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Tools Needed
- Spark plug socket (16mm or 21mm depending on car)
- Ratchet with extension bar
- Torque wrench
- Compressed air or vacuum (to clean plug wells)
- Gap gauge (if plugs aren’t pre-gapped)
Step-by-Step
- Let the engine cool completely (hot plugs can strip aluminium threads).
- Remove the engine cover if fitted.
- Disconnect the ignition coil connector and remove the coil (one per plug on modern cars).
- Clean around the plug well with compressed air to prevent debris entering the cylinder.
- Use the spark plug socket to unscrew the old plug anti-clockwise.
- Inspect the old plug — note its condition (colour, electrode wear, deposits).
- Check the new plug’s gap matches specification (most come pre-gapped).
- Do not apply anti-seize. Modern NGK and Denso plugs are supplied with a trivalent zinc-chromate or nickel plating that already prevents seizing in an aluminium head. NGK states that anti-seize acts as a lubricant and can alter the torque you actually apply by up to 20% — enough to strip the cylinder-head thread. Fit the plug dry.
- Thread the new plug in by hand first to avoid cross-threading.
- Torque to specification — check your handbook, and check which seat your plug has. On an aluminium head NGK specifies roughly 24–29 Nm for a 14mm flat (gasket) seat plug, but only about 10–20 Nm for a 14mm conical (taper) seat plug.
- Refit the ignition coil and connector.
- Repeat for remaining cylinders.
Common Mistakes
- Using the wrong torque for your seat type — a flat (gasket) seat and a conical (taper) seat take very different figures; check which yours is before setting the wrench.
- Using anti-seize — NGK and Denso both advise against it on plated plugs, because it lubricates the thread and causes over-tightening.
- Not using a torque wrench — over-tightening strips the thread; under-tightening causes leaks.
- Changing plugs hot — aluminium cylinder heads expand when hot, making stripped threads more likely.
- Dropping debris into the cylinder — always clean the plug well first.
- Wrong gap — incorrect gap causes misfires. Verify even on ‘pre-gapped’ plugs.
Frequently Asked Questions
How long does it take to change spark plugs?
30-60 minutes for a 4-cylinder engine. Some engines have difficult access (especially BMW and Subaru boxer engines) which can take longer.
Can I change just one spark plug?
You can, but it’s best practice to change all plugs at once. They wear at similar rates, and inconsistent plugs can cause rough running.
What torque should spark plugs be?
It depends on the seat type as well as the thread size, and the two are very different. NGK’s figures for an aluminium head are roughly 24–29 Nm for a 14mm flat (gasket) seat plug, 15–20 Nm for a 12mm flat seat, and only about 10–20 Nm for a conical (taper) seat plug of either size. Cast-iron heads take higher figures. Always check your specific engine’s torque spec — over-tightening is the most common DIY mistake.
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How we check fitment for this guide
We cross-reference fitment figures against manufacturer specifications and OE-supplier catalogue data where available, and matched to the standard codes that actually govern fit — battery type/group sizes, wiper lengths and fitting types, bulb fitment codes, tyre size markings, and oil specifications. We work by make, model, year, engine, and build date, because a single model can take different parts across its life. Where the right part depends on your exact vehicle, we tell you to confirm by registration or VIN. More on our method →