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Understanding The Aircraft Spark Plug

Posted May 30, 2017

The mighty Aircraft Spark Plug gets a lot of blame for many things that can go wrong in an Aircraft engine or any simple two stroke. The first assumption is to replace the spark plug and all is well and carry on with your business. This is simply not always the answer and while your engine might run well for a small amount of time; you have not fixed the root cause of the problem. The dilemma can very well be from the spark plug but a host of problems arise that will cause a secondary effect on the plug that is somewhat residual.

Image credit: skeeze via Pixabay, CC0 Public Domain

Troubleshooting Spark Plug Problems

Usually the first step in any assumed problem in this area is an overall visual inspection of the condition of the spark plug and maybe replacing the plug with a newer or different one to see if the problem is fixed. Knowing what you are looking for on the spark plug can also help you identify what the engine is doing wrong that is essentially causing the change in the appearance of the spark plug.

Temperature changes

The rise in temperature directly affects the spark plug and can happen from many things in the engine. A poorly fitting or damaged engine baffle can create a leak and will then cause cool air to escape. This escaped, cool air can cause an uneven temperature balance and one or more of the cylinders can be at sufficient, higher temperatures. The insulator of the plug may have a shiny white appearance and black deposits with irregular electrode structural patterns from the higher temperatures.

Image credit: cegoh via Pixabay, CC0 Public Domain

Visual Inspection of the Insulator Tip

The tip may have different types of deposits and this can cause many problems with the flow of alternating current from each electrode.

Understand the Deposits

  • Carbon Fouling- is a misfire that is most likely from the carburetor or a vacuum leak.
  • Oil Fouling- is usually a problem from the head gasket or piston ring.
  • Lead Fouling– symptom will present as yellowish-brown deposits and one of or more of the cylinders does not fire correctly when under high revolutions per minute.
  • Conductive Material- can be any conductive deposit that has made some type of bridge from electrode to electrode and causes a short. Inspecting the deposit can help in identifying the problem.

Understand the Structure

  • Broken- insulator maybe broken by different things to mostly involve some type of extreme thermal changes. If the electrode is broken this can happen from a mechanical collision and also vibration problems may happen from improper installation.
  • Melting-center or ground electrode is melted and is usually from improper installation resulting in uneven transfer of heat from the tip. It can also be directly impacted from uneven heat sources from the combustion areas in the cylinder head or pistons.
  • Lead Erosion -is when there is too much lead in the gas and the lead reacts with the electrodes at higher temperatures which then causes a chemical reaction. This reaction will in turn start to break down the electrodes and make them less dense.

Understand the Gaps

Understanding the change in the electrode gap and what has caused the problem is very important to maintaining the life of the spark plug. A considerable gap will cause the cylinder to not fire appropriately which will then decrease the temperature of the spark plug and cause deposit buildup.

Cleaning the Spark Plugs

In order to minimize casual deposit buildup; The spark plugs need to be maintained every 100 hours and cleaned by a sufficient cleaner.


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