Spark Plug: Description and Operation

The spark plugs protrude into the combustion chambers and provide the spark that ignites the compressed air/fuel mixture. There are many different types of spark plugs for the many different applications and each type has specific advantages for its applications. Different features include:

Typical Spark Plug Construction:

1. THREAD SIZE - The diameter and pitch of the threaded portion of the spark plug.

2. REACH - The length of the threaded portion and how far the electrode protrudes into the combustion chamber.

3. FLAT SEAT - Flat seat spark plugs have a flat area where the spark plug seats against the cylinder head. These plugs are used with a metal gasket that often must be threaded onto new spark plugs before installation.

4. TAPERED SEAT - Some spark plugs have a tapered area where the plug seats against a tapered recess in the cylinder head. No gasket is used with tapered seat spark plugs and correct tightening torque is especially critical, particularly when installed in aluminum cylinder heads.

Spark Plug Heat Range Illustrated:

5. HEAT RANGE - The ability of the spark plug to cool its electrodes.

a. Hot plugs have longer insulators around the center electrode and less cross-sectional area for heat to transfer from the electrode to the cylinder head. Heat is conducted away from the center electrode more slowly and the electrode tip stays at a higher operating temperature.

b. Cold plugs have shorter insulators around the center electrode and a larger cross-sectional area for heat to transfer to the cylinder head. Heat is conducted away from the center electrode quickly and the electrode remains at a cooler operating temperature.

6. RESISTOR TYPE - Most spark plugs are resistor type plugs. These have a carbon resistor or auxiliary air gap built into them to provide radio static suppression and to ensure a higher sparking voltage is reached in the secondary ignition circuit before the spark occurs. Resistor type spark plugs are always used with high energy electronic ignition systems.

7. NON RESISTOR TYPE - These are rarely used any more due to their poor compatibility with electronic ignition systems. Spark plugs without auxiliary resistance built in will allow the coil to discharge at much lower voltages (5K-10K volts) resulting in a weaker (lower energy) spark than electronic ignitions are capable of. These are used in older breaker point type ignition systems that are not designed to produce the high sparking voltages required by today's emission controlled gasoline engines. Non-resistor plugs also produce more radio-static interference.

8. PLATINUM OR PALLADIUM PLUGS - Some spark plugs have center electrodes coated with or made of special alloy metals to reduce wear.

NOTE: There are many different manufacturers of spark plugs and in most cases any brand will do. Always use the correct plug type for the application, as prescribed by the vehicle manufacturer.