Electrode: nickel, platinum or iridium?

The metal at the very tip of the centre electrode is the single spec that decides two things at once: how long the plug lasts and how reliably it lights the mixture. Nickel, platinum, iridium — the choice is not marketing, it's metallurgy. Here is what each one really buys you.

Why the tip material matters at all

Every time the plug fires, a tiny arc jumps from the centre electrode. That arc is brutally hot, and over millions of cycles it slowly erodes the metal away. As the tip wears, the gap widens, the voltage needed to fire climbs, and ignition gets less reliable. So the harder and more heat-resistant the metal, the longer the gap stays stable — and the longer the plug stays good.

There is a second effect, and it's the clever one: a thinner tip concentrates the voltage. With a fine point, the electric field is denser, so the spark forms at a lower voltage. A thin tip also touches less of the surrounding gas, so it steals less heat from the young flame kernel (this is called the quenching effect). Less quenching means the flame survives and spreads more easily — especially when conditions are unfavourable.

Good to know

A flame born next to a thick, cold metal electrode can be "smothered" before it grows. That's why a 0.4 mm iridium tip lights a lean or cold mixture far better than a 2.5 mm nickel one — the spark has more room to breathe.

The three materials

Nickel (standard, often with a copper core)

The classic, cheapest electrode. The visible tip is a nickel alloy; many are built around a copper core for excellent heat conduction (copper is a superb conductor but melts too easily to be exposed, so it's buried inside). Nickel is soft and wears relatively fast, so the tip is left thick to survive. Typical life: ~20,000–30,000 km. Perfect for older engines that were designed for it.

Platinum (harder, slower wear)

Platinum is far harder and has a much higher melting point than nickel, so the tip resists erosion and keeps its gap much longer. It also allows a finer tip than nickel. Typical life: ~60,000–90,000 km. It became the standard when long-life service intervals and coil-on-plug ignition arrived.

Iridium (the finest tip, the best ignitability)

Iridium has one of the highest melting points of any metal (~2,450 °C) and is extremely hard. That lets makers grind the centre tip down to just 0.4–0.6 mm while keeping it durable. The result: the lowest required firing voltage, the best ignition of lean, cold or high-compression mixtures, and the longest life — typically ~90,000–120,000 km and beyond.

Side by side

MaterialTip diameterTypical lifePriceIgnitability
Nickel (copper core)~2.0–2.5 mm~20–30k kmLowestGood
Platinum~0.6–1.1 mm~60–90k kmMediumBetter
Iridium~0.4–0.6 mm~90–120k km+HighestBest (esp. lean/cold)

Upgrade, or keep what's fitted?

This is where electrode material is special. Heat range, thread, reach, seat type — those must match exactly. But the tip material is the one spec that may legitimately differ between two equivalents. You can fit an iridium plug where a nickel one was originally used, as long as the thread, reach and heat range are identical. The engine simply gets a longer-lasting, better-igniting plug.

The reverse is the trap. Many modern engines — coil-on-plug and wasted-spark systems in particular — were designed around platinum or iridium and even rely on their long service interval and low firing voltage. On those, respect the spec.

Good to know

Don't downgrade iridium → nickel where iridium was specified. You'll get a much shorter life, harder cold starts and weaker ignition of the lean mixtures these engines run. And do the maths: an iridium plug may cost 3× a nickel one but last 4–5× as long — the price per kilometre often favours iridium, before you even count the labour of changing plugs more often.

The common mistake

Buying on price alone and replacing a factory iridium with a cheap nickel plug "because they look the same". Example: a modern coil-on-plug petrol engine specced for a fine-wire iridium plug, refitted with a thick nickel one. Within a few thousand kilometres the owner gets misfires under load, a rough cold start and a check-engine light — and ends up buying plugs again far sooner. The nickel plug wasn't faulty; it was simply the wrong material for that ignition system.

In short