Originally Posted by lacey

Use a masonry drill with a carbide tip
Grind it to the same shape as a std drill
First hole drill smaller than you need and work your way up
To size
Run the drill slow with some oil to stop it burning out
Try and tap the hole to a std size right through
Then put a grub screw in the hole with a shed load of ptfe
( if you helicoil you may have trouble sealing it )
But after you have fitted the grub screw you can helicoil the damaged hole
Ready for the new bolt