Know your spark plug
Spark plugs vary in design to suit a number of applications
such as loading and speed range of an engine, compatibility
with the cumbustion chamber design, tolerance for varying mixture
strengths, compression ratios and operating temperatures.
Spark plugs are basically classified according to heat range,
i.e., the ability to transfer heat from the firing tip of the
spark plug to the central electrode and to dissipate this heat
to the engine's cooling system. This classification divides
spark plugs into two categories, namely 'cold' plugs
and 'hot' plugs.
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Cold
Plug
A 'cold' plug has the advantage of quicker heat
transfer. It has a shorter insulator and hence heat
has to travel a shorter distance. Ideal applications
are in highly tuned engines and for continuous speed
running conditions. In a cooler engine, it would
get fouled up. |
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Hot Plug
A 'hot' plug has a longer insulator, thereby
heat travel to electrode from firing tip takes longer.
This enables it to operate at higher temperature
to compensate for the cooler running engine. It
is unsuitable for highly tuned engines as the high
temperatures involved would cause overheating and
pre-ignition.
Spark plugs are made in differing ranges from 'hot'
to 'cold' to ensure dependable performance in any
engine. |
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Long reach |
Short reach |
Long
reach and short reach plugs
Another base for classification depends on the length
of the threaded portion. This varies according
to the thickness of the cylinder head above the
combustion chamber. The length of the thread is
known as the 'reach' of a plug. A spark plug
with longer threads should never be fitted onto
a short reach head as the excess length could damage
the valve or piston. Similarly, a short reach plug
should not be fitted in a long reach head as the
exposed threads of the cylinder head can get damaged
during combustion and make it difficult to fit on
a correct reach plug later.
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Side-
Gap plug
often used in highly tuned engines
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Three-
gap plug
has three earthed electrodes (one obscured in drawing)
to give longer plug life. |
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Tapered-shoulder
Plug used instead of a plug with a gasket.The
conical shoulder is tightened into recess to provide
a gas-light seal.
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