How to Replace Spark Plugs: Gap, Torque & Intervals
Spark plugs are small, cheap and often overlooked — but worn ones cause rough idling, misfires, hard starting and a noticeable drop in fuel economy. Replacing them is one of the most satisfying jobs you can do yourself, and on most petrol engines it's straightforward with basic tools. Get the gap and torque right and you'll feel the difference the moment you turn the key.
What you'll need
- A set of spark plugs matched to your engine
- A spark plug socket (usually 16mm or 21mm) with a rubber insert
- A ratchet, extension bar and a torque wrench
- A feeler gauge to check the gap
- A little dielectric grease and an anti-seize compound (sparingly)
When should you change spark plugs?
Intervals depend entirely on the plug type. Copper/nickel plugs typically last 20,000–30,000 miles. Platinum plugs run to around 60,000 miles, and iridium plugs can last 60,000–100,000 miles. Always fit the type your engine was designed for — don't downgrade an iridium engine to copper. Check your handbook, but if you're getting misfires, a flickering engine warning light or worse MPG, it's worth pulling one to inspect it.
Step-by-step replacement
- Work on a cold engine. Removing plugs from a hot aluminium head risks damaging the threads. Let it cool fully.
- Access the plugs. Remove any engine cover. On modern engines each plug usually has its own ignition coil (coil-on-plug) sitting on top — unclip the wiring connector and unbolt the coil. On older engines you'll have HT leads instead; remove one at a time so you don't mix up the firing order.
- Clean around the plug. Blow out any dirt from the recess so nothing drops into the cylinder.
- Remove the old plug. Use the socket and extension, turning anticlockwise. They can be tight — go steady.
- Check the new plug gap. Most plugs come pre-gapped, but always verify with a feeler gauge against your engine's specified gap (typically 0.7–1.1mm). Adjust gently only if needed — never lever on the iridium tip.
- Fit the new plug. Start it by hand to avoid cross-threading. A dab of anti-seize on the threads (sparingly) helps future removal, and a smear of dielectric grease inside the coil boot keeps moisture out.
- Torque correctly. This matters. Tighten to the manufacturer's spec with a torque wrench — usually around 20–30 Nm for 14mm plugs. Too loose and they blow out or overheat; too tight and you'll strip the head. If no torque wrench is to hand, the rule is hand-tight plus a quarter to half turn for a gasket plug.
- Refit coils or leads in the correct order and reconnect everything.
Reading your old plugs
Before you bin them, the old plugs tell a story. A light tan/grey tip means healthy running. Black and sooty suggests a rich mixture; oily and wet can point to worn valve seals; white or blistered hints at overheating. If they all look uniform and tan, your engine's happy. If one looks very different, that cylinder may need a closer look.
Spark plugs and ignition coils go together
If you're chasing a misfire, remember a weak ignition coil produces the same symptoms as a worn plug. It's common to replace plugs and find one cylinder still stumbles — a failing coil is often the cause. If you've a high-mileage engine and the coils are original, it can be worth doing both together. Quality plugs from trusted names like NGK and Bosch are a sound investment for reliable starting.
Match plugs by your make, model and engine, or by the OE/manufacturer part number stamped on the old plug. For older or classic engines where the right heat range is hard to find, our request-a-part team can help source the correct plugs — just send your registration or the old part number. Shop spark plugs with code SAVE10 for 10% off and tracked UK delivery.
Frequently asked questions
- How often should spark plugs be replaced?
- It depends on the type: copper plugs around 20,000–30,000 miles, platinum about 60,000 miles, and iridium up to 100,000 miles. Always fit the type your engine specifies.
- Do I really need a torque wrench for spark plugs?
- Yes, ideally. Over-tightening can strip the cylinder head threads and under-tightening causes overheating or blow-out. Most 14mm plugs torque to around 20–30 Nm.
- Can a bad spark plug cause a misfire?
- Absolutely. Worn or fouled plugs cause misfires, rough idling and poor fuel economy. A failing ignition coil produces similar symptoms, so check both if a misfire persists.