spark plugs

Here is an extensive thread that discusses the use of 18mm plugs in EXPERIMENTAL AIRCRAFT.

vansairforce.net/threads/why-dont-we-use...lug-adapters.229170/

Remember the use of non approved plugs in certified aircraft isn’t necessarily safe or legal.

These are the top plugs I use and have on ALL my amateur built, Lycomings as well as Continental.I have never had an issue.  Please be aware that I am in Canada and our rules are different