Spark Plugs

1

• Topics covered in this presentation
• Spark Plug Construction
• Spark Plug Reach, Seating and Heat Range
• Spark Plug Servicing

2
The Spark Plug
The spark plug

• Ignites the air and fuel mixture.

• Removes heat from the combustion chamber.

Typical spark plug components
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3
The Steel Shell
The shell is threaded so it can be screwed into
the cylinder head.
It is machined with a hexagonal section, for plug
removal and fitting.
To minimize corrosion, its surface is
electroplated with nickel.
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Question
1
How many sides does a spark plug have on the part
designed for plug removal and replacement?
Enter your answer and press SEND.
5
The Ceramic Insulator
Purpose

• To prevent high voltages from shorting to ground.

• To remove heat from the combustion chamber.

It is made from aluminium oxide and glazed to
discourage dirt.
Ribs increase the surface area of the insulator
to reduce flashover to ground.
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2
Question
What material is the spark plug's ceramic
insulator made from?
A) Aluminium nitrate
B) Aluminium hydroxide
C) Aluminium oxide
D) Aluminium carbonate
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Ground and Centre Electrodes
Electrodes are made from nickel alloy or platinum.
They must be resistant to high temperatures and
corrosion.
The ground electrode is welded to the shell.
The centre electrode has a machined tip.
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3
Question
In a spark plug, what is the ground electrode
connected to?
A) Shell
B) Ceramic insulator
C) Centre electrode
D) Insulator nose
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Spark Plug Dimensions
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.
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4
Question
Spark plugs are made in four different thread
diameters. Is this true or false?
Answer True or False.
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Spark Plug Dimensions
Spark Plug Reach
Incorrect reach can cause engine running problems
or spark plug damage.
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Spark Plug Seating
Flat Seating
Spark plugs must be fitted with a metal washer to
seal combustion chamber and remove heat.
Conical Seating
No washer required as plug is sealed by its
conical seat.
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Spark Plug Temperature
Ideal spark plug tip temperature 500 to 850C.
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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Question
5
What temperature, in degrees C, would be within
the ideal range of operation?
Enter your answer and press SEND.
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Heat Dissipation
2
The combustion process produces heat.
Dissipation occurs as follows

• 20 is absorbed by intake stroke fresh air.

10

• 58 is absorbed by the cylinder head walls.

10

• 20 is absorbed by the insulator and spark plug
  side walls.

58

• 2 is absorbed by spark plug wires.

20
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6
Question
Approximately what percentage of heat from the
combustion process is absorbed by the cylinder
head walls?
A) 2
B) 20
C) 25
D) 58
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Spark Plug Heat Range
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 distance from tip of
insulator to where it meets the metal shell.
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Spark Plug Heat Range
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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Other Types of 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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Resistor Spark Plugs
Spark plug with carbon compound resistor in
centre core.
The resistor suppresses radio frequency
interference (RFI).
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7
Question
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
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Spark Plug Removal
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 Plug Conditions
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8
Question
What is this spark plug suffering from?
A) Carbon fouled
B) Glazed deposits
C) Gap bridges
D) Oil fouled
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Spark Plug Gap Adjustment
Typical Procedure

• Obtain gap information from shop manual.

• Check gap between electrodes with a feeler gauge.

• It should be a sliding fit.

• If necessary, adjust gap using feeler gauge tool.

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9
Question
What device is normally used to measure the spark
plug gap?
A) Micrometer
B) Ruler
C) Feeler gauge
D) Microscope
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Spark Plug Fitting
Typical Procedure

• Apply grease to spark plug threads.

• Hand screw spark plugs into cylinder head.

Flat seat

• Tighten plugs as per shop manual.

• Refit spark plug leads.

Conical seat
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Summary
You should now be aware of

• Spark Plug Construction

• Spark Plug Reach, Seating and Heat Range

• Spark Plug Servicing

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