Editing Spark plug (section)

===Spark plug gap===

[[File:Spark plug gauge.jpg|right|thumb|''Gap gauge:'' A disk with a tapered edge; the edge is thicker going counter-clockwise, and a spark plug will be hooked along the edge to check the gap.]]

Spark plugs are typically designed to have a spark gap which can be adjusted by the technician installing the spark plug, by bending the ground electrode slightly. The same plug may be specified for several different engines, requiring a different gap for each. Spark plugs in automobiles generally have a gap between {{convert|0.6|and|1.8|mm|in|abbr=on}}. The gap may require adjustment from the out-of-the-box gap.

A ''spark plug gap [[Gauge (engineering)|gauge]]'' is a disc with a sloping edge, or with round wires of precise diameters, and is used to measure the gap. Use of a [[feeler gauge]] with flat blades instead of round wires, as is used on [[distributor]] points or [[poppet valve|valve]] lash, will give erroneous results, due to the shape of spark plug electrodes.{{citation needed|date=May 2012}} The simplest gauges are a collection of keys of various thicknesses which match the desired gaps and the gap is adjusted until the key fits snugly. With current engine technology, universally incorporating solid state ignition systems and computerized [[fuel injection]], the gaps used are larger on average than in the era of [[carburetor]]s and breaker point distributors, to the extent that spark plug gauges from that era cannot always measure the required gaps of current cars.{{citation needed|date=April 2012}}<ref>For example, in the 1967 Champion spark plug catalog, the "Deluxe Gap Tool & Gauges" on p. 38 is designed to handle gaps from {{convert|0.015|to|0.040|in|mm|abbr=on|order=flip}}, which is less than the gap required by many modern cars.  As for older cars before c. 1960, notice the vintage vehicle section of the 1997 AC Delco Spark Plug Catalog, page 250 to 264.  Gaps in the 1920s for many makes were often {{convert|0.025|in|mm|abbr=on|order=flip}}.  However, many modern cars have gaps not much larger, such as those made by Volvo from 1967 to 2014 normally had gaps of {{convert|0.028|to|0.030|in|mm|abbr=on|order=flip}}.  See this make's listings in the 2015 Champion Master Spark Plug Application Catalog, pp. 333 to 339, for which the only exception were some 4.4 liter engines.</ref>  Vehicles using compressed natural gas generally require narrower gaps than vehicles using gasoline.<ref>For example, the Ford Crown Victoria's 4.6 liter engine required a {{convert|0.044|in|mm|abbr=on|order=flip}} gap when using CNG, but requires a {{convert|0.054|in|mm|abbr=on|order=flip}} gap when using gas.  See the 2015 Champion Master Spark Plug Application Catalog, p. 124; a technical explanation is found on p. 825.</ref>

The gap adjustment (also called "spark plug gapping") can be crucial to proper engine operation. A narrow gap may give too small and weak a spark to effectively ignite the fuel-air mixture, but the plug will almost always fire on each cycle. A gap that is too wide might prevent a spark from firing at all or may misfire at high speeds, but will usually have a spark that is strong for a clean burn. A spark which intermittently fails to ignite the fuel-air mixture may not be noticeable directly, but will show up as a reduction in the engine's power and [[fuel efficiency]]. Gap adjustment is not recommended for iridium and platinum spark plugs, because there is a risk of damaging a metal disk welded to the electrode.<ref>{{Cite web |date=2022-01-27 |title=How to Choose Proper Spark Plugs for Your Engine |url=https://vinsonar.com/help/spark-plug-guide/ |access-date=2022-05-12 |website=VIN Sonar {{!}} help - Automotive Guides and Online Tools |language=en-US}}</ref>