Spark Plug

The spark plug is the "final actuator" of the ignition system. Its job is to discharge high-voltage electricity generated by the ignition coil across a tiny electrode gap inside the cylinder, producing an electric arc that ignites the compressed air-fuel mixture.

Core Construction 

Centre and ground electrodes: The area where the electric arc is created. Electrode materials (such as iridium, platinum, or nickel) dictate durability and ignition stability. 

Ceramic insulator: Isolates tens of thousands of volts of high-voltage electricity, preventing current from leaking sideways across the metal shell. 

Threaded shell: Threads securely into the cylinder head, serving both mounting and heat dissipation functions.

Key Spark Plug Parameters

Heat Range: The most critical specification when selecting spark plugs. 

Cold-type spark plug: Dissipates heat quickly; ideal for high-rpm, high-load engines. 

Hot-type spark plug: Dissipates heat slowly; suited for low-rpm, low-load engines.

Consequences of mismatch: Using the wrong heat range can cause pre-ignition (engine knocking, which severely damages the engine) or rapid carbon fouling.

Plug Gap: The clearance between electrodes. A gap that is too wide strains the ignition coil and causes misfires, whilst a gap that is too narrow produces a weak spark, resulting in incomplete combustion. 

Spark Plug "Self-Diagnosis" (Reading Plug Colours)

Inspecting a removed spark plug reveals key insights into the engine's operating condition:

Light brown/greyish-white: Optimal engine condition with complete combustion.

Black carbon deposits: Overly rich air-fuel mixture, prolonged idling, or a spark plug heat range that is too cold.

Wet oil fouling: Worn valve stem seals or worn piston rings, allowing engine oil into the combustion chamber.

Melted/glazed white electrodes: Engine overheating, over-advanced ignition timing, or poor-quality fuel causing knocking.

Replacement Advice and Pitfalls to Avoid

Stick strictly to OEM specifications: Avoid blindly swapping to so-called "multi-electrode" or "spark-boosting" aftermarket plugs. The factory electrode positioning and reach are calibrated through fluid dynamics; tampering with them can disrupt the in-cylinder air-fuel charge flow.

Installation torque is critical: An under-torqued spark plug can cause combustion gas leaks (loss of cylinder compression), while over-tightening can strip the cylinder head threads. Always use a torque wrench instead of relying on gut feel. 

Keep the spark plug wells clean: Ensure the plug wells are free of dirt, debris, or engine oil before installation; otherwise, high-voltage current can "flashover" along the outside of the insulator, causing misfires or ignition failure. 

Replace on schedule: Even if the plugs look "still usable", replace them according to the service booklet intervals. As electrodes wear down over time, the gap widens, which will significantly shorten the lifespan of costly ignition coils.