OK you  convinced me.  In light of what you just said what merit would
smaller  plugs have?
I was only interested in them because they would fit better in  a dual
plug insert.
Obviously a smaller plug means a smaller cavity in  the area near the
inner liner.  Is that such a
big deal.  For  instance when the leading apex seal crosses that hole is
there a transfer  of gases   from the presiding exhaust cycle?

Doug in  Japan



Doug,
The smaller cavity does both. It marginally improves compression and the
smaller opening has less gasses bypassing through it. A smaller plug gets the
actual spark closer to the combustion chamber as well.
Bill Jepson