Spark Plugs: Copper vs Platinum vs Iridium, and Why the OE Plug Matters

A spark plug looks like a length of metal with a porcelain insulator. Inside that simple shape sits one of the most carefully-engineered components in any petrol engine. The centre electrode, the ground electrode, the gap, the heat range, the resistor value — each of these is matched to the engine designer’s combustion strategy. Substituting a generic plug for the OE part can cause symptoms that no OBD code will properly diagnose.

What the heat range actually controls

Heat range is how quickly the plug dissipates heat into the cylinder head. A "hot" plug runs hotter at the firing tip and self-cleans deposits well at low loads. A "cold" plug runs cooler and resists pre-ignition under high loads. Each engine is designed for a specific heat range; using the wrong one causes fouling at one end or pre-ignition at the other.

The numbering system varies by brand — NGK numbers run lower for hotter; Denso, Bosch and Champion have their own scales. Match the OE part number rather than translating across systems.

Electrode materials and lifespan

Copper electrodes ignite fuel well but wear quickly — typically 15,000–25,000 mile life. Platinum electrodes last 40,000–60,000 miles. Iridium electrodes last 60,000–100,000 miles, sometimes longer. Double-precious (platinum on the ground electrode, iridium on the centre) extends life further.

The longer life of precious-metal electrodes isn’t magic — it’s electrode-mass durability. Each spark slightly erodes the electrode tip; precious metals erode at a much slower rate than copper. Over a 60,000-mile interval, the gap on a copper plug widens noticeably while an iridium plug stays close to its original specification.

Gap and why "just bend it a bit" can ruin a plug

The gap is the distance between centre and ground electrodes. Most modern engines specify 0.9–1.1mm. The factory plug arrives at the right gap; iridium plugs in particular shouldn’t be re-gapped because the fine iridium tip is brittle and breaks easily under gap-adjustment pliers. Check the gap with a feeler gauge but don’t adjust it unless you’re fitting a copper plug and the gap is obviously wrong.

Symptoms of worn plugs

Misfire under load (especially uphill or accelerating), rough idle, poor cold-start, fuel economy creeping down. Modern engines compensate so well that the symptoms can be subtle for thousands of miles before fault codes appear. Replacement at the manufacturer’s recommended interval — usually 40,000–60,000 for platinum, 60,000–100,000 for iridium — keeps the engine running clean.

Brand and OE matching

NGK is OE on most Japanese and Korean engines (Toyota, Honda, Mazda, Hyundai, Kia, Subaru, Nissan, Mitsubishi). Denso is also OE on much of the Japanese parc and is often used by Toyota as the original. Bosch is OE on most European cars (VAG, BMW, Mercedes, Porsche). Champion is OE on many older European and American applications. Beru is OE on premium German performance engines.

The "OE-equivalent" claim from generic plug brands is usually overstated; the price savings rarely justify the risk of misfires or unexpected fuel-trim drift. For a 4–6 cylinder service, the cost of premium iridium plugs from the right OE brand is £40–£80 — small money for an interval of 60,000+ miles.

Find spark plugs confirmed to fit your engine on the Spark Plugs collection. Match the OE brand and you’ll have a smoother engine, better economy, and no surprise warning lights.