The Car Care Guide
Keeping Your Car Healthy
Structure and Terminology
The main parts of a spark plug are the insulator, the metal shell, and the two electrodes. The insulator
is made of sintered aluminum oxide (Al2O3) and filler material in order to provide good heat
conduction, mechanical strength, resistance to thermal shock, and anti-corrosion. In addition, the insulator must
provide a high electrical resistance, about 1 Megohm at 400ºC (750ºF), between the center electrode and the shell.
The metal shell has six sides (hexagonal) for use with a socket wrench during installation and removal and
plating for anti-corrosion. The lower section of the shell is threaded for attaching the spark plug to the cylinder head
. The length of the threaded section (called the reach, which is 19mm in the 6G72 engine) must closely match the thickness
of the cylinder head as shown below. The electrodes are a nickel-steel alloy containing chrome, manganese,
and silicon, which provide very good anti-corrosion and anti-erosion properties. The ground electrode is
welded to the threaded section of the shell. The center electrode is welded to a copper core that runs the
length of the electrode and connects to the spark plug wire terminal at the top. The stock spark
plugs have a small piece of platinum welded to the tips of both electrodes.
