What are Spark Plugs?
Small igniters threaded into the cylinder head that spark to set the fuel-air mixture alight.
Simplified animation — not to scale.
In plain English
Each cylinder has a spark plug that fires a high-voltage spark across a tiny gap, igniting the compressed fuel-air mixture at exactly the right moment to drive the piston down.
“Like the flint in a lighter, except it strikes precisely in time with the engine, up to a hundred times a second at high revs.”
How it works
The ignition coil turns 12 volts into tens of thousands of volts, sent to the plug at the top of the compression stroke. The spark jumps the gap between the electrodes and ignites the mixture. Worn electrodes or a fouled tip create a weaker, less reliable spark.
Signs of trouble
- ⚠Rough idle or a noticeable misfire
- ⚠Hesitation or flat spots under acceleration
- ⚠Harder starting, especially from cold
- ⚠Poor fuel economy
£20–£100
Always get a written quote. Prices vary by bike, region, and parts brand.