I've gone to candle wax. Gentle heating is all you need. You can literally watch it go into the threads.Generic WD40 won't help with a seized bolt or nut. Use a penetrating oil. Liquid Wrench, PB Blaster, Kroil, Knock'er Loose etc..
As stated before turn the nut until the stud backs out of the block. Working as a tech have done it many times and have never broken anything. Just when reinstalling the stud in the block be sure the coil hasn't moved just match the marks in the coil and the stud.Just my 2cents, that stud I believe holds the ignition coil in place, be careful that you don't break the housing on the motor that holds that stud. Best answer is dremel a slot in top of stud and try and break the set on the nut. Otherwise, I'd cut the nut off to preserve the stud mount.
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If you saw the picture you see he is working on a HONDA the starter and fan cover are held on by the three studs. Often for some reason the nuts do not release and turn to stud out. Common problem and happens often. As I stated before keep turning the nut until the stud backs out of the block, then holding the stud remove the nut. As for the stater Honda uses two plastic lugs that are camed ant go out when the stater is pulled. Sometimes are overeager person ca destroy the stater by pulling quickly before the dogs are locked in. You know plastic doesn't take much abuse.Just pull the whole engine shroud off so you can get to the underside and grab the stud. Recoils are repairable if you want to screw with it. After the recoil assy is off take it apart but watch how the spring is wound so you can rewind the spring. If it is not grabbing, cranking the engine, open it up. Their 3 or 4 balls in there that grab the spurs on the crank shaft. They usually get dirty and stuck and don't engage. Clean the inside thoroughly, dry and lube with graphite, and reassemble. Wind up the spring and re-install the rope and spring pulls it in. If it doesn't pull in all the way, start over and wind the spring some more. Oh, put on a new rope while you are at it!