The adjustment I make is to bend the "knuckle" on the arms of my bobbins outward so the "buttons" that go into the ends of the spool are perfectly parallel. This allows the thread spool to turn easier, keeps it from popping out, plus it fits in my hand better.
I do this bending while I hold the arms together up near the tube. This is so I don't inadvertently spread the arms too far apart and break the bobbin.
Once I get the buttons parallel, I'll pop in a spool to check the fit and if I need more tension, I'll squeeze the arms IN above the knuckle with a spool in place or the fingers of my other hand holding the buttons apart.
When I'm done, the tension is perfect for any size thread I use and if I want more, I just squeeze in on the bobbin arms.
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