if you are to check an iridum, you should very very veyr very very carefully use a wire gauge, not a stepped gauge, and carefully check the opening and im saying use a table and chair and to carefully. you can be the one regretting shoving it into a step gauge or you can trust ngk to produce to the spec and then trust ac delco to do remotely any QC and then all the hands it passed through till you cheaper than ruining 6 plugs and buying more lol.
Not comforting, correct, but be careful is the key word