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(Contributed by Dave Pound)

Contrary to what many have previously been led to believe the following is an explanation of how a glow plug functions in a motor.
The plug is initially heated by applying a voltage (typically 1.5 volts) to it. This is to cause it to glow so as to ignite the fuel at compression and start the internal combustion cycle.
Once the cycle has started the power source can be disconnected as with the heat generated at combustion the catalytic reaction generated between the methanol and platinum in the plugs coil becomes sufficient to keep the process going.
The catalytic reaction is a reaction whereby platinum will glow in the presence of methyl alcohol vapour. This will happen without any external power source being applied!

How do you select the correct plug for your application and why?

To do this you need to understand a little more of the theory behind the process. In glow fuel the catalytic reaction is generated between the methanol and platinum only. Castor oil, synthetic oil, nitro methane etc do not generate a catalytic reaction with the platinum.
Next you need to understand that a certain surface area of platinum is required to generate a sufficient catalytic reaction to keep the internal combustion process going.
Also it is necessary to allow extra surface area for the reaction to be great enough when it diminishes with the available methanol dropping as in the case at motor idle.
Simply put, cold plugs are manufactured using a thicker wire to give greater surface area to facilitate a greater glow and thus the required catalytic reaction where less methanol is present in the fuel mixture.
So! More nitro means less methanol which in turn means a greater surface area of platinum will be required to generate the required catalytic reaction.
Correspondingly a lesser oil content can mean more available methanol and lesser surface area of platinum would be required to generate a sufficient catalytic reaction.
Suddenly it all makes sense! To work out which temperature plug to use you need to know how much methanol is in your fuel not how much nitro or oil. As a rough rule of thumb:
80% methanol or above use a hot plug .
70% ~ 75% use a medium plug .
60% ~ 75% use a cold plug.
65% or less use a very cold plug.

Idle Bars and Other Stuff

Again contrary to what many believe the idle bar on a glow plug is not necessarily what its name would suggest. It is in fact to stop any fuel not vapourised from dousing the platinum coil of the glow plug by dispersing it away from the coil.

Why are plated coils not at good as platinum alloy coils?

Plated coils suffer from very quick degeneration as the plating breaks down under operating conditions. As bits of plating come off the coil the plug is effectively becoming a hotter and hotter unit until in a comparatively short time it is no longer able to perform its function.
Conversely a platinum alloy coil will still degenerate but as it is platinum alloy throughout, the surface remains as platinum alloy and the plug continues giving much the same characteristics for quite a very long time