TerminalWhere the ignition lead or coil plugs on. The high voltage comes in here.
InsulatorCeramic, usually alumina, that holds the centre electrode. It keeps the voltage from reaching the shell.
HexThe six-sided part of the shell that a spark plug socket fits.
Metal shellThe steel body round the insulator. It screws into the cylinder head and grounds the plug.
ThreadsScrew the plug into the cylinder head.
Ground electrode (side electrode)The bent arm welded to the shell. The spark jumps to it from the centre electrode.
Spark gapThe space the spark jumps across. It is set by bending the ground electrode.
Centre electrodeCarries the high voltage from the terminal to the tip. The spark leaves it there.
From outside 2
RibsRidges that lengthen the path over the insulator. The spark then cannot jump down its outside (flashover).
GasketA crush ring that seals the plug to the cylinder head. So no combustion gas leaks out.
Cut in half 1
ResistorCuts the radio interference the spark would make.
How this list was compiled: Read 2026-09-26. (1) NGK (Niterra), Parts of a spark plug, https://www.ngkntk.com/newsroom/feature/emea/parts-of-a-spark-plug/ : terminal, ceramic insulator of aluminium oxide, resistor for interference suppression, centre electrode joined to the terminal, ground (side) electrode, gasket ring sealing combustion gas, metal shell with its hex head for a socket, ribs against flashover. (2) Wikipedia, Spark plug, https://en.wikipedia.org/wiki/Spark_plug : threaded metal shell isolated from the central electrode by a ceramic insulator; that electrode may contain a resistor; terminal at the top; ribs lengthen the path against flashover; external seal usually a crush washer; spark gap adjusted by bending the ground electrode; thread and nut sizes. Licence: own work
The drawing: Drawn for this site by script, 2026-09-26. Own work, nothing traced. Licence: own work
How it works
These are the parts of a spark plug, first from outside, then cut in half to show the resistor and centre electrode inside the ceramic. Start with the 8 main parts.
Read the full guide
Top to bottom
The ignition lead clips onto the terminal. Below it the white ceramic insulator, ridged with ribs, runs down into the metal shell. A socket grips the hex; the gasket seals against the cylinder head, and the threads screw into it.
Where the spark jumps
High voltage runs down the centre electrode, through the resistor, to the tip. There it jumps the spark gap to the ground electrode, the bent arm on the shell, and lights the fuel.
A mechanic sets the spark gap by bending the ground electrode slightly, closer to or further from the centre electrode.
Questions and answers
What does the resistor in a spark plug do?
It cuts the radio interference the spark makes, which could upset the car's electronics. Most cars need resistor plugs.
What are the ribs on a spark plug for?
They lengthen the path over the outside of the insulator so the high voltage cannot jump down it to the metal shell, called flashover.
What is the ground electrode?
The bent metal arm welded to the shell at the tip. The spark jumps to it from the centre electrode. Some plugs have two, three or four.