Spark Plugs
Topics covered in this presentation:
 Spark Plug Construction
 Spark Plug Reach, Seating and Heat Range
 Spark Plug Servicing

Spark Plugs
The spark plug:
 Ignites the air and fuel mixture.
 Removes heat from the combustion chamber.
Typical spark plug components:
Centre electrode
Ground electrode
Terminal
Ribbed insulator
Shell
Conductive seal
Insulator nose
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Spark Plugs
The shell is threaded so it can be screwed into the cylinder head.
Hex
It is machined with a hexagonal section, for plug removal and fitting.
To minimize corrosion, its surface is electroplated with nickel.
Thread
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Spark Plugs
How many sides does a spark plug have on the part designed for plug removal and replacement?
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Spark Plugs
Purpose:
 To prevent high voltages from shorting to ground.
 To remove heat from the combustion chamber.
Ribs
It is made from aluminium oxide and glazed to discourage dirt.
Ribs increase the surface area of the insulator to reduce flashover to ground.
Nose
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Spark Plugs
What material is the spark plug's ceramic insulator made from?
A) Aluminium nitrate
B) Aluminium hydroxide
C) Aluminium oxide
D) Aluminium carbonate

Spark Plugs
Electrodes are made from nickel alloy or platinum.
Ground electrode
They must be resistant to high temperatures and corrosion.
The ground electrode is welded to the shell.
The centre electrode has a machined tip.
Centre electrode
Spark erosion
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Spark Plugs
In a spark plug, what is the ground electrode connected to?
A) Shell
B) Ceramic insulator
C) Centre electrode
D) Insulator nose

Spark Plugs
Thread Diameter
Spark Plugs are made in
10, 12, 14, and 18 mm thread diameters.
Spark Gap
The gap must be correct.
If it is too small, a weak spark is produced.
If it is too large, the mixture will burn inefficiently.
Thread diameter
Reach
Spark gap
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Spark Plugs
Spark plugs are made in four different thread diameters.
Is this true or false?
Answer T rue or F alse.

Spark Plugs
Spark Plug Reach
Incorrect reach can cause engine running problems or spark plug damage.
Too short
Too long
Correct reach
Cylinder head
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Spark Plugs
Metal washer
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Spark Plugs
Ideal spark plug tip temperature = 500 to 850 °C.
If temperature = low, carbon and combustion chamber deposits will not be burnt off.
If temperature = high, the plug may overheat, damaging the electrodes.
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Spark Plugs
What temperature, in degrees C, would be within the ideal range of operation?
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Spark Plugs
The combustion process produces heat.
Dissipation occurs as follows:
 20% is absorbed by intake stroke fresh air.
 58% is absorbed by the cylinder head walls.
 20% is absorbed by the insulator and spark plug side walls.
 2% is absorbed by spark plug wires.
10%
10%
58%
20%
2%
100%
Fresh air
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Spark Plugs
Approximately what percentage of heat from the combustion process is absorbed by the cylinder head walls?
A) 2%
B) 20%
C) 25%
D) 58%

Spark Plugs
Heat range = spark plug's ability to remove combustion chamber heat.
Determined by:
 Insulator nose length.
 Gas volume around the insulator nose.
 Composition of the insulator and the centre electrode.
Insulator nose length
Insulator nose length = distance from tip of insulator to where it meets the metal shell.
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Spark Plugs
Hot Plug:
 Long insulator nose, exposing a large surface area to combustion gases.
 Dissipates heat slowly.
 Firing end heats up quickly.
Cold Plug:
 Shorter insulator nose, minimizing surface area exposed to combustion gases.
 Dissipates heat quickly.
 Firing end heats up slower.
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Spark Plugs
Some spark plugs have multiple ground electrodes.
They provide multiple spark paths which reduces spark erosion.
This surface discharge plug can be used with CDI systems.
It provides good sparking, even when fouled.
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Spark Plugs
Spark plug with carbon compound resistor in centre core.
Plug with carbon resistor
The resistor suppresses radio frequency interference (RFI).
Resistance material
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Spark Plugs
What can be incorporated into a spark plug to suppress radio frequency interference?
A) Ferrite beads
B) Wire wound resistor
C) Capacitor
D) Carbon resistor

Spark Plugs
Typical Procedure:
 Remove the lead from each plug, by using the rubber boot.
 Slacken each spark plug.
 Use an air line/brush to remove all debris from around each plug.
 Remove all plugs and place on bench in cylinder order.
 Inspect each spark plug.
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Spark Plugs
Normal use
Overheated
Gap bridged
Cracked insulation
Pre-ignition
Carbon fouled
Deposit fouled
Splashed deposits
Oil fouled
Glazed deposits
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Spark Plugs
What is this spark plug suffering from?
A) Carbon fouled
B) Glazed deposits
C) Gap bridges
D) Oil fouled

Spark Plugs
Typical Procedure:
 Obtain gap information from shop manual.
Feeler gauge
 Check gap between electrodes with a feeler gauge.
 It should be a sliding fit.
Tool to alter gap
 If necessary, adjust gap using feeler gauge tool.
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Spark Plugs
What device is normally used to measure the spark plug gap?
A) Micrometer
B) Ruler
C) Feeler gauge
D) Microscope

Spark Plugs
Typical Procedure:
 Apply grease to spark plug threads.
 Hand screw spark plugs into cylinder head.
 Tighten plugs as per shop manual.
 Refit spark plug leads.
Flat seat
Conical seat
90 °
15 °
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Spark Plugs
You should now be aware of:
 Spark Plug Construction
 Spark Plug Reach, Seating and Heat Range
 Spark Plug Servicing
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