This is very interesting. Three different engines running at a "good" temp and the all of them deformed the glow plug coil.
As said before the first problem is that Piccos do not use an OS or Nova plug. Put a Picco plug on the EMX and one problem is solved.
What about the other two? Two engines with lower head clearance? Too lean or too rich setting with 230F?
Fuel? I think that is a good question.