Editing Spark plug (section)

===Sealing to the cylinder head===
[[File:Spark plugs old and replacement 7058.JPG|thumb|Old spark plug removed from a car, new one ready to install.]]
Most spark plugs seal to the cylinder head with a single-use hollow or folded metal washer which is crushed slightly between the flat surface of the head and that of the plug, just above the threads. Some spark plugs have a tapered seat that uses no washer. The torque for installing these plugs is supposed to be lower than a washer-sealed plug.<ref>For example, notice the torque recommendations on p. 823 of the 2015 Champion Master Spark Plug Application Catalog.</ref>  Spark plugs with tapered seats should never be installed in vehicles with heads requiring washers, and vice versa.  Otherwise, a poor seal or incorrect reach would result because of the threads not properly seating in the heads.