any damage. If the engine doesn't misfire when you first start it then it will crap itself in a short space of time, if your unlucky. Don't take the risk. The pictures below show two tools used to open the plug gaps safely without damaging the centre electrode. Most prepackaged plugs come pre-gapped luckily. All you have to do is check them. When closing the gap, tap the electrode very gently as the electrode bends very easily. Also check that the HT lead terminal is screwed on tight. If left loose this can cause radio interference and sometimes misfiring of the plug.

Ensure that the seat area is perfectly clean before installing. Any dirt that jams in between the plug seat and the plug will change the heat range of the plug by making the heat path longer thereby making the plug hotter and eventual failure will occur. When this happens an incorrect diagnosis of the plug will surely happen and will lead to, someone to richen the carb jets or install a colder plug by error. The best approach to this problem is to use compressed air. I always loosen the plugs out about three quarters of the way and then use an airgun to blowout any debris & dirt between the plug and the seat area.

The specs below are for a Bosch plug. Other manufacturers may have different specs.

A problem I have come across quite often is that some plugs are extremely difficult to remove due to seizure of the threads. This generally happens when the plugs have not been changed out on a regular service. Sometimes it is the brand of spark plugs used and the material the body is constructed from. When they seize, you are faced with a dilemma. Do I force the plug out or not. Decisions, decisions. If you force it out and the plug breaks you are in deep shit, if the plug hole is difficult to get to. The head may have to be removed to get the old plug out. If the plug does come out, the tread will definately be damaged and will require repair. The top photo is a thread chaser costing less than $10. It does a good job of restoring the thread. Ensure you place adequate grease on the thread of the tool to catch any metal cut from the threads.

To stop spark plugs seizing in the head I highly recommend to LIGHTLY coat the threads with a high temp anti-seize compound. The compound will not interfere with heat dissipation of the plug and therefore will not affect the heat range.

By the way, if you intend reusing a flat washer type plug, then use a torque wrench to tighten it to the plug manufacturers recommended torque. The reason for this is that the washer will have already been compressed during the first installation, compressing it again by a quarter of a turn will over tighten the plug and possibly lead to breakage now or when you next go to remove them and over tightening may cause damage to the thread.