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In this how-to article, we will cover the removal, inspection and installation of spark plugs.  Spark plugs are covered in most manufacturer maintenance programs and are generally supposed to be changed every 2 years.  Having fresh, properly installed spark plugs will help increase mileage, power output, and reliability of your vehicle.

Tools / Resources: Consumables:
  • metric / imperial socket set
  • torque wrench
  • spark plug gap tool
  • spark plug socket
  • new spark plugs
  • anti-seize compound
  • dielectric tune up grease
  • disposable rags

Before you start this procedure, ensure you have all of the tools and consumables listed above at your disposal.  Be sure you purchase the proper spark plugs as indicated in your owner's manual.  If you don't already have anti-seize compound, be sure you purchase the high temperature variety (copper).  Similarly, a tube of dielectric (tune up) grease is an excellent investment at the same time.

Once your tools are assembled, the first step will be to record your vehicle's firing order.  This is the order in which the spark plug wires are installed both in the engine (spark plugs) and from the distributor (or coils).

Do not rely on your factory manual for this information - Mazda has the wrong firing order in their manual for the MX-3.

Factory spec. for a Mazda MX-5 Miata is:

spark plug: { NGK BKR5E-11 }

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figure 1

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Take a picture if possible .. otherwise, make a detailed drawing and use white-out or a marker to label all of the spark plug wires + their respective connection points.

This is critical in the event that you decide to remove all of the spark plug wires later on in your mechanical projects.  We are introducing this concept now so that you begin to familiarize yourself with your engine's electrical system

Once you have made a record of the proper firing order, etc .. remove the first spark plug wire and have a look down in the spark plug well.

You are looking for a pool of oil, debris, etc. which you do not want to introduce into the combustion chamber.

If there is any debris, etc. be sure you remove it before continuing further.  If you find oil, this may indicate that you are about due to change your valve cover gasket.

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figure 3

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Not all engines will have spark plugs in such an easy to manage location (most V-engines for example) .. so be sure you make any needed arrangements for ease of access.

Start by removing the spark plug using whatever combination of extensions and sockets required.

We highly recommend that you use a dedicated spark plug socket for this procedure.

When removing the spark plug, depending on who did the installation, you may need to exert moderate force.  We caution you - do not use excessive force to remove a spark plug - if you need to apply more force than perhaps 30 pound-feet, something is wrong.  If this is the case, spray a generous amount of penetrating oil (not WD-40) in the spark plug well and try again in 24 hours.

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Once you have removed the spark plug, inspect the portion which was in the combustion chamber.

A seasoned mechanic can look at a spark plug and tell you whether or not you are having any mechanical problems - an amateur however may not have the same eye.

All you are looking for is a nice tan colour - this indicates nearly ideal combustion.

If you haven't already done so, remove a spark plug from its packaging and place it in the spark plug socket.

Refer to your factory service manual for the appropriate spark plug gap range.

Factory spec. for a Mazda MX-5 Miata is:

spark plug gap: { 1.0 - 1.1 mm \ 0.040 - 0.043 in. }

The spark plug seen in figures 7 and 8 has been cleaned and reused for demonstration purposes.

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figure 7

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figure 8

Be sure you use a gap tool similar to the one seen in figure 8 and not a flat feeler type gauge.  This is important because some spark plugs are made in such a way that using a flat feeler gauge will not give you the true gap.

Se sure you take your time and set all spark plug gaps to the same range.  This is crucial for both mileage and engine output (power).

Once the gap has been set to within spec. remove the spark plug from the socket and apply a small amount of dielectric tune up grease to the insulator.

Use your finger to spread the dielectric grease around the ceramic portion of the spark plug.  This will help maintain a dry environment for the spark plug to wire connection.  The grease will also make reinstallation of the plug wires easier.

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figure 9

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Be sure that you only use the appropriate grease during spark plug installation.  Furthermore, be sure to read all product manufacturer's directions for proper application.
Next, apply a small amount of dielectric grease inside the spark plug wire.  This will also help with proper fitment of the spark plug wire over the spark plug terminal during reinstallation.

Do not use too much grease inside of the spark plug wire - it can be messy during your next spark plug change.

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figure 11

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Now place the spark plug back into the spark plug socket, exposing the threads.  At this point, it is advisable that you apply a very thin coat of copper anti-seize compound.  Be sure to wipe off any excess anti-seize compound.

Using copper anti-seize compound on your spark plug wires is particularly advisable if your engine head is made of aluminum.  It is not uncommon for dissimilar metals to cold-weld without proper precautions.  Think of this as cheap insurance against costly head-work.

It is extremely important that you do not cross-thread the spark plug into the engine - this can cost you dearly in both dollars and hours!  To ensure you do not cross-thread the spark plug, use only your hands during the initial installation.

Take your time and be cautious until you are confident with your abilities - though you should never use anything but your hands to start the installation of spark plugs.

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Once the spark plug is fully seated by hand, use your torque wrench to tighten the spark plug to factory spec.

When installing a new plug for the first time, it is advisable that you tend towards the lower torque numbers.

Factory spec. for a Mazda MX-5 Miata is:

torque: { 15-22 N/m \ 1.5-2.3 k/m \ 11-16 lb/ft}

Once the spark plug has been installed, reinstall the spark plug wire ensuring that the wire is fully seated onto the spark plug.  If you used dielectric tune up grease, this portion should be relatively easy.

Feel for a small "click" type feeling which should indicate that the metal collar in the spark plug wire has fully engaged the spark plug lead.

Repeat the above directions for all remaining spark plugs and you are done!

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figure 15

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