Check to see if you don't have any overlapping surfaces, or 'non-manifold' surfaces. I think stl-check in max can check for things like that, but I'm not certain. In any event, it looks like you may have doubled geometry somewhere, or at least an edge with more than two faces.
edit: and if you used a bolean operation to create that collar, you should always keep in mind that more often than not, there will be cleanup to do.
Thanks, STL Check seemed to have picked up a lot if not all the problem points. I'll update when I've fixed everything it highlighted and see if that solved the problem
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edit: and if you used a bolean operation to create that collar, you should always keep in mind that more often than not, there will be cleanup to do.
I think u may find whats needed there.