Sparking Respect: Unique facts about the aviation spark plug

Sparking Respect Unique facts about the aviation spark plug By Harry Fenton and Dick Podiak September 1999 Imagine this engineering project: Design an electromechanical device for the aircraft engine that will spark between 90 to 150...

Typical spark plug gap for the aviation plug is .016-in to .021-in., as opposed to as much as .050-in. for automotive type plugs. Why the big difference? The tighter gap of the aviation spark plug requires less voltage to arc across the gap, reducing the chance of misfire, and lower voltage means less electrode wear due to material transfer loss during the arc event. If operating stress is reduced, reliability is improved.

The heart of the spark plug is the aluminum oxide, ceramic insulator material that extends from the firing end of the plug in the combustion chamber up to the top of the plug where the ignition harness is connected. The composition, glazing, and manufacturing control of the insulator are critical in determining the reliability of the spark plug.

The percentage of aluminum oxide in the insulator affects the mechanical strength of the material, and is a determining factor in the ability of the insulator to accept and reject heat from the combustion process. If the composition is incorrect, the ceramic material may crack, resulting in damage to engine components. A poor quality ceramic insulator may cause the electrodes to remain at too high of a temperature, which can result in pre-igntion of the fuel/air mixture. The glaze applied to the firing tip of the insulator ceramic improves the mechanical strength of the ceramic surface. The glaze applied to the ceramic at the ignition lead end of the spark plug provides a smooth surface for easy cleaning.

Inside the insulator and inline with the electrode is a resistor. This resistor serves to limit the peak electrical discharge current when the plug fires, thereby reducing electrode wear. The resistor of the plug can be manufactured in a number of different ways: either as a multi-part assembly or in a monolithic fashion — "doped" into the glass seal. The monolithic resistor is a more modern technology and offers improved reliability, longer life, and is more economical to manufacture.

Sparking Respect

Unique facts about the aviation spark plug

By Harry Fenton and Dick Podiak

September 1999

Selecting the proper plug
Plug selection is typically straightforward due to extensive documentation and application information provided by the spark plug and engine manufacturers. But, for any given application, there may be more than one spark plug approved to allow for alternatives to optimize performance.

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Some plugs offer identical fits, but different heat ratings, so don't be fooled into thinking that "as long as it fits, it's ok." A spark plug with a heat rating incorrect for a particular engine can cause significant, if not catastrophic damage to an engine. Always use the spark plugs found in the spark plug or engine manufacturer's FAA-PMA approved application listing.

Spark plug manufacturers use a part numbering system that describes the qualities of the plug. For instance, within the spark plug part number, there is a code for the heat rating of the plug. Intrinsically, the value of the heat rating numbers reads like a thermometer. High numbers denote "hot" plugs and low numbers denote "cold" plugs. A hot plug features a longer insulator nose, and transfers heat more slowly back to the engine, therefore remaining physically hotter. Inversely, a colder plug has a shorter insulator nose and transfers heat more rapidly, operating at a lower overall temperature than a hotter plug. As a rule, hot plugs are used in cold running engines and cold plugs are used in higher performance running engines.


The differences in heat ratings offer distinct advantages in terms of anti-fouling and lead scavenging. Generally speaking, the plug needs to be hot enough to burn off carbon deposits from the tip, but not so hot that the plug tip retains enough heat to initiate pre-ignition. If the plug is too cold, carbon deposits will not be burned off the tip. If the plug is too hot, lead salts that accumulate on the insulator nose will cause a lead fouling.

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