the worst cases I have encountered have only succumbed to ;
- pedal spindle in the bench vice
- length of something added to the crank for extra leverage
- multiple hot/cold cycles using blowtorch or heat gun
- using ATF instead of penetrating oil on the hot crank
Most oils become very fluid at high temperatures and penetrate well. Once the part is hot, solvent-based penetrating fluids just boil and are of little value.
ATF starts to burn (char) at slightly over 200C which means that you ought to see if you overheat the crank or not (the metal will quickly change irrevocably at higher temperatures than this). At lower temperatures the water tied up in the oxide will bubble out through the oil, and you know you are doing some good. Alternating hold/cold cycles cause (net) the oxide in the thread to lose water and gain oil.
Even with the best and most thorough treatment there is always a fair chance that when the crank starts to move, it will have torn the threads in part of the threaded length. When this happens the threads in the crank will be damaged, depending on where the tear starts. If it is near the back of the crank then the crank thread may be totally destroyed and a helicoil is necessary. If it is near the front of the crank then you may lose one or two threads only in which case the crank is still usable.
IME the threads are most likely to bind near the front of the crank especially when pedal spindles without a full shoulder are used (these let the grot into the threads and help the grease/copper-ease to be washed out). The undercut in the spindle threads can either harbour copper-ease or corrosive grot; the choice is yours. However during removal the damage to the crank is severe should the threads be torn starting from the back, so it makes most sense to have the pedal spindle in the bench vice so that the back of the crank is uppermost, and to introduce penetrant from there, heat/cold cycle assisted.
When you have got the pedal out, the spindle threads may be full of aluminium etc. Under no circumstances should you screw such pedals back into a nice set of cranks; you will wreck the crank threads. The spindle ought to be hard, too hard to address with a file (if you want the file not to go blunt very quickly). You can appear to clean the threads out with a wire brush, but IME the only 100% safe and effective method of cleaning the threads is to scrape the grot out using a pointed tool. I usually employ the point of a Stanley knife blade, and examine the result using about x10 magnification. If you want to 'test' a pedal spindle then there is something to be said for screwing it into an old steel crank.
IME if you adopt the above approach then the pedal spindle can always be saved (*), and the crank (if the seizure was bad enough to start with) is more likely than not to be damaged. However it is never so badly damaged that it cannot be repaired using a thread insert.
(*) the exception to this may be stupid pedals with allen-key-only spindle fittings; 6mm sockets round out too easily, and 8mm sockets tend to split lengthwise when high torque is used.
hth
cheers
