This is what it looks like in action.  You lock the cutter to the bolt with a jam nut.  Use copious amounts of mineral spirits as a cutting lubricant to keep the cutter blades from loading up with aluminum.  Remove the cutter frequently to clean the aluminum chips out of the blades.  In a few minutes you will have a nice, flat spot face.

This sucker takes a lickin and keeps on tickin.  You can rest assured in the knowledge that your head gasket will get pinched adequately to keep the oil where it belongs.

Another reason is that the 12mm threads in the aluminum cylinder head are giving up.  How do I know that?  I keep finding my studs/inserts proud.

This is my Stage 3 head after a little over 6500 miles and several nut tightenings.  The tool bit catches on the edge of the stud.  It was well below the sealing surface when I installed the head.  So, the stud insert now interferes with proper head gasket crush.  No good.

You can see how the insert is bearing hard on the head gasket.

What’s goin on?

IMO, another case of fatigue.  The condition is aggravated by removing the stud to resurface the head.  The stud, by design, has a class 5 interference fit into the aluminum head.  Remove the stud to resurface the head and you end up with a class 3 or 4 fit.  Not as robust.  Now increase the cyclic stress by bumping up the compression 2 points from 8.5:1 to 10.5:1.  The horsepower goes way up and so does the break mean effective pressure.  The head gets subjected to a lot more force.  Now open up that exhaust port and decrease the structural integrity of the head.  Takes a lot less force to bend it.  Throw in a pile of red-hot exhaust gas, get the whole shebang cycling about 2000 to 3000 times per minute, and guess what gives up.

All is not lost, hope springs eternal.  Maybe the thing can be salvaged, possibly even improved.

We start with our trusty valve seat cutting tool.  Let’s make a counterbore inside the exhaust port and install an insert the other way, with the shoulder on top.  So now, the shoulder does all the work, not the threads.

Take the old stud out.  That can be hard.  I had to drill mine out.  Don’t exceed a 27/64” drill and make sure you are perfectly centered.  Then clean up the threads with a M12-1.25 tap.  Not too bad.  Should work.

Install the guide in the stud hole.  Use washers/spacers as shown to set the depth.  You want a nylon or fiber washer in contact with the head surface to prevent damage to the sealing area.

You can drive the cutter assembly with a cordless hand drill set to slow speed.  You turn it in reverse (CCW).  Again, use copious amounts of mineral spirits to prevent the aluminum from loading up the cutter, and clean the cutter blades frequently.  Only takes a few minutes to do the job, even with the interruptions for cleaning.  You end up with this.  Nice.