The retaining clip (snap iron pieces) is included to eliminate the possibility of the screw tap from falling into the cylinder if you tap too far! The roll pin in the twist drill/screw tap is the only stop needed for it.
38900 Spark Plug Repair Kit has 8 solid bushing type inserts. The inserts lip design prevents them from going too far and falling into the cylinder.
Its recommended to use a video inspection camera to verify all metal shavings are out of the cylinder before installing the spark plug.