Ha! ... about the only problem I have with my 7518 ... and one I have done nothing about either.
The problem is the heat buildup during use causes the metal to expand, tightening the nut beyond human ability to un-tighten without pliers ... something I keep handy for just that.
There is a solution (which I haven't bothered with because I tend to forget the frustration between uses) that involves using a bicycle seat clamp to replace the current locking assembly. You can find more about it at:
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luck ... and let us know if you do the fix, and what parts you needed. A regular here has a picture on their website, but can't remember who it is ... hopefully they, or someone with a better memory, will chime in.