A spark plug is the small part screwed into the cylinder head of a petrol engine that makes the spark which lights the fuel and air mixture, thousands of times a minute. A worn or dirty one shows up as hard starting on cold mornings, jerks when you open the throttle, poor mileage and a blackened exhaust tip.
In our workshop the spark plug is one of the first things we pull out when a bike will not start after standing through the monsoon. This page walks through the parts, the types, how to read an old plug and when to replace it, and links to our model pages. For tyres, batteries and other parts, visit the main AutoTyreSize index.
At a glance
- What it doesMakes the spark that ignites petrol and air in the cylinder
- Engines that use itPetrol, CNG and LPG engines; diesel engines do not
- Main typesCopper core with nickel tip, precious metal tip, iridium
- GapSet by the engine maker; see the owner's manual
- Healthy tip colourLight tan or grey
- Change intervalAs per the service schedule in the owner's manual
What a spark plug does and its main use
Inside a petrol engine, the piston squeezes a mix of air and fuel. At the right instant, the ignition system sends a high voltage pulse down to the plug. The current jumps across a small gap between two metal electrodes and makes a spark. That spark starts the burn that pushes the piston down and turns the crank. Without it, a petrol engine cannot run at all.
The plug also has a second job that riders rarely think about: it carries heat away from its firing tip into the cylinder head. If it runs too cool, carbon builds up on it. If it runs too hot, the tip can glow and fire the mixture early, which damages the engine. That balance is called the heat range, and it is why you should fit only the plug type listed for your engine.
Spark plug diagram: the parts named
| Part | Where it is | What it does |
|---|---|---|
| Terminal | Top end | The plug cap or coil boot clips on here |
| Ceramic insulator | White body | Stops the high voltage leaking to the engine |
| Hex | Metal nut shape | Where the spark plug spanner grips |
| Sealing washer or seat | Below the hex | Seals combustion pressure |
| Threaded shell | Lower metal body | Screws into the cylinder head and carries heat away |
| Centre electrode | Tip of the insulator | Carries the current to the gap |
| Ground electrode | Bent arm at the bottom | The spark jumps to it across the gap |
Picture the plug standing upright: cap end on top, white ceramic in the middle, metal hex and thread below, and the two electrodes facing each other at the very bottom. The gap between them is measured in fractions of a millimetre and is printed in the owner's manual.
Spark plug coil and how the spark is made
A battery gives only 12 volts, far too little to jump a gap under compression. The ignition coil steps that up to many thousands of volts. On most commuter bikes the coil sits under the fuel tank and connects to the plug through a thick high tension lead and a rubber cap. Many cars and some newer bikes use a coil that sits directly on top of each plug.
When a bike misfires, the plug is not always to blame. A cracked plug cap, a loose lead or a weak coil gives the same symptoms. Fit a known good plug first; if the misfire stays, have the coil and cap tested.
Do not hold a plug lead with your bare hand and crank the engine to see the spark. The voltage can give a painful shock, and a loose lead sparking near the fuel line is a fire risk.
Does a diesel engine have a spark plug
No. A diesel engine squeezes air so hard that it becomes hot enough to light the diesel the moment it is sprayed in. This is called compression ignition, and it needs no spark. Many diesel cars and small diesel vehicles carry glow plugs instead. These are small electric heaters that warm the combustion chamber so a cold engine starts easily. They do not fire every stroke and are not a replacement name for spark plugs. Petrol, CNG and LPG engines all need spark plugs.
Types of bike spark plug
- Copper core, nickel tip: the standard plug in most commuter bikes and scooters. It sparks well but its nickel tip wears faster, so it needs replacing more often.
- Precious metal tip: a harder metal disc welded on the electrodes so they wear slowly. Fitted in many cars.
- Iridium: a very fine, hard centre electrode that needs less voltage to spark and keeps its shape for longer. See our iridium spark plug guide for when it is worth fitting.
The thread size, reach (how far it goes into the head) and heat range must match your engine whichever type you choose. A plug that is too long can hit the piston; a shorter plug leaves threads exposed to carbon. Match the old plug's code or the code in the manual, and ask the parts shop to cross check it.
Spark plug change interval and signs of wear
Every bike and car maker lists a spark plug check and replacement point in the service schedule at the back of the owner's manual. Follow that. Plugs on bikes usually get checked and cleaned at routine services and replaced at a set distance; iridium plugs are often given a longer life by the plug maker, but some iridium plugs made for performance last no longer than a standard one.
Replace the plug early if you notice:
- The engine needs many kicks or long cranking on a cold morning.
- Jerks or a stutter when you open the throttle at low speed.
- Mileage falling for no other reason.
- Electrodes worn round, a gap clearly wider than the manual figure, or a cracked insulator.
Reading an old plug: what the tip colour tells you
| Tip looks | Likely meaning | What to do |
|---|---|---|
| Light tan or grey, dry | Engine running normally | Clean, check gap, refit if within life |
| Black, dry and sooty | Rich mixture, dirty air filter, too much short city use | Clean or change air filter, have the fuel system checked |
| Black and wet with oil | Oil reaching the chamber, worn rings or valve seals | Engine inspection by a mechanic |
| White, blistered or melted | Overheating or wrong heat range | Stop long rides until checked; confirm plug type |
| Wet with petrol | Flooded after repeated cranking | Dry the plug, wait, start with throttle closed |
A tip that looks a little light in colour is not, on its own, proof of a lean engine. Look for blistering or melted metal before blaming the fuel setting.
How to remove and fit a spark plug safely
- Work on a cold engine. Hot aluminium threads strip easily.
- Pull the plug cap by its rubber body, not by the lead.
- Blow dust and grit out of the recess around it first; debris dropped into the cylinder scores the bore.
- Unscrew with the correct spark plug spanner held straight.
- Screw the replacement in with your fingers for the first few turns. If it binds, stop and restart; a cross threaded plug ruins the head.
- Tighten to the figure in the manual. A new plug with a crush washer needs a little extra turn after it seats.
- Push the cap back on firmly; you should feel it snap home.
Do not regap a fine iridium plug with a flat feeler blade or by levering on the centre pin. If the gap is out of specification, use a wire type gauge and bend only the ground electrode, or simply fit a new plug.
Spark plug pages for popular bikes and scooters
| Page | Covers |
|---|---|
| Iridium spark plug | How iridium differs, double iridium, when it helps |
| Spark plug spanner | Socket sizes and using the right tool |
| Iridium spark plug for Hunter 350 | Plug type and fitting notes |
| Iridium spark plug for Classic 350 | Plug type and fitting notes |
| Iridium spark plug for GT 650 | Twin cylinder, two plugs to change |
| Bullet spark plug | Plug type for the Bullet |
| Ntorq spark plug | Scooter plug type and access |
Owners of the Meteor 350, Splendor and other models asking about an iridium spark plug should check the plug code in the owner's manual first, then compare it against the iridium part that the plug maker lists for that engine.
Questions owners ask
What is a spark plug in simple words?
It is a small electrical part in a petrol engine that makes a spark across a tiny gap. The spark lights the fuel and air mix so the engine can run. Diesel engines do not have one.
How often should a bike spark plug be changed?
Go by the maintenance chart printed in the bike's handbook, which lists both a check point and a replacement point. Change it early if the bike starts hard, jerks or loses mileage, or if the electrode looks worn round.
Does a diesel engine have spark plugs?
No. Diesel engines light their fuel by the heat of compressed air. Many have glow plugs, which are heaters that help cold starting, not spark makers.
Can I clean a spark plug and reuse it?
A dry, lightly sooty plug can be cleaned with a soft brass brush and refitted if its electrodes are not worn. An oily, cracked or melted plug must be replaced, and the cause found.
Will a costlier spark plug increase mileage?
Only if the old plug was worn. A new plug of the correct type restores lost performance. Upgrading a healthy plug rarely changes mileage you can measure.
What is a spark plug coil?
It is the ignition coil, which raises the 12 volt battery supply to the very high voltage the plug needs. A failing coil or cracked plug cap can cause misfires even with a new plug.