How to change spark plugs

Why change your spark plugs ?

You may not need to change your spark plugs, but as part of looking after your car and servicing it, it is useful to know how to take them out and inspect them.  Perhaps give them a clean and check the electrode gap.

Checking or even changing your spark plugs is an easy and quick job.
Giving them a clean or changing them can help improve the performance and economy of your car. 20 minutes doing this job can save you money at the petrol pumps in no time at all and can reduce your emissions too !!

What tools do I need ?

flexi spark plug tool
Flexi spark plug tool
spark plug spanner
Spark plug spanner

spark plug tool
Spark plug socket
spark plug socket for fitting to ratchet
I have a few of the above spark plug tools from working on different cars.
Not all spark plug spanners work on all cars.
Most cars have 14mm spark plugs, that is the thread diameter, not the size of the hexagonal end.


Wire brush for cleaning off deposits if you refit the old spark plugs.
Gap gauge feeler gauges for checking the electrode gap.

So how do I change the plugs then ?

We have selected some videos below that best explain how to do this job.
We recommend having a look at all of them, and also have a look at your cars user/owner manual or Haynes Manual where you should find the correct electrode gap for your specific car.

What to look for when the plugs are out of the car.

Even if you are fitting new plugs and not renewing the old ones, it is important to inspect the old plugs as this will tell you how good or bad your engine is running.
Here are some images with explanations ;

spark plug removal
This is how a good spark plug should look.
The colour (Grey-tan to white) shows that the engine is running well.

how to replace spark plugs
This plug is showing excessive electrode wear.
It could cause an engine missfire when accelerating and could also cause difficulty starting the engine. Old age is normally the cause, should have changed it sooner.

how to change spark plugs
Notice that the insulator has cracked and chipped off. A common cause is incorrect electrode gap, but sometimes it can be caused by engine detonation. Detonation can be due to incorrect octane fuel being used or incorrect ignition timing.
Electrode gap is important, ensure yours are set as per your Haynes Manual

inspecting spark plugs
Excessive electrode wear.
This could be due to the fact that the incorrect spark plug has been fitted. Spark plugs have different heat ranges, fit the wrong one and this happens.

how to check spark plugs
A spark plug fouled with oil.
Oil cannot get into the combustion chamber (where the plug sit) unless other parts of the engine are worn.  These could be worn valve guides, worn cylinder linings or worn piston rings

inspecting spark plugs
Fouled with carbon deposits.
This can be a common thing on hard driven cars.  It indicates a rich fuel mixture, weak ignition or wrong plug (too cold). Think about a tune-up or mini-service consisting of new air filter, ignition leads, carb clean etc.

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