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How-To Guide
How to Change Spark Plugs
Changing spark plugs restores something you did not notice losing. Worn plugs degrade gradually — a slightly rougher idle, a little hesitation, fuel economy drifting down over a year — so the improvement after a change is often larger than people expect.
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It is also the job with the most expensive single mistake on this site. Aluminium cylinder heads strip if you over-tighten a plug, and removing plugs from a hot aluminium head can pull the threads out on the way. The engine must be completely cold. That one rule prevents the failure that turns a modest job into a cylinder head repair.
What You Need
- The correct plugs for your engine: heat range, reach and gap are all engine-specific. Use the part number from your manual, not a general recommendation — our spark plugs guide, or check on Amazon
- A socket set with a spark plug socket: a plug socket has a rubber insert that grips the ceramic and stops you dropping the plug — our socket set guide, or check on Amazon
- A torque wrench: this is the job where guessing costs a cylinder head. Plug torque figures are surprisingly low — our torque wrench guide, or check on Amazon
Changing Spark Plugs, Step by Step
Start with a completely cold engine
Not warm. Cold — ideally left overnight. Aluminium expands more than the steel plug threads, and pulling a plug from a hot head can bring the threads with it. That failure means a thread repair insert or a cylinder head, and it is entirely avoidable by waiting.
This is the single most important instruction in this guide. If the car has been driven in the last few hours, do it tomorrow.
Get access and photograph everything first
On a four-cylinder the plugs are usually along the top of the engine under a plastic cover. On a V6 or V8 one bank is often behind the intake, and it may be a long job. Take photographs before disconnecting anything — connector routing is much easier to photograph than to remember.
Remove the engine cover, then the coil pack or lead for the first cylinder only. Working one cylinder at a time removes any chance of mixing up leads on an engine with plug wires.
Blow out the plug well before removing anything
The recess around each plug collects grit, and anything sitting there falls straight into the cylinder the moment the plug comes out. Compressed air is ideal; a vacuum with a narrow nozzle works. Do this for each cylinder before you loosen its plug.
Debris in a cylinder means removing the head to get it out. Thirty seconds of prevention.
Remove the old plug and read it
Fit the plug socket squarely with an extension so you are pulling straight rather than at an angle. Break it loose, then unscrew by hand once it is free.
Look at the tip before you throw it away, because it is a free diagnostic. Light tan or grey is a healthy engine. Black and sooty suggests running rich. Oily means oil is reaching the combustion chamber, usually valve stem seals or rings. White and blistered can indicate running lean or overheating — worth investigating rather than just fitting a new plug.
Check the new plug rather than re-gapping it
Modern plugs come pre-gapped for their application. Check the gap gently against your specification if you like, but do not re-gap fine-wire iridium or platinum plugs with a conventional tool — the precious-metal tip chips or bends and the plug is ruined.
If the gap is significantly wrong out of the box, you almost certainly have the wrong part number rather than a plug needing adjustment. Stop and check.
Thread the new plug in by hand, always
Start every plug by hand, using a length of rubber hose over the plug end if access is tight — the hose grips well enough to turn the plug and slips if it starts to cross-thread, which a socket will not do.
Never start a plug with a socket and a ratchet. Cross-threading an aluminium head is the second way this job becomes expensive, and hand-starting makes it nearly impossible.
Torque to specification, which is lower than you think
Once the plug seats, torque it to the manufacturer's figure — typically somewhere between 15 and 25 ft-lb for most plugs, which is far less than most people apply by feel. Over-tightening stretches or strips the threads; under-tightening lets combustion gases past the seat and lets the plug overheat.
A small amount of anti-seize is sometimes recommended and sometimes explicitly forbidden by the plug manufacturer. Follow the plug maker's instruction, and if you do use it, reduce torque slightly because it changes the friction.
Refit the coil and repeat, one cylinder at a time
Refit the coil pack or lead for that cylinder before moving to the next. A dab of dielectric grease inside the boot helps it come off cleanly next time and keeps moisture out.
Work through the remaining cylinders the same way. When all are done, refit the cover, start the engine and let it idle. A brief rough patch as the fuel trims settle is normal; a persistent misfire means a coil not seated properly, so recheck rather than driving on it.
Safety: The engine must be completely cold before any plug is removed. Pulling plugs from a hot aluminium cylinder head can strip the threads out with them, which is a cylinder head repair rather than a service. If the car has been driven recently, wait.
Common Mistakes
Working on a warm engine
The expensive one. Aluminium heads and steel plug threads expand at different rates, and a plug removed hot can take the threads with it. Cold means genuinely cold, not just cool enough to touch.
Starting the plug with a ratchet
Cross-threading an aluminium head happens in the first turn. Hand-start every plug, using a length of rubber hose if you cannot reach — the hose slips instead of forcing a misaligned thread.
Re-gapping iridium plugs
Fine-wire iridium and platinum tips are delicate and a conventional gapping tool chips or bends them. Modern plugs arrive pre-gapped for the application. If yours is visibly wrong, it is the wrong part number.
Not cleaning the plug well
Grit sits in the recess around each plug and drops into the cylinder as soon as the plug leaves. Getting it out again means removing the cylinder head. Blow or vacuum each well before loosening anything.
Cold engine, clean the well, hand-start, torque to specification, one cylinder at a time. Do those five things and this is a straightforward service that will make the car feel noticeably better than it did that morning.
Frequently Asked Questions
How often should spark plugs be replaced?
It depends on the plug material and what your manufacturer specifies. Copper plugs may need changing every 20,000 to 30,000 miles, platinum around 60,000 and iridium often 100,000 or more. Symptoms — rough idle, hesitation, misfire codes, gradually worsening economy — frequently show up before the interval is reached.
Can you change spark plugs on a warm engine?
You should not. Aluminium cylinder heads expand more than the steel plug threads, and removing a plug while hot can pull the threads out with it. That turns a routine service into a thread repair or a cylinder head. Let the engine cool completely, ideally overnight.
Do new spark plugs need gapping?
Most arrive pre-gapped for their listed application and need no adjustment. Fine-wire iridium and platinum plugs should not be re-gapped with a conventional tool at all — the tip damages easily. You can check the gap gently, but a significantly wrong gap out of the box usually means the wrong part number.
How tight should spark plugs be?
Lower than most people assume — commonly between 15 and 25 ft-lb, but use your manufacturer's figure. Over-tightening strips aluminium threads; under-tightening lets combustion gases escape past the seat and causes the plug to overheat. This is a job worth using a torque wrench for.
What do the old spark plugs tell you?
A great deal. Light tan or grey means healthy combustion. Black and sooty suggests a rich mixture. Oily deposits point at oil entering the combustion chamber, often valve stem seals or piston rings. White and blistered can mean running lean or overheating. Read them before binning them — it is free diagnostic information.
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How We Write Guides
Every product named in the “what you need” section is one we have researched and written about separately, and each links to that guide so you can see the reasoning rather than taking a recommendation on trust. We publish no prices, because they change faster than we can update pages. Where a job carries genuine risk we say so plainly rather than burying it, and where a job is beyond a home mechanic we say that too. Last reviewed August 10, 2026.
A note on safety: Never remove spark plugs from a warm or hot engine, and always start the new plugs by hand. Both rules exist to protect aluminium cylinder head threads.