AIRCRAFT SPARK PLUGS

A spark plug is a device that is inserted in the head of an internal combustion engine cylinder that
ignites the fuel air mixture by means of an electrical spark.

Spark plugs are manufactured in a variety of sizes and shape each to fit a particular engine,
according to operating temperature and the design of the cylinder head.

GENERAL CONSTRUCTION

The modern spark plug consists mainly of three parts; the electrodes, the ceramic insulator and the
metal shell.

(a) High tensile steel body which screws into the engine cylinder.

(b) Central electrode which is encased in a ceramic insulating material and secured inside the
body.

(c) Earth electrodes one or more of these protrude from the body towards the central electrode
to form a spark gap.

NB: the other end of the central electrode terminates in a contact which is connected with
the magneto distributor via the HT leads.

(d) Resistor: which is interposed between the contact and central electrode; helps to:

i. Reduce corrosion

ii. Reduce erosion

iii. Reduce radio interference by eliminating ‘spikes’ generated by the ignition system.

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NB

1. Modern spark plugs which are fitted with suppression screens with the screen tube braced
to the body are not supposed to be dismantled like the common spark plugs.

2. The screws on earlier types were insulated by mica. Modern plugs incorporate a ceramic
sleeve insulate which may be integrated with or separate from the electrode insulator.

3. The HT lead is secured to the spark plug by means of a cable connector and electrical contact
with the central electrode is obtained through a spring attached to the end of the cable core.

4. The types of plugs approved for use on a particular engine are specified in the relevant
manuals and no other types should be used. For identification purposes, the type number is
always marked on the spark plug body.

5. It is preferable for an engine to have all plugs of the same type and make.

CLASSIFICATION OF SHELL THREADS

These are classified as 14 or 18mm diameter, long reach or short reach, thus:

DIAMETER LONG REACH SHORT REACH

14mm ½ ‘’ 3/8’’

18mm 13/16 ‘’ ½ ‘’

The reach of the spark plug is the length of the threaded portion which screws into the cylinder head
and is measured from the shoulder to the end of the thread.

The designation numbers for spark plugs provides an indication of the characteristics of the plug.
Champion spark plugs have letters to indicate whether they contain a resistor, the barrel type,
mounting threads, reach, hex size, heat rating, gap and electrode style e.g.

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 1. No letter or an ‘R’, R=resistor type

2. No letter, E or H

• No letter –unshielded

• E-shielded 5/8’’ – 24 threads

• H-shielded ¾ ‘’ -20 threads

3. Mounting threads, reach and hex size

• A-18mm, 13/16’’ reach, 7/8’’ stock hex

• B-18mm, 13/16’’ reach, 7/8’’ milled hex

• D-18mm, ½‘’ reach, 7/8’’ stock hex

• J-14mm, 3/8’’ reach, 13/16’’ stock hex

• L-14mm, ½ ‘’ reach, 13/16’’ stock hex

• M-18mm, ½ ‘’ reach, 7/8’’ milled stock.

4. Heat rating range

• 26 to 50 indicate coldest to hottest spark plugs.

• 76 to 99 indicate special application aviation spark plugs.

The term heat range refers to the spark plug ability to transfer heat from the firing end of the spark
plug to the cylinder head. "Hot" plugs have a long, slim insulator nose that creates a long heat
transfer path, whereas "cold" plugs have a relatively short insulator to provide a rapid transfer of
heat to the cylinder head

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 Fundamentally, an engine which runs hot requires a relatively cold spark plug, whereas an engine
which runs cool requires a relatively hot spark plug. If a hot spark plug is installed in an engine which
runs hot, the tip of the spark plug will be overheated and cause preignition. If a cold spark plug is
installed in an engine which runs cool, the tip of the spark plug will collect unburned carbon, causing
fouling of the plug.

5. Gap and electrode style

• E=two prong aviation

• N=four prong aviation

• P=platinum fine wire

• B=two prong massive, tangent to centre

• R=push wire, 900 to centre

SPARK PLUG MAINTENANCE AND INSPECTION

Spark plug operation can often be a major source of engine malfunctions because of lead, graphite,
or carbon fouling and because of spark plug gap erosion. Most of these failures, which usually
accompany normal spark plug operation, can be minimized by good operational and maintenance
practices. Spark plugs should therefore be serviced strictly IAW manufacturer’s instructions at the
intervals specified in the maintenance manual.

a. CARBON FOULING

Carbon fouling from fuel is associated with mixtures that are too rich to burn or mixtures that are so
lean they cause intermittent firing. Each time a spark plug does not fire, raw fuel and oil collect on
the non firing electrodes and nose insulator. These difficulties are almost invariably associated with
an improper idle mixture adjustment, a leaking primer, or carburetor malfunctions that cause too
rich a mixture in the idle range. A rich fuel/air mixture is detected by soot or black smoke coming
from the exhaust and by an increase in r.p.m. when the idling fuel/air mixture is leaned to "best
power." The soot that forms as a result of overly rich, idle fuel/air mixtures settles on the inside of
the combustion chamber because the heat white fumes of evaporating and burning oil coming from
the exhaust.

b. LEAD FOULING

Lead fouling of aviation spark plugs is a condition likely to occur in any engine using "leaded fuels."
Lead is added to aviation fuel to improve its antiknock qualities. The lead, however, has the

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undesirable effect of forming lead oxide during combustion. This lead oxide forms as a solid with
varying degrees of hardness and consistency. Lead deposits on combustion chamber surfaces are
good electrical conductors at high temperatures and cause misfiring. At low temperatures the same
deposits may be good insulators. In either case, lead formations on aircraft spark plugs prevent their
normal operation. To minimize the formation of lead deposits, - ethylene dibromide is added to the
fuel as a scavenging agent which combines with the lead during combustion.

c. ELECTRODE EROSION

Erosion of the electrodes takes place in all aircraft spark plugs as the spark jumps the air gap
between the electrodes. The spark carries with it a portion of the electrode, part of which is
deposited on the other electrode and the remainder is blown off in the combustion chamber. As the
air gap is enlarged by erosion, the resistance that the spark must overcome in jumping the air gap
also increases. This means that the magneto must produce a higher voltage to overcome the higher
resistance. With higher voltages in the ignition system, a greater tendency exists for the spark to
discharge at some weak insulation point in the ignition harness. Since the resistance of an air gap
also increases as the pressure in the engine cylinder increases, a double danger exists at takeoff and
during sudden acceleration with enlarged air gaps. Insulation breakdown, premature flashover, and
carbon tracking result in misfiring of the spark plug, and go hand in hand with excessive spark plug
gap. Wide gap settings also raise the coming in speed of a magneto and therefore cause hard
starting. Spark plug manufacturers have partially overcome the problem of gap erosion by using a
hermetically sealed resistor in the centre electrode of some spark plugs. This added resistance in the
high-tension circuit reduces the peak current at the instant of firing. This reduced current flow aids
in preventing metal disintegration in the electrodes. Also, due to the high erosion rate of steel or any
of its known alloys, spark plug manufacturers are using tungsten or an alloy of nickel for their
massive electrode plugs and platinum plating for their fine wire electrode plugs.

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 d. GRAPHITE FOULING

As a result of careless and excessive application of thread lubricant to the spark plug, the lubricant
will flow over the electrodes and cause shorting. Shorting occurs because graphite is a good
electrical conductor. The elimination of service difficulties caused by graphite is up to the aviation
mechanics. Use care when applying the lubricant to make certain that smeared fingers, rags, or
brushes do not contact the electrodes or any part of the ignition system except the spark plug
threads.

REMOVAL AND SERVICING PROCEDURE OF SPARK PLUGS

Spark plugs should be removed for:

(a) Inspection and maintenance at intervals recommended by the manufacturers.

(b) Inspection and testing when it is suspected that the plugs are the cause for engine
malfunctioning.

1. Before removing the spark plugs, the ignition harness must be disconnected. The shielded
terminal connectors of the HT leads are removed by loosening the elbow nut with the
proper size crowfoot or open end wrench.

CAUTION: care must be taken to avoid damaging the elbow. Pull the lead straight
with the centre line of the plug barrel, because if a side load is applied, damage to
the barrel insulator and the ceramic lead terminal may occur.

2. When the ignition harness has been removed, using a proper socket and applying a steady
pressure with one hand on the hinge, holding the socket in alignment, with the other hand
remove the plug.

NB: in course of engine operation, carbon and other products of combustion will be
deposited across the spark plug and cylinder, as a result, a high torque is generally
required to break the spark plug loose.

This factor imposes a shearing load on the plug and if the load is great enough, the
plug may break off leaving a section in the cylinder spark plug hole. If this happens,
great care must be exercised in the removal of the broken section. Normally it will
be necessary to remove the cylinder from the engine and remove the broken section
in accordance with overhaul manuals.

As each spark plug is removed, it should be placed in a tray with numbered holes so that the engine
cylinder from which the spark plug has been removed can be identified. This is important, because
the condition of the spark plug may indicate impending failure of some part of the piston or cylinder
assemblies.

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Immediately after the spark plug has been removed, they should be given a careful visual inspection
and all unserviceable plugs should be discarded. Spark plugs with cracked insulators, badly eroded
electrodes, damaged shells, or damaged threads should be rejected.

SPARK PLUG CLEANING

All oil and grease should be removed from both the interior and the exterior of the spark plug by
using approved solvents such as Stoddard. After cleaning the solvent is dried using compressed air or
drying oven.

Lead and carbon deposits which form on the ceramic core, the electrodes and inside the plug shell
can easily be removed by using an abrasive blasting machine especially designed for cleaning spark
plugs.

INSPECTION AFTER CLEANING

Cleaned spark plugs must be inspected visually for:

• Distortion

• Cracks in insulator

• Damaged threads

• Excessively eroded electrodes

If any of these exists, the spark plug should be rejected.

The electrodes must be examined for:

(i) Security

(ii) Erosion

Any plug with loose electrodes or with erosion exceeding manufacturer’s recommendation must be
rejected.

As a general rule, erosion at the sparking points which reduces the cross section area of the central
electrodes by less than 50%, or which reduces the cross section area of the earth electrodes by
necking less than 33% is acceptable.

Spark plug erosion is usually if not always accompanied by an increase in spark-gap.

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SPARK PLUG GAP SETTING

The spark gap must be set within the specified limits by bending the earth electrodes. Care must be
taken to ensure that they do not fracture. The central electrode must never be bent.

The specified limits are obtainable by bending the earth electrodes using a spark plug setting tool
while checking with a spark plug gap tool or feeler gauge. 0.011” should go through, 0.0015”
shouldn’t go through.

NB: if the gap of a four-prong or two-prong spark plug has been closed beyond the limit, no effort
should be made to open the gap. In such a case the plug should be discarded or returned to the
manufacturers for adjustments.

Single-electrode and two-prong wire electrode plugs are so constructed that the gap can be either
opened or closed without danger or cracking the ceramic insulator.

SPARK PLUG TESTING

After the plug has been serviced, it requires being tested IAW manufacturer’s instructions. Spark
plugs are tested by applying high voltage, equivalent to normal ignition voltage, to the spark plug
while the plug is under pressure.

A bench test cannot simulate conditions in the combustion chamber; therefore, a special ignition
tester must be used.

A bench test, however, does establish that plug insulation is satisfactory and that all electrical
conductive deposits have been removed.

A typical spark plug test can be carried out as follows:

1. Connect the tester to an electrical power supply of the correct voltage; also connect the air
supply to the tester.

2. Screw the spark plug to the tester finger tight.

3. Connect the high voltage lead to the spark plug.

4. Open the air valve until the test pressure is indicated on the pressure gauge.

NB: although an air pressure of 80lbs/in2 is considered satisfactory for most spark plugs,
some manufacturers may stipulate an air pressure which is related to the spark gap.

5. Press the tester button for a few seconds (3.5) and observe that regular sparking occurs, if
not discard the plug.

LEAK TEST

A test for gas leakage may sometimes be specified, to ascertain that the seal in a detachable plug or
the joint in a non-detachable plug is sealing satisfactory.

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The plug under test must screwed in a pressure chamber and the other end immersed in a container
of white spirit so that the liquid covers the joints. Then raise the air pressure in the chamber to
700KN/m2 and observe leakage in form of bubbles.

SPARK PLUG INSTALLATION

1. Carefully coat the threads thinly/sparingly with a graphite base anti-seize compound, leaving
2-3 threads towards the electrodes.

During the application of the compound, be very careful that none of the compound drops
onto the electrodes.

2. To install a spark plug, start it into the cylinder without using a spanner and turn it by hand
until the plug is seated on the gasket. Finish to specified torque using a torque wrench. The
usual torque for 18mm spark plugs is 360 to 420 in-lbs, and the torque for 14mm plugs is
240 to 300 in-lbs.

Over tightening a spark plug may damage threads and make the plug difficult to remove or,
in extreme cases, may change the gap setting. Over tightening will also cause the spark plug
to stick in the cylinder.

NB: don’t forget to rotate the plugs. This helps to assure even electrode wear due to alternate
polarity firing in the standard factory-mag setup. See the simple rotation diagram bellow.

STORAGE

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Spark plugs which have been cleaned and tested but are not put to immediate use must be inhibited
against corrosion by coating the nose with a suitable storage oil and fitting thread protectors to the
plug body and screen.

If not intended for long time storage, place them in racks and keep them warm and dry preferably in
a cupboard as precaution against condensation. Plugs which are meant for long term storage or for
transit, package as follows:

(a) Place each plug in a polythene tube and label clearly

(b) Plugs should be stored in conditions which are:

• Clean

• Dry

• Even temperature

• Well ventilated

• Free from corrosive fumes

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