Depending on the design of the shell, the spark plug has a sealing ring or a captive gasket (washer). Some plugs are designed with a tapered shell and don't use a gasket to seal the cylinder bore. After the insulator is inserted into the spark-plug shell, it is swaged and heat shrunk in position using inductive heating under high pressure.
Wear on the electrodes is caused by erosion, the burning away due to arcing, and corrosion, which is a chemical and thermal attack. These two factors cannot be treated separately in relation to their effect on electrode wear. Degradation increases the required ignition voltage. In addition, the electrodes must have good heat dissipation. These requirements call for different electrode shapes and materials, depending on operating conditions and applications.
The ground electrode is welded to the shell and usually has a rectangular cross-section. Its service life is dependent on its heat dissipation. Both the ground and center electrode use composite materials to improve thermal dissipation and extend the service life. Another factor affecting service life is the ratio of the thermally exposed surface to the thermally conductive cross-section. Minimal dimensions, special designs, and partial coverage of the center electrode are all ways to obtain an optimal arc pattern at the ground electrode.
To enhance performance, the external surface and the contours of the area facing the center electrode can also be modified. Some spark plugs include a varied number of ground electrodes (two, three or four) and ground electrodes of different dimensions. Both thicker ground electrodes and multiple electrodes can extend spark-plug life. Since electricity takes the path of least resistance, plugs with multiple ground electrodes provide additional electrical paths. This contradicts aftermarket advertising claims that dual-electrode plugs provide an arc from both electrodes. Any first-semester electrical-engineering student knows that isn't possible.
The center electrode on conventional spark plugs (one that has an air gap between the insulator bore and the center electrode) is melted gas-tight into the insulator with a special conductive seal. It has a slightly smaller diameter than the bore in the insulator nose to compensate for the different internal expansion rates of the electrode material and the insulator ceramic. This air gap is subject to close tolerances and is of great importance to heat range (see sidebar).
A cylindrical center electrode projects from the insulator nose. Center electrodes made of precious metals are smaller in diameter than the compound electrodes, which have a copper and nickel-alloy jacket.
Electrode shape affects heat dissipation, resistance to wear, the ignition voltage requirement, and how the arc is transferred to ignite the mixture. The shape depends on the type of spark plug and its position relative to the cylinder-bore center.
An electrode gap is the shortest distance between the center electrode and the ground electrode: The smaller the electrode gap, the lower the ignition voltage needed to arc the plug. A narrow electrode gap reduces the voltage necessary to produce an arc, but the short, low-voltage spark can transfer only minimal energy to the mixture, and ignition misfire can easily occur. Higher voltages support an arc across a larger gap. This type of gap transfers energy to the mixture, but if an improper coil is used, the associated reduction in ignition-voltage reserves increases ignition miss at high engine speeds and cylinder pressures.
Pontiac engines usually use an electrode gap between 0.035 inch to 0.080 inch, depending on the application. For a stock or almost-stock engine, check the shop manual for your engine's optimized electrode gap. On a highly modified Pontiac where it's now more a function of spark-plug design, ignition-coil potential, maximum engine-speed, and peak cylinder-pressure, find the ideal gap by trial and error. Occasionally, with a specially designed spark plug such as the Bosch Platinum+4, which has four side electrodes, the manufacturer's specifications supercede the Pontiac specification.
Not all cylinder heads position the electrode tip in the same location relative to the cylinder-bore center. The ideal is to have the ionization (the arcing of the gap) occur in the bore center so that the flame can expand evenly to the perimeter of the cylinder. This often takes a backseat to ease of installation or lower manufacturing costs. It's best to have the flame start in the most turbulent area of the bore, the center. The exhaust valve is the second choice due to its higher temperature.