Hot answers tagged rust-removal
Looks like the threads on the bolt are crushed/damaged, so you're only choice might be to drill it out or cut it off. Put on a pair of good work gloves. Remove the blade from a hack saw. Then work the blade between the nut, and the plastic washer. It will likely be slow, hard work, but eventually you should be able to cut the bolt. Alternatively, you ...
The most expedient and practical solution is to drill out the bolt. Probably access is better from the top through the tank, but it can be done from either side. The goal of "drilling out" is to weaken the shaft of the bolt enough so that either the friction on the nut is reduced to the point where it turns, or the shaft breaks and separates into two ...
I'd use an angle grinder with a thin cutoff wheel and basically just cut either the locking nut or the whole cylinder in half. If you're reasonably careful, you should be able to do this and barely touch the fiberglass (if at all). You're much more likely to cause cracks or other damage if you are putting torque on something, and it's much easier to have ...
Why not leave the lock in place, drill a hole for a new lock and move the locking mechanism a few inches to the right or left?
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