I have used YLI nylon filament thread (very fine, .003mm I think) in the bobbin of my Bernina with no problems. I do wind the bobbins slowly, with little tension, so it doesn't stretch. It is also advisable to use metal bobbins with it; plastic bobbins can distort. You never want to overfill a bobbin with this thread either; it's best to slightly underfill.