Spark Plug Function and Removal
The spark plug is the ignition source for the compressed air-fuel charge in each cylinder. It consists of a steel shell threaded into the cylinder head, a ceramic insulator, a center electrode (nickel, platinum, or iridium), and a ground electrode. A high-voltage pulse (12,000–45,000 volts) from the ignition coil arcs across the electrode gap (typically 0.028"–0.060" depending on engine specification), igniting the mixture. Spark plug replacement intervals vary dramatically by electrode material: copper-core plugs every 30,000 miles; platinum single-electrode plugs every 60,000 miles; iridium plugs every 100,000 miles. Iridium's extreme hardness maintains the precise electrode geometry over high mileage, preserving fuel economy and cold-start performance. Removal requires a spark plug socket (has a rubber insert to grip the ceramic insulator and prevent breakage) and an extension. Critical: the engine must be cold or at least cooled down for 30 minutes before plug removal. Hot aluminum cylinder heads have expanded threads that can seize and pull plug threads out of the head when torqued in a hot state. Spray a small amount of penetrating oil around each plug's base before removal on high-mileage engines — worn or corroded plugs can seize in aluminum heads. Loosen each plug 1/4 turn, spray, wait 2 minutes, then remove fully. If a plug spins loosely (thread pull-out), stop immediately — a HeliCoil thread repair insert is needed.