Originally Posted by
Gresp15C
+1 for gaining access from the inside if possible and screwing it outward. Can you temporarily remove the lever?
+1 If you decide to drill it out, drilling from the lever side will work with a regular right-hand drill bit. Start with a smallish bit and gradually go bigger; it will likely unscrew by itself as you progress.
If, as suggested, the hole ID has been crushed undersize in a spill, you may need to rethread it to get a new adjuster to thread in.