Editing Spark plug (section)

====Metal case/shell====
The metal case/shell (or the ''jacket'', as many people call it) of the spark plug withstands the torque of tightening the plug, serves to remove heat from the insulator and pass it on to the cylinder head, and acts as the ground for the sparks passing through the central electrode to the side electrode. Spark plug threads are cold rolled to prevent thermal cycle fatigue.  It's important to install spark plugs with the correct "reach," or thread length.  Spark plugs can vary in reach from {{convert|0.0375|to|1.043|in|cm|abbr=on|order=flip}}, such for automotive and small engine applications.<ref>For examples, see the listing in the plug type chart of the 2015 Champion master spark plug application catalog, pp. VI</ref> Also, a marine spark plug's shell is double-dipped, zinc-chromate coated metal.<ref>{{cite web|title=Marine Spark Plug Savvy|url=http://marineenginedigest.com/diy/sparkplugsavvy.htm|publisher=MarineEngineDigest.com|date=29 April 2012|access-date=1 December 2012}}</ref>