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Mismatched swingarm spool thread pitch? too. You might have 1.25 or 1.5 pitch threads (or both, if you're really lucky). You can buy pitch measuring tools, or just try M10-1.25 and M10-1.5 bolts and see which one works.
You can use a tap to clean the threads, but it shouldn't require any effort if the tap matches the threads. If it's hard to turn, you're using the wrong tap and starting to cut new threads through the existing ones.