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How to fit spark Plugs

Without spark plugs, your petrol engine won’t start or run. But, for the engine to run smoothly they need to be maintained at regular intervals (your owners’ manual will tell you how often this is needed). Warning signs that your spark plugs need to be replaced include difficulty in starting the car, a rough sound when idling, misfiring, reduced engine power and increased fuel consumption.

If you suspect your spark plugs need to be replaced, continue reading to find out what you need to do.

Tips and tricks when fitting spark plugs

Before we get to work, here are some useful tips and tricks to  help you when fitting spark plugs.

  • Only work on one spark plug at a time – remove it, inspect it, clean it, and (if it’s salvageable) gap it. Then replace it and move onto the next spark plug in cylinder order. This maintains the correct firing order and ensures you won’t connect spark plugs to the wrong wire.
  • Always use the correct tools and techniques for this job. Watch the video to see the techniques in action.
  • Good spark plugs use fuel efficiently. This means that as they become more worn, they take a toll on engine performance and fuel consumption. When you calculate the additional fuel consumption, early replacement of your spark plugs makes sense.

Tools needed to fit spark plugs

Before you start work, collect together the tools and parts you’ll need for the job:

  • Spark plug socket
  • Torque wrench
  • Ratchet
  • Socket extension
  • Spark plug gap gauge
  • Compatible spark plugs
  • Needle-nose pliers
  • Rags
  • Spark plug wire puller
  • Short length of flexible hose
  • Dielectric grease

You might also need:

  • Anti-seize compound
  • Spark plug wires

How to remove spark plugs

1. Locate the spark plugs in your car

If you’re not sure where your spark plugs are, consult your owners’ manual. Your owners’ manual will also tell you how many spark plugs you have, the correct ‘gap’ and the right socket to use to remove them.

It’s recommended to only work on one spark plug at a time and then move onto the next in cylinder order. This ensures the spark plugs and wires are always connected to the right place when you replace them, as spark plugs fire in a specific order and crossing wires will cause the engine to run roughly, as well as potentially damaging your engine. This is also a good time to check the wires for any damage or cracks. If you do remove more than one wire at a time, number th em to ensure that you reconnect them to the correct place afterwards.

2. Let your engine cool before removing the spark plugs

Wait until the engine is cool enough to touch before attempting to remove the spark plugs. Then, clean around the spark plugs with an old paint brush or a compressor. This will prevent dirt from falling through the plug hole into the engine.

3. Remove the first spark plug

Spark plug socket

Carefully remove the wire plug or cap by gripping it as closely to the bottom as possible and slowly working it off. It’s important not to tug it out as this could damage more than just the spark plug.

Use your socket wrench with the extension bar and ratchet to remove the spark plug carefully and slowly.

4. Measure the gap of the spark plug

Check for the correct gap between the tip and the electrode. An incorrect gap can lead to misfiring, which can increase fuel consumption, raise exhaust emissions and damage the oxygen sensors and catalytic converter.

While most spark plugs are pre-set according to the specific spark plug model number and application, it’s worth double checking and consulting your owners’ manual for the correct gap for spark plugs for your car.

spark plug measurement

5. Check the spark plug for wear

Your old spark plugs can give you a good idea about the overall health of your ignition system. You can take a look into our Trouble Tracer Charts in our Maintenance category.

6. Clean around the threads

This is a good time to check and clean the threads of your spark plugs and holes, and to ensure the screw-on terminals (if there are any) are tight.

How to install spark plugs

1. Use the correct replacement spark plug

Spark plugs are not interchangeable, as their size, shape and function can vary. So, it’s always best to check your owners’ manual to see exactly what type of spark plug you need.

2. Lubricate the new spark plugs

Lubricate ceramic insulator of the new spark plug with dielectric grease to prevent electrical noise and interference. Be careful not to get any grease on the centre or side electrodes.

3. Insert the new plugs

Carefully ‘seat the plug’, this means starting to thread the spark plug into the engine by hand, in a clockwise direction for two full rotations. It ensures your spark plug goes in straight without ruining the threads on the plug or in your engine.

Continuing tightening with a special spark plug socket to the recommended torque -always available on the packaging- (check your owners’ manual for this). If you use anti-seize compound on the plug threads, reduce the recommended torque by 10%.

If you under-torque your spark plugs, your spark plugs will have poor heat transfer and cause elevated combustion chamber temperatures and pre-ignition and detonation.

And, if you over-torque your spark plugs, you will stress the metal of your spark plugs and distort the inner gas seals. Your spark plugs will also have bad heat transfer and cause elevated combustion chamber temperatures and pre-ignition and detonation.

Spark plug socket

4. Reattach wires, plugs and caps

Reconnect each wire to the spark plugs, checking you have put them in the correct place.

5. Test your work

Lastly, start your engine to check  everything is working correctly.

If you want to learn more about how to correctly fit your spark plugs, watch our video: 


For informational purposes only. We are not liable for any damages resulting from your reliance on this content.