Heating
When heating and removing bolts depending on the size of them determines pretty much the measures you use at removing the bolt. You need to know a few things about the bolting.
What are the materials the bolt is in? Is the bolt held in with a heli-coil? Size and material of the bolt? What application is the bolt used for lube oil system, or water coolant system? Is it a hex head, or an allan head?

Removal of allan head bolts have some advantages over hex head bolts. Firstly, if the allan head starts to strip it can have a small peice of allan stock welded into the allan socket. This ensures you can use impact tooling effectivly on the bolt. Depending on the thread diamater of the bolt will determine the size of the impact used on the bolt. The thread diamater of of hex bolting can be determined because the head of the bolt for "standard sizes" is exactly one and a half the thread diameter size. A simple example can be seen in a hex bolt that accepts a 3/4" socket it will have a thread diamater of 1/2".

Knowing the materials of the bolting and what it is bolted to are crucial if you decide to go with heat. Reason being is dissimiliar metals have differing properties of heat transferance and disipation. If the bolt is in an aluminum block I would suggest you soak it in penetrant for several hours if not days. I do not suggest using that PB junk others have suggested I dont know how well it works but can tell you that KROIL is simply amazing. w w w.kanolabs.c om/google/ .. I use this all the time when working it does what it says and saves allot of knuckles from being skinned from splipping on a wrench under full load. If you do use heat on a mild or hardened steel bolt after it has been soaked in penetrant you have a limmited window of opportunity to remove it since the aluminum will absorb the heat and be the same heat as the bolt rather quickly. I suggest that you get the head of the center of the bolt cherry red and soon as it goes black from being cherry then you work on getting it out I would suggest using impact tooling and heat both together. If you do not have an impact gun you can use a slugging wrench as suplement if you have room to swing a hammer and strike the wrench.

If you start moving the bolt shortly after heating it I suggest you work quickly "not carelessly" on removing the bolt reason being is once the bolt and the block are at the same thermal growth the bolt may just simply stop moving from being extracted. If the extraction of the bolt simply stops halfway out and wont budge anymore a few posibilities may have occured the bolt may have developed a gaul or simply the heat transferance and expansion of the materials are the same. Either case try a little more force if it dont budge any longer then call it a day soak it again and then resume heating the bolt again once both materials are cooled.

Broken bolting
Many times broken bolts if ground flush can be extracted with a small hammer and a prick punch. If the shaft of the bolt is not corroded and used on say a lubrication system once you start tapping on the bolt it will work its way out. You would do this with a small ball peen hammer and the prick punch set at roughly a 45degree angle. You may have to make a few stake marks and keep changing position in the bolt shaft so you have the best swing and still be comfortable. Again penetrant and using finesse will get these out.

Broken bolting that is corroded can be removed how some have suggested by welding a head of a bolt onto the broken shaft. Only problem that may happen with disimiliar metal properties would be potential heat transfer and expansion growths. If this method does not work after you let the bolt and block cool you should consider grinding the bolt flat to the block or structure and use a various amount of cutting tools.

FME
It is important that if you do resort to using cutting, grinding or drilling tooling that you practice good FME=Foreign Material Exclusion. You must cover any ports, holes or crevices with barriers. Your barriers can be sponges with lanyards attatched to plug holes. Taped off poly plastic and other various plugs. I suggest you keep an good FME log sign everything in and at the end sign everything out. Each barrier should be numbered and logged this will prevent you from leaving something where it shouldnt. Each tool should be logged in this will keep you from losing something where it doesnt belong.


Grinding and Cutting Tools.
So you have a broken bolt cant weld anything to it and it is flush to the surface of the block. No worries if you have the tools to do the job. Some people prefer drilling holes into bolts with a mag base drill and then using easy outs to extract the broken bolt. There are advantages and disadvantages to this. The advantage is if the bolt is not overly hard to turn you can farily easily use an easy out. If it is BALLS TIGHT then you risk the chance of breaking a hardened steel easy out inside an equally messed up bolt. If you do break the easy-out inside the bolt then you need to extract the peice by drilling or probing it with a dentist pick.

If you know the bolt is BALLS TIGHT then there are other options you can use. You can use left hand drill bits by using a centre punch you need to get dead centre of the bolt shaft from there you need to gradually increas the size of the drill bit. Eventually with a left hand drill bit the bolt will just walk out of the hole. If this does not happen then eventually you will wear the bolt so thin you can collapse it in the hole using a prick punch and a ballpeen hammer.

The other option you can use is drill the center of the the bolt and then use a die or pencil grinder and slowly move circles inside the shaft of the bolt. The end result will be you caving the bolt in with a hammer and a prick punch.

When using your drilling and cutting tools I would suggest for FME purposes barrier off the area below where you are cutting and or drilling have a helper use a shop vac to suck up any shavings of cuttings as you work. put a high strength magnet near the bolt being worked on so to catch any shavings. Use a small pen magnet to remove any shavings or chips as your working I would suggest doing that periodically.

Heli-Coils.

The removal of bolting from heli-coils are pretty much like removing any other bolting. The difficult thing about heli-coils are that they tend to make extracting corroded bolting a bit more difficult. For the most part if you use heat get the head of the bolt cherry red and then soon as the red turns black perform your impact extraction. If the heli-coil does start backing its way out with the bolt do not panick many times a heli-coil will back out slightly but not all the way. If a heli-Coil extracts itself less than a thread do not try pushing or putting the heli-coil back in the hole. Instead you can use a grinder with a wafer wheel and cut off the peice of the heli-coil protruding out of the flush hole. If the heli-coil does extract out of the hole more than a few threads you should consider a few optional guidlines. Does the heli-coil have one and a half 1+1/2 the thread diamater inside the hole. If this is true then cut off the portion that has backed out and you should be able to safely re-use the heli-coil. If it does not then you should find a replacement for the heli-coil.