Spark plug: produces an electric spark in the combustion chamber to ignite the air-fuel mixture. Located in each cylinder head, screwed into the engine. Structure: central electrode (high voltage), ground electrode (return path), ceramic insulator (between electrodes), metal shell (grounds to engine). High voltage (~15,000-40,000V) from the ignition system jumps the gap between electrodes, creating a spark.
Spark plug gap: distance between central and ground electrodes; typically 0.028-0.060 inch depending on engine; affects spark intensity; checked with feeler gauge.
Heat range: how quickly the plug dissipates heat; 'hot' plugs run hotter (good for cold engines, low-speed); 'cold' plugs run cooler (high-performance, high-RPM); using wrong heat range causes problems: too hot = pre-ignition / detonation, too cold = fouling. Spark plug condition reveals engine health: light brown/tan deposits = normal; oily/wet = oil leak past rings or valve seals; sooty black = rich mixture (too much fuel); white/blistered = lean mixture or overheating. IGNITION SYSTEM components: (1) BATTERY — provides 12V DC; (2) IGNITION COIL — transformer that steps up 12V to 15,000-40,000V; (3) DISTRIBUTOR (older systems) — routes high voltage to correct cylinder in firing order; modern engines use DISTRIBUTORLESS or COIL-ON-PLUG (each plug has its own coil) — fewer parts, more reliable, precise timing; (4) IGNITION CONTROL MODULE / ENGINE CONTROL UNIT — controls timing electronically; (5) SPARK PLUG WIRES (older systems) — high-voltage cables from coil/distributor to plugs; modern coil-on-plug eliminates wires.
Ignition timing: spark fires SLIGHTLY BEFORE the piston reaches top dead center (before TDC) on compression stroke; allows time for combustion to develop pressure as piston is at top; typically 10-30° BTDC at idle; advances with engine speed. Faulty spark plugs cause misfires (cylinder doesn't fire properly), rough idle, hesitation, poor fuel economy, hard starting. Modern long-life plugs (iridium, platinum) last 100,000+ miles; older copper plugs need replacement every 30,000 miles.