nuts & bolts
building basics
Spark Plugs
You can’t star t without ‘em
RICHARD KOEHLER , EA A 161427
N
ext 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 iridium 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 electrodes, the ceramic insulator, and the metal shell.
The center of the spark plug is the inner electrode
assembly consisting of the terminal contact, spring, resistor, 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 manufacture 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 harness. 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.
Always install
the type of plug
approved for your
particular engine.
Resistor-type spark plugs are designed to reduce the burning and erosion of electrodes in engines with shielded harnesses. The capacitance between the high-tension cable
and the shielding is sufficient to store electric energy in
quantities that produce a comparatively high-current discharge at the spark plug electrodes. The energy is considerably greater than is necessary to fire the fuel-air mixture.
Therefore, it can be reduced by means of a resistor to provide greater spark plug life. Unshielded spark plugs are still
used in a few light-aircraft engines.
The spark plug firing tip can be one-, two-, three-, or
four-pronged, and either massive or fine wire. It may have
a projected core nose for reduced lead fouling. The tip will
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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 cylinder 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 3/4inch (all weather). Another improvement, which leads to greater dependability and longer life, is the use of
iridium-alloy firing 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 classification of the spark plugs
according to their ability to transfer heat from the firing end of the
spark plug to the cylinder head. Spark
plugs have been classified as hot, normal, 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 insulator is designed to be the hottest part
of the spark plug, its temperature is
related to the temperatures for preignition (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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AUGUST 2007
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 iridiumalloy plugs.
the insulator tip drops below about
800°F. Thus, your spark plugs must
operate between these two temperatures. 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 settings. Plug performance depends on
the operating temperature of the
insulator nose, which should ideally
be between 1,000°F and 1,250°F.
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.
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 certificatEAA Sport Aviation
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building basics
Iridium-alloy firing tips provide greater
dependability and a longer life at about
triple the cost.
ed engines, the manufacturer lists the
plugs that may be used on a particular engine and whether they’re FAA
approved in the Type Certificate Data
Sheet (TCDS). 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. Generally, a number in
the plug part number designates the
heat range of the plug. The larger the
Simplistically, an
engine that runs hot
requires a “cool” heat
range plug, and an
engine that runs cool
requires a “hot” plug.
number, the “hotter” the plug. Typical 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 homebuilt 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 conductiv120
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 flight advisor.
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