How quickly the plug conducts heat away from its tip - not how hot the spark is.

The tip must run hot enough to burn off deposits and cool enough not to become an ignition source in its own right.

Heat range is set mainly by the length of the insulator nose: a long nose retains heat, a short one sheds it into the head quickly.

A plug too hot for the duty glows and ignites the mixture before the spark, which is pre-ignition and can destroy a piston in minutes under load.

A plug too cold never reaches its self-cleaning temperature and fouls with deposits, giving misfires and poor cold running.

Manufacturers specify the range for the engine and its expected duty; changing it is a decision for sustained unusual loads, not a general upgrade.

It extends service life and eases the demand on the coil - it does not make meaningful extra power on a standard engine.

Iridium and platinum have very high melting points and resist erosion, so the electrode keeps its shape far longer.

That allows a very fine centre electrode, which needs less voltage to fire and so reduces the load on the ignition coil.

The main practical benefit is interval: several times the life of a conventional nickel plug.

Fitting a nickel plug where iridium is specified works, but the plug will be badly worn long before the vehicle's service schedule looks at it again.

Fitting iridium where nickel is specified is harmless but rarely pays for itself unless the labour to reach the plugs is high.

Many are pre-gapped and some must not be adjusted at all - fine-wire iridium and platinum electrodes are easily damaged by a gapping tool.

Conventional nickel plugs are gapped with a feeler gauge and adjusted by bending the ground electrode.

Fine-wire precious-metal plugs have delicate centre electrodes and tips that can be knocked off entirely by careless adjustment.

Manufacturers frequently supply them pre-set and state that no adjustment should be made.

Where checking is permitted, use a wire gauge rather than a flat feeler, and never lever against the centre electrode.

Check the gap against the engine's specification rather than the plug's box, since the same plug is sometimes specified at different gaps for different engines.

Because a wider gap needs a higher voltage to jump, and that extra demand stresses the coil at every firing.

Electrodes erode with use, so the gap widens progressively through the plug's life.

The voltage required rises with the gap, and it rises further under load when cylinder pressure is high.

The coil is asked to deliver near its limit repeatedly, which accelerates breakdown of its insulation.

So a coil failure on an engine with overdue plugs is usually a consequence rather than a coincidence.

Replacing a coil without replacing overdue plugs leaves the cause in place, and the new coil goes the same way.

Clean the well, fit by hand first, torque to specification, and never over-tighten into an aluminium head.

Blow or vacuum debris out of the plug well before removing the old plug, since anything there falls straight into the cylinder.

Start every plug by hand to be certain it is not cross-threading; aluminium heads are easily damaged and the repair is substantial.

Torque matters in both directions: too loose and the plug cannot conduct heat into the head, too tight and the thread or the plug's own seal is damaged.

Anti-seize compound is specified by some manufacturers and prohibited by others, because it changes the effective torque - follow the engine maker's instruction.

Fit new washers where the plug uses a separate one, and never reuse a crush washer.

A great deal - colour, deposits and erosion each indicate something specific about that cylinder.

A light tan or grey insulator nose indicates normal combustion at the right heat range.

Black dry soot indicates a rich mixture, a plug too cold for the duty, or a lot of short-journey running.

Black oily deposits indicate oil reaching the chamber, from valve stem seals or rings.

A white, blistered or glazed insulator with electrode erosion indicates overheating - a plug too hot, a lean mixture, or over-advanced timing, and it is a warning worth acting on before a piston is damaged.

Compare all the plugs from an engine side by side: one that differs from the others identifies the cylinder with a problem.

Yes - they wear together and a mismatched set gives uneven combustion between cylinders.

All the plugs in an engine have done the same work for the same time.

One new plug among worn ones fires more readily than its neighbours, producing different combustion in that cylinder.

The labour to access the plugs is the same whether one or all are replaced, so there is no economic case for doing one.

Fit the same make, type and heat range across the engine; mixing brands within a set introduces small differences for no benefit.

Where one plug has failed early, read it and the others carefully - the cause is usually specific to that cylinder and worth finding.