Well, so much for the MAPP gas idea. I had this lower plenum with a twisted off thermostat housing bolt, so thought, in light of the above discussion, that I'd prove that I could get it out easily. Here's the twisted-off bolt before my attempts:
And, after a round of MAPP gas and Vise Grips well and truly applied the only difference was that the paint was gone.
So, out came the welder and I welded the nut to the stump of the bolt. Lots of penetration and heat, and the nut glowed red for quite a while when I was done. And it looked like this:
After it cooled and I'd flooded it with penetrating oil and coaxed it with lots of tapping, I gently started tapping on a wrench with my hand. Pretty soon I started getting movement and thought I was "there", only to learn that the stump was now 1 turn shorter.
So I did it again, this time ensuring that the weld was as hot as I could make it to both get as much penetration as thermal shock on the threads. And, now I have two nuts with about 1 turn of the bolt in them:
So, now there's only enough thread showing to fill a nut, and I have Loctite Red in there and will let it sit overnight before seeing if that will work.
Gary, AKA "Gary fellow":
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Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI