Spark plugs: intervals, and reading the old plug

Standard nickel spark plugs want replacing every 10,000–15,000 km (6,000–9,000 miles); iridium and other precious-metal plugs run to 40,000 km (25,000 miles) and beyond. Your owner's manual has the exact figure, and it wins. And before the old plugs go in the bin, read them — the colour of the firing end is a free report on how the engine has been running.

Why plugs wear out

Every combustion cycle, the spark erodes a trace of metal from the electrodes. The gap slowly widens, the voltage needed to jump it climbs, and eventually the coil can no longer deliver reliably — first under load, where cylinder pressure is highest, then everywhere. Nickel electrodes are soft and erode quickly. Iridium is far harder, which is the whole reason those plugs last three to four times longer; the fine iridium centre electrode also needs less voltage to fire, which helps marginal cold starts.

Distance drives plug wear, not time. A bike that sits all winter ages its oil and brake fluid on the calendar, but the plugs only care about kilometres burned.

The interval, and remembering it

Plan on 10,000–15,000 km for standard plugs, 40,000 km or more for iridium, and shorten either if the engine burns oil or spends its life at full throttle. Factory schedules usually pair the plug change with a major service, which is convenient — on many bikes the tank and airbox are already off for other work.

Plugs are also the classic forgotten job. The interval is long enough that two years on, nobody remembers whether they were done. Write the change into a maintenance log with the odometer reading and the question never comes up again — nor do you pay a shop to replace plugs with 4,000 km on them.

Signs the plugs are tired

Harder cold starts, needing more cranking than the bike used to. A stumble or misfire under load — top gear, low revs, throttle rolled open is where a worn plug shows first. Fuel economy drifting down over months. An idle that turns ragged and never quite settles. None of these are exclusive to plugs — a clogged air filter produces similar symptoms from the intake side — but plugs are the cheapest thing to rule out, so they go first.

If the engine cranks and refuses to fire at all, that is a different diagnosis. Work the no-start checklist in order before blaming the plugs.

What the old plug tells you

Pull the old plugs and compare them against this table before the new ones go in.

Firing end	Engine state	What to do
Tan to light brown	Mixture and heat range correct	Nothing — fit the new plug
Black, dry soot	Running rich	Check the air filter, then fuelling
Black, wet and oily	Oil entering the chamber	Compression test — rings or valve seals
White, blistered or glazed	Running lean or too hot	Find the cause before riding hard
Rounded electrode, wide gap	Worn out, nothing more	Replace, shorten the interval slightly

On a multi-cylinder engine, read each plug separately. One odd plug among matched siblings points at that cylinder specifically — its injector, coil, or valves. The same colour across the whole set describes the general state of tune instead.

White or blistered is the reading that should stop you. A lean, overheating cylinder can damage a piston; find the air leak or fuelling fault before the next hard ride. Heat range matters here too — the number in the plug code describes how quickly the plug sheds heat, and the specified grade is the correct one. A step hotter or colder is a tuning decision for a modified engine, not an upgrade.

You also do not have to wait for replacement day. On any bike with accessible plugs, pulling one at half-interval is a ten-minute health check that costs nothing and catches a rich or lean drift early.

Fitting new plugs without wrecking the head

The plug is steel and the head is aluminium, so the threads always lose. The discipline:

• Clean around each plug well before removal — compressed air or a stiff brush — so grit cannot fall into the cylinder.
• Work on a cold engine. Hot aluminium threads strip with very little help.
• Check the gap against spec with a feeler or wire gauge, even on pre-gapped plugs. On iridium plugs, never lever against the fine centre electrode.
• The anti-seize debate: most modern plugs have plated shells, and the manufacturers say fit them dry. Anti-seize lubricates the threads and makes it easy to over-torque. If your manual calls for it anyway, use a trace and back the torque off accordingly.
• Thread the plug in entirely by hand until it seats. Then either the manual's torque figure, or the turn printed on the box — typically a half to two-thirds of a turn for a new plug crushing its fresh washer, far less when refitting a used one.

A stripped plug thread means a thread insert at best, and on some engines the head coming off. Snug is enough.

Whether iridium is worth it

Iridium plugs cost three to four times more — $8–20 (€7–18) each against $3–6 (€3–5) for nickel — and add no power to a healthy engine. What you are buying is interval, and interval matters in proportion to access. On a V-twin with the rear plug under the tank, or a faired four where reaching the cylinders means an hour of bodywork, quadrupling the interval is worth every cent. On a naked single with five-minute access, nickel is fine and always was.

If the plugs are buried, batch the job with the other deep services. A valve clearance check reaches the same part of the engine — one disassembly, two jobs, one bill.