I pulled a XR600 apart yesterday and one of the head/cylinder bolts woudn't come out. Oh-well. I had to get it apart. I just snapped it off.
Took it to Engine Dynamics in Petaluma and he drilled it out and Helicoiled it WHILE I WAITED. Then we measured it and found it out of spec so I left it for an over bore piston. Should be done in about a week.