Tools Needed
- Spark plug socket (16mm or 21mm depending on car)
- Ratchet with extension bar
- Torque wrench
- Anti-seize compound
- 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).
- Apply a small amount of anti-seize to the new plug threads.
- Thread the new plug in by hand first to avoid cross-threading.
- Torque to specification (typically 20-30 Nm — check your handbook).
- Refit the ignition coil and connector.
- Repeat for remaining cylinders.
Common Mistakes
- 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?
Typically 20-30 Nm for 14mm thread plugs and 15-20 Nm for 12mm thread. Always check your specific engine’s torque spec — over-tightening is the most common DIY mistake.
