I sometimes find that if i change spools of thread, the thread will break. But i don't think it's a thread problem for me, i just think each spool of thread sometimes effects your tension just slightly differently. This is how you make sure your tension is right. Hold a threaded bobbin case in the air, grab the thread and let go of the case, you should be able to jiggle the thread up and down, and the thread should unwind from the case with each jiggle, it should neither just run free, or be frozen, and that is what the little screw on the bobbin case is for. I check my bobbin tension with each new bobbin i put in, it only takes seconds, and you just use your thumbnail to turn the screw a tiny bit either way, until the bobbin case drops easily, but not free-run, with each jiggle of the thread. Ok, once thats done, if your thread is still breaking, what is happening when your thread breaks? If my upper thread breaks, and it is just a quick loud snap, then the upper tension is just a pinch too tight, if the thread breaks, but you also notice that the spool of thread made a spinning noise when it broke, and the spool of thread is kind of spun in circles and un-spun some thread, then the tension is just a pinch too loose. I could never figure out tension for years lol, but have just figured it out last few months, and makes a huge difference. As for thread, my machine is a 40 year old kenmore, and the thread it prefers the best is uhmmm, lol, yep, wal-mart 3 for a buck thread. I know most people cringe at this, but this is what works for me :)