Well......First time in my life I have ever seen such a thing!
A design flaw by Honda in my humble opinion. The 4 studs between 1-2 and 3-4 have openings in the head that allow water/debris to collect at the cylinder level with no way to drain or flush. After 34 years, the cylinder can become bonded, like ONE, to those 4 studs. This particular one sat in Aero Kroil for over 3 months. It would not budge. I was able to remove all the other studs - no problem. I tapped 1/2-20 threads in all the surrounding cylinder holes. Then put set screws in the case threads. Started cranking down on the 1/2-20's till it let go and boy did it! Hard to imagine just how much pressure was applied but it was obviously a lot.
Machine tool all OK. I was using a tooling plate made for SOHC cases to hold it. Rather expensive workbench but solid. But the machine tool is the answer for the next one that's stuck. Need to saw those 4 studs off close to the head and machine them out completely. Fixing to
break tear disassemble another one today and see what we get. The center 2 studs between 2 and 3 had no support under them and I am somewhat concerned that the cases on this one could have been cracked/damaged in the process.
You should see the contraption the guys down in NZ use for this problem. However, I seriously doubt it would have worked on this one.