If there is a threaded metal collar pressed into the plastic tank, and the bolt threads into this collar, then the collar is probably freely spinning (it shouldn't, it should firmly be held in place by the plastic), and this is likely the reason you can't unscrew the bolt. I don't know for sure if this is the case here, but I have seen problems with this type of design elsewhere.
If this is the case, then sawing off the head of the bold will allow you to remove the tank, but you will still need to get a new tank, as gasaholic had to do.
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Last edited by 3th3r; 03-12-2013 at 02:18 AM.