I used to be a boat mechanic, so believe me when I tell you I have extracted
a lot of busted bolts. I have a matco
DEK10 extractor set with the left handed drill bits. It has served me well.
Pro tip:
Hands down, the best oil I have ever used for removing busted bolts is Rapid-Tap
If it's in a place that you can heat it up with a torch, heat the busted bolt until it's cherry red.
start dropping rapid tap on the busted bolt
At first it will smoke, then when it is the correct temp something magic will happen
it will start sucking down into the threads. Just keep dripping it on 'till it stops bubbling
Let it cool to room temp
If possible, grind, sand or file the busted shank flat
use a center punch to dimple the center of the bolt before you drill.
60% of the time, it works every time.
-Russ