Car Spark Plugs: Iridium, Platinum or Copper, and When to Change Them

Spark plugs are the one service item on a modern car that owners either change far too often or never at all. The old habit of new plugs every service comes from an era of copper plugs and points ignition; the new habit of ignoring them comes from plugs that genuinely last a hundred thousand kilometres. The right answer depends on what the engine was built to use, and the plug that is in it now is not always that.

Copper, platinum or iridium?

The names describe the tip of the centre electrode, and the tip decides how long the plug lasts and how well it fires.

Standard nickel plugs — usually called copper, for the copper core — have a relatively thick centre electrode that erodes with every spark. They are cheap, they fire well when new, and they wear. Roughly 20,000 to 40,000 kilometres is the range most makers quote, and the engine will tell you when they are tired.

Platinum plugs weld a small platinum disc to the centre electrode, and sometimes to the earth electrode as well. Platinum does not erode the way nickel does, so the gap stays where it was set for far longer: sixty to a hundred thousand kilometres is normal. These were the original long-life plug, and a great many cars from the late nineties onward were built around them.

Iridium plugs use a fine wire of iridium, a metal harder and more heat-resistant than platinum, as the centre electrode. Two things follow. The fine wire needs less voltage to jump the gap, so the plug fires more reliably, especially at high revs and under lean, high-pressure conditions. And it barely wears, so intervals of a hundred thousand kilometres or more are routine. Almost every engine designed in the last fifteen years, and every coil-on-plug engine with a wide gap, was designed for an iridium plug.

Do not downgrade

The most common spark plug mistake we see is a car that came with iridium or platinum plugs being fitted with standard ones because they were cheaper on the shelf. The engine will run. It will also misfire under load sooner than it should, the gap will open up in a few thousand kilometres, and on a coil-on-plug engine the coils are being asked to jump a gap they were not designed for. Fit the grade the maker specified or better, never worse. The upgrade the other way — iridium into an engine specified for standard plugs — is legitimate and often worthwhile, as long as the heat range and the physical dimensions match.

How to tell they are due

A worn set of plugs is rarely dramatic. It is a slightly rougher idle, a hesitation off the line, a cold start that takes an extra crank, fuel economy that has drifted a few per cent, or a misfire code on a car with a warning light. Because it comes on slowly, owners adapt to it. If the plugs are past the interval in the service schedule and the car has any of those symptoms, the plugs are the first, cheapest thing to change.

Pull one and read it. The insulator should be light tan; black is rich or an oil problem, white is lean or hot, and one plug different from its neighbours is that cylinder's problem. Our guide to reading spark plugs was written for outboards, and the colours mean exactly the same thing in a car.

Fitting them properly

• Check the gap, even on plugs sold as pre-gapped, but do it gently on an iridium or platinum plug — never lever against the fine-wire tip.
• Thread by hand until it seats, then tighten to the torque the plug maker specifies. Overtightening stretches the shell; undertightening lets the plug run hot because it is not transferring heat into the head.
• No anti-seize on a plated plug. NGK plugs are plated for the purpose, and anti-seize on top changes the torque and leads to overtightening.
• Look at the coil boots on a coil-on-plug engine while you are there. A cracked or carbon-tracked boot will misfire a brand-new plug.
• Replace as a set. One new plug among five old ones fixes nothing for long.

Finding the right plug

If you have the number off the old plug and it is a Bosch, a Denso or a Champion, our cross-reference tables give the NGK equivalent for each: start with the Bosch cross reference or the Denso cross reference, and there are separate tables for Champion, Autolite and Motorcraft, and ACDelco. An interchange is a starting point, not a guarantee — check the heat range and gap against the car's specification before you fit. If you are matching a plug by its physical size, the NGK plug dimension tables list thread, reach, hex and seat for the whole range. Every code in those tables links to the plug itself in our NGK spark plug range.

Plugs are part of the service work we do on cars and four-wheel-drives at Kadina through automotive servicing and repairs. If the car is misfiring and you are not sure whether it is plugs, coils or something else, ring 0412 532 984 and we will find out rather than guess.