A friend came over with the cylinder head from a 1983 Honda Goldwing GL1100 Interstate that had a busted valve cover bolt. Used my micro mill and a left-hand drill bit to eventually get it out. Flattened the top of the busted fastener with a carbide endmill, then center-drilled a divot so the left-hand 3/16″ drill wouldn’t wander. A few moments later and the broken bolt spun out of the hole. Success!