Yeah in my mechanical experience. Seems your really have to get the hardware near cherry red hot. The quicker you can get the heat to part the better to avoid the heat transfer. You want to cause the nut to expand while the threads stay near the same. I've used the little brazing torches don't find them powerful enough. They waste a whole tank of oxygen in about 5-8 mins at full power and at $8 or more a tank it begins to get a little expensive. For example it may be cheaper to just break the bolts and replace them with fresh ones. But, they do work as a home torch setup when you really need a torch without the large space requirements or worries of gas fill facility's. When dealing with smaller hardware or brazing they are perfectly adequate. However, a real gas tank type torch set works best for cutting and heating metal as you have the gas volume to get work done.
The plain MAPP torch set has been something I have used in the past as well I'm poor and want heat. But, as I've said you want something that can heat NOW not 8 minutes of being aimed at the fastener. The Triflame benzomatic torch heads are the industry standard. Using the single flame torch heads like what is generally sold for plumbing you'll find it just won't get hot enough. Mapp is better then propane but not by much.
Been a big fan of Liquid Wrenches new formula when it comes to penetrates. Used to be PBblaster crazy. But, not anymore.
Brute force, cheater pipes, breaker bars, impact guns and the like always seem to be my solutions to the stuck fastener issue.
