Google: Spark Plug Reach"Be sure to get a clear understanding of what each plug manufacturer means by the tern "spark plug reach". Definition seems to vary.
For instance, NGK GR4 2635 plug has a 3/4 in. reach. On this plug the 3/4 in. reach is measured from the
surface of the body of the plug where the gasket sits
to the bottom of the metal housing encapsulating the ceramic tip.
https://www.jegs.com/i/NGK-Spark-Plugs/739/GR4/10002/-1
It's shown on the NGK Long Reach Plug in this link.
http://www.ngk-sparkplugs.jp/english...q13/index.html