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Our car recently suffered from a stripped spark plug. Plugs had been replaced about 6 months previously. Could this serious fault have been caused by a new plug being incorrectly fitted?

Are you able to tell me what alerted you to the fact the spark plug thread was stripped?

Did the car start to develop a problem which was traced back to a stripped thread or was the plug removed for a particular reason and the damaged noted at that time?

I am assuming the damage was to the thread in the cylinder head itself rather than just the spark plug, can you confirm?

These sorts of jobs can be a mechanics worst nightmare and they can easily become the innocent party simply because they were the last or second to last person to work on the engine.

Can you also provide the make/model of vehicle and the repair cost please.

Look forward to your reply.

Many thanks for your response. The vehicle in question is a 2002 Ford Escape with about 70,000kms on the clock and was being driven at the time the plug blew. The fault was traced to a stripped thread in the cylinder head and has since been repaired with a spark plug thread repair tool. Costs of repair include $560.00 labour (8 hrs) and $161 for materials. I would be interested to know if the stripped thread could have been caused by the incorrect (cross threading) placement of the spark plug when all plugs were replaced back in late January.
Sorry but I forgot to add that the repair was carried out in the same workshop that replaced the plugs 7 months previously.

A cross threaded plug can often be tightened sufficiently not to cause a problem, provided no attempt is made to remove it. In some cases, the addition of some sort of locking compound can also cover up somebody’s mistake and make the plug secure for a reasonable time.

Often the extent of the damage to the cylinder head thread is only made obvious when the cross-threaded spark plug is removed during the next service etc.

Damage to the thread can also be caused by the plug being over tightened (depending on the type of plug) or the thread in both the cylinder head and plug not checked for debris etc before a plug is refitted or replaced. This can create damage to the cylinder head thread on removal of the plug.

Good workshop practice is to always fit a plug using hand pressure only and screwing it in a reasonable amount before a wrench is used. This way a mechanic knows the thread is in good condition. Some garages go the extra mile and use a torque wrench to ensure the plug is refitted correctly.

When a plug blows out of a cylinder head the first suspicion is, it was loose. Over time a loose spark plug can unscrew itself further by the natural vibration of the engine. With the pressure applied from within the combustion chamber to the base of the plug, it would eventually created a situation where the plug is almost ‘fired’ out of the cylinder head complete with thread.

One easy inspection would be to check the other plugs. Its highly likely if one plug was loose then the others would be as well.

I don’t want to cast suspicion on your garage but it would be interesting to know what they believe to be the cause of the stripped thread.

We have made contact with both Ford NZ and their dealer network to try and determine if this is a common problem or weakness with this particular model. They say no, which has confirmed the thoughts of the technical heads within the AA.

Again, many thanks for your response. The garage in question assurred me that replacement of the plugs previously had nothing to do with the thread being stripped. I was interested in getting another opinion and so my approach to the AA. The garage told me they believed it was "just one of those things" and could/would not say what they believed caused it. They did add that it was highly unusual in this make and model.

Hardly a response that fills one with confidence I have to say.

If you can put your hand on your heart and say nobody else has worked in that area of the engine since the plugs were replaced, then the spot light is still focused on the garage to provide a better explanation as to why this happened.