Big mistake, drilling out broken manifold bolt on exhaust and drilled to far into water jacket. How do I fix this

Say you have snapped the head off a rusty bolt on the car, its infuriating!

But, there is a fantastic tip from a mechanical engineer that works every time, and is easy to do!

Lets assume the bolt is a 10mm exhaust stud or similar, take a 4mm drill bit (sharp one) and a 4mm hex (allen) key, (especially a t-bar type or socket mounted one) - drill a pilot hole right down into the bolt with the 4mm drill, as far as you can without snapping the drill bit in the bolt!!! Use light pressure, a sharp drill bit and let the drill do the work, and keep it straight....then when you pull the drill out, tap the hex key into the hole you made with a hammer, get it in as far as you can, it will be a tight fit as the 4mm hex key measures 4mm from flat to flat across it, and the corners of each flat are slightly more than that so it bites very well into the bolt, spray plenty of release fluid or wd40 around the exposed bolt thread to help loosen it, leave for 10 minutes if rusty, then turn the hex key with either a socket and wrench or the t-handle on it, and it will turn the bolt without slipping!

And hex keys are VERY VERY strong, much stronger than a bolt as they are tempered high tensile steel - the bolt will unscrew with ease! Forget useless screw and bolt extractors! And simply pick a drill size that gives a decent hole leaving some metal at the sides of the bolt but not too small a size, and keep the hex key to the same size in mm as the drill you use, if you made a mistake just go up a size or 2 on both drill bit and hex key and you get another try, but should not need it!