A spotfacer is required for C Series pins installed into through holes.  Spotfacers cut a recess into the surface of the casting to accept part of the shoulder of the pins.  Some of the shoulder must remain below the surface after it is ground or machined flat.  The shoulder is what holds the Spiralhook™ threads tight.  Spotfacers cut an angle into the surface that matches the negative angle under the shoulder of the corresponding pin.  This creates additional holding power.  Approximately 50% of the shoulder should be below the surface whenever possible.  However, in very thin material keep the spotface shallow to leave as much room for threads as possible.  Approximately 0.020" should serve as a minimum for spotface depth.  If an invisible repair is not an issue, you can leave approximately 50% of the shoulder protruding above the surface of the completed repair for additional strength.  Use a slow speed drill with all spotfacers to improve life of the cutting edges.  (Note:  C Series pins can be installed into blind holes without embedding the shoulder into the surface.  When the pin hits the bottom of a blind hole, the threads are locked into place.)