The spark plug is found in most of the more recent Stationary Engines powered by petrol (gasoline) or kerosene. The spark plug ignites the fuel when it is energised from a coil or magneto. In this article we'll deal with:
  • Types of plug
  • Cleaning a plug
  • Identifying plug condition and cause


Types of Plug

Spark plugs come in two basic forms: Those which can be disassembled into component parts "separable" and those which are in one piece "integral". The latter being typical of todays modern plug. The design goal for a spark plug is to deliver a clean spark in the heat and pressure of the combustion chamber. To do this it must be made of non flammable heat resistant materials.
seperable-plug integral-plug


A steel core carries the high voltage current from the contact on the tip of the plug to the arc point at the base of the plug. Surrounding the core is an insulating material. In historic plugs this may have been mica surrounded by porcelain ceramic. Today it is a silicone impregnated ceramic. Surrounding the ceramic insulator is a steel shell which is threaded on one end for fitting into the hole in the engine cylinder. The spark arcs (jumps) from the steel core electrode to the anode which is usually integral with the shell. A modern point will likely have a single anode, older plugs may have one, three or five. The insulator both prevents electrical leakage between the core and shell, and governing the operating temperature of the plug.


Typical Sizes

The typical sizes you will encounter are:
  • 0.5" straight with 18 tpi (threads per inch)
  • 0.5" tapered with 18 tpi
  • 7/8" straight with 18 tpi
  • metric: 18mm with 1.5mm pitch
  • metric: 14mm with 1.5mm pitch
  • metric: 10mm with 1.5mm pitch
correct-plug The length of the plug body or shell varies widely from plug to plug. A spark plug works best when the electrode is within the combustion chamber, if too short a thread is used then the electrode will be outside the combustion chamber and combustion will occur late if it occurs at all. You should choose a length which reaches into the chamber but does not interfere with the piston or valve operation.

Hot and Cold Plugs

plug-lengths The length of the steel core and insulator govern the temperature of the plug. A short insulator cools more quickly than a longer insulator. In situations where either the plug is situated badly (through a design fault) or where the plug might foul (eg. the fuel is dirty such as in a hit-n-miss or 2 stroke) a higher temperature plug would be used in an effort to keep the plug clean. Pre-ignition can occur when too hot a plug is selected as the plug retains so much heat that the combustion mixture ignites as it heats over its flash point in the vicinity of the plug.

Cleaning a Plug

Alcohol or other non oily solvent should be used in preference to fuel. If fuel is used an oily residue will remain on the plug which will attract carbon deposits which is what we're trying to eliminate in the first place. It is very important that the ceramic is not scratched as this will cause electrical leakage when the scratches fill with carbon. I use a tooth pick to scrape out the built up oil and carbon from the base of the plug then clean with alcohol and a small brush. I usually clean the electrode and the anode with 600 grit paper to smooth them. As part of the cleaning I set the gap to the engine manufacturers recommendation. If there is no recommendation or you do not have the information I set the gap to 0.025" for buzzcoil or 0.020" for rotary magneto since this is suitable for low compression engines. Note that engines with a magneto require a smaller gap as when the engine is running slowly the magneto produces less energy. Trip magnetos produce the same energy regardless of speed so can use the larger 0.025" gap.

Identifying Plug Condition and The Cause

A clean plug should be bright and shiny, possibly with a very thin coating of fine white ash.
  • black and oily - there is lubricating oil within the combustion chamber, possibly a sign of scored cylinder bore, bad piston rings or leaky valve stems. Note that in a 2-stroke engine oil is expect and is countered by using a hotter plug.
  • black and sooty - the combustion mixture is too much rich, ie. there is more fuel than is required for efficient combustion
  • burnt or melted electrodes - this indicates that the plug is operating at too higher temperature which could be caused by either: too higher current provided to the plug; or the plug extents too far into the combustion chamber.
  • red/rusty - water is getting into the combustion chamber.

When should the spark be timed?

The spark should be timed so that it fires when the combustion mixture is under the maximum pressure, that is immediately prior to TDC. This will allow the combustion to complete while under the best circumstances and utilise most of the expansion force as the combusting gas expands forcing the piston away from TDC towards BDC.

If the spark occurs too soon before TDC, the combustion will take place too early robbing the piston of power and possibly even reversing its direction. If the spark is

If the spark occurs at or immediately after TDC the expansion of gas will not be complete when the piston reaches BDC meaning the piston has to fight against combustion to complete the stroke.

Note that in a 4 stroke engine the spark should not fire every time the piston reaches TDC as it may ignite any combustibles left in the exhaust causing combustion within the exhaust.

The Spark Intensifier

spark-gap The spark intensifier is supposed to generate a better hotter spark at the plug point since it has more energy. This is a wives tale as much energy is wasted crossing the first spark gap. It is however a good visual test of sparking during fault finding.