Editing Spark plug (section)

====Insulator tip====
[[File:Candela rotta.jpg|thumb|right| Two spark plugs in comparison views in multiple angles, one of which is consumed regularly, while the other has the insulating ceramic broken and the central electrode shortened, due to manufacturing defects and / or temperature swing]]
On modern (post 1930s) spark plugs, the tip of the insulator protruding into the combustion chamber is the same sintered aluminium oxide (alumina) [[ceramic]] as the upper portion, merely unglazed. It is designed to withstand {{convert|650|°C|-1|abbr=on}} and 60&nbsp;kV.

Older spark plugs, particularly in aircraft, used an insulator made of stacked layers of [[mica]], compressed by tension in the centre electrode.

With the development of [[Tetraethyllead|leaded petrol]] in the 1930s, lead deposits on the mica became a problem and reduced the interval between needing to clean the spark plug. Sintered alumina was developed by [[Siemens]] in Germany to counteract this.<ref name="Banks" >
{{cite book
  |title=I Kept No Diary
  |author=Air Commodore F. R. Banks
  |year=1978
  |page=113
  |publisher=Airlife
  |isbn=0-9504543-9-7
}}</ref>  Sintered alumina is a superior material to mica or porcelain because it is a relatively good thermal conductor for a ceramic, it maintains good mechanical strength and (thermal) shock resistance at higher temperatures, and this ability to run hot allows it to be run at "self cleaning" temperatures without rapid degradation.  It also allows a simple single piece construction at low cost but high mechanical reliability. The dimensions of the insulator and the metal conductor core determine the [[#Heat range|heat range]] of the plug. Short insulators are usually "cooler" plugs, while "hotter" plugs are made with a lengthened path to the metal body, though this also depends on the thermally conductive metal core.