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nu t s & bo l t s

building basics

Spark PlugsYou can’t star t without ‘em

RICHARD KOEHLER, EAA 161427

Next to oil, the lowly spark plug seems to be one of the most misunderstood engine components. How long should your plugs last? I know folks

who change them every 100 hours. Some claim that iridi-um plugs can last 2,000 hours. At prices ranging from $17 to more than $50, and eight to 12 plugs per engine, spark plugs become a high cost item.

The spark plug is the part of the ignition system where the electric energy of the high-voltage current produced by the magneto is converted to the heat energy required to ignite the fuel-air mixture in the engine cylinders. It provides an air gap across which the high voltage of the ignition system produces a spark to ignite the mixture. An aircraft spark plug consists of three major parts: the elec-trodes, the ceramic insulator, and the metal shell.

The center of the spark plug is the inner electrode assembly consisting of the terminal contact, spring, resis-tor, brass cap and conductor, and the nickel-clad copper electrode. The insulator, between the electrode assembly and the shell, is made in two sections. The main section extends from the terminal contact to a point near the electrode tip. The barrel-insulating section extends from near the top of the shielding barrel far enough to overlap the main insulator.

The outer section of the spark plug is a machined-steel shell. This shell is often plated to eliminate corrosion and to reduce the possibility of thread seizure. To prevent escape of high-pressure gases from the cylinder of the engine through the spark plug assembly, internal pressure seals, such as the cement seal and the glass seal, are used between the outer shell and the insulator and between the insulator and the center electrode.

The shell of the spark plug includes the radio-shielding barrel. In some spark plugs, the shell and shielding barrel

are made in two sections and screwed together. The two parts should never be disassembled because during man-ufacture the correct pressure is applied to provide a gas tight seal. Any disturbance of the seal may cause leakage.

The shell and the radio-shielding barrel complete the ground circuit for the radio shielding of the ignition har-ness. It is externally threaded on both ends so it can be joined to the radio shielding of the ignition harness at the top and be screwed into the cylinder head at the bottom.

Spark plugs are manufactured with many variations in construction to meet the demands of aircraft engines.

Resistor-type spark plugs are designed to reduce the burn-ing and erosion of electrodes in engines with shielded har-nesses. The capacitance between the high-tension cable and the shielding is suffi cient to store electric energy in quantities that produce a comparatively high-current dis-charge at the spark plug electrodes. The energy is consid-erably greater than is necessary to fi re the fuel-air mixture. Therefore, it can be reduced by means of a resistor to pro-vide greater spark plug life. Unshielded spark plugs are still used in a few light-aircraft engines.

The spark plug fi ring tip can be one-, two-, three-, or four-pronged, and either massive or fi ne wire. It may have a projected core nose for reduced lead fouling. The tip will

Always install

the type of plug

approved for your

particular engine.

118 AUGUST 2007

building basics

have a heat range depending on the heat rejection properties of the joint between the core and the shell. The threads that hold the plug in the cyl-inder may be 14 or 18 millimeters and either short or long reach. The threads on the top of the barrel may be either 5/8-inch (standard) or 3/4-inch (all weather). Another improve-ment, which leads to greater depend-ability and longer life, is the use of iridium-alloy fi ring tips, at about triple the cost.

Besides having to get the right size shell threads (14 or 18 mm), the right reach (long or short), and the right terminal threads (regular, 5/8-24, or all-weather, 3/4-20), you need to get the right heat range.

The term “heat range” refers to the classifi cation of the spark plugs according to their ability to trans-fer heat from the fi ring end of the spark plug to the cylinder head. Spark plugs have been classifi ed as hot, nor-mal, and cold. However, these terms may be misleading because the heat range varies through many degrees of temperature, from extremely hot to extremely cold. Since the insula-tor is designed to be the hottest part of the spark plug, its temperature is related to the temperatures for pre-ignition (very hot) and fouling (not hot enough).

Pre-ignition is likely to occur if the surface areas in the combustion chamber exceed a critical limit or if the spark plug core nose temperature exceeds 1,630°F. At the other end, fouling or short-circuiting of the plug due to carbon deposits can occur if

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the insulator tip drops below about 800°F. Thus, your spark plugs must operate between these two tempera-tures. The ideal plug should operate at as hot a temperature as possible when the engine is at low power and as cool as possible at high power set-tings. Plug performance depends on the operating temperature of the insulator nose, which should ideally be between 1,000°F and 1,250°F.

Simplistically, an engine that runs hot requires a “cool” heat range plug, and an engine that runs cool requires a “hot” plug. If you switch them, you run the chance of pre-ignition on the hot engine and fouling on the cold engine. Remember that hot and cold are relative terms.

Different heat range plugs cannot be substituted arbitrarily, as is often done in the family car. On certifi cat-

If you have a minimally

modified certificated

engine, then the TCDS

is a good source of

information on spark

plugs that will work in

your engine.

The spark plug firing tip can be one, two,

three, or four pronged, and either massive

or fine wire. On left is an example of a

massive wire plug, next to it is a fine wire

example. Above are examples of iridium-

alloy plugs.

ed engines, the manufacturer lists the plugs that may be used on a particu-lar engine and whether they’re FAA approved in the Type Certifi cate Data Sheet (TCDS). If you have a minimal-ly modifi ed certifi cated engine, then the TCDS is a good source of informa-tion on spark plugs that will work in your engine. Generally, a number in the plug part number designates the heat range of the plug. The larger the

number, the “hotter” the plug. Typi-cal numbers are from 27 to 41. A 27 is used on the high-revving TCM Tiara engines, and a 41 on low horsepower 80-octane engines. A typical home-built engine uses 37, 38, or 40 heat range plugs.

The principal factors governing the heat range of aircraft spark plugs are the distance between the copper sleeve around the insulator and the insulator tip, the thermal conductiv-

building basics

Iridium-alloy firing tips provide greater

dependability and a longer life at about

triple the cost.

Simplistically, an

engine that runs hot

requires a “cool” heat

range plug, and an

engine that runs cool

requires a “hot” plug.

120 AUGUST 2007

ity of the insulating material, thermal conductivity of the electrode, the rate of heat transfer between the electrode and the insulator, the shape of the insulator tip, the distance between the insulator tip and the shell, and the type of outside gasket used. Other features of spark plug construction may affect the heat range to some extent, but these seven factors are of primary consideration. Always install the type of plug approved for your particular engine.

Richard Koehler is associate professor of Aerospace Sciences at the University of the District of Columbia and has been an EAA member since 1980. He is an active airframe and powerplant mechanic with inspection authorization, a commercial pilot with instrument and multi-engine ratings, and a technical counselor and fl ight advisor.

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