Well, they're inexpensive to purchase anyway, but you generally don't have to replace them unless they are scored, and that doesn't happen unless you have put a needle in improperly several times, and it marked the case. Before replacing it, I would test the bobbin by rolling it down a flat surface to see if it wobbles; if it is, it's warped. I would also try another bobbin because it's easy to get an incorrectly wound bobbin, which won't allow the thread to unwind smoothly. Take the bobbin and case out of the machine as a unit, making sure that the thread is where it usually is while sewing, and pull the thread. There should be SOME resistance, but not a lot. It could also actually be your TOP tension that is too tight. In your manual, there are instructions and drawings to show you what your stitches should look like, and how to take corrective action if they are not. While you CAN adjust the bobbin tension, if it doesn't need it you can really get yourself into trouble, and they generally don't--thread resistance is generally caused by other factors.
on Jan 22, 2014