I use 50wt Aurifil which is a 2 ply, so thinner than most 50wt, but not quite as thin as a 60wt. I have used Mettler 60wt as well ... in fact just picked up 4 large cones of Bottom Line 60wt in my "piecing colors" but I haven't put them on the machine yet. I got them at a going out of business sale and couldn't resist the price of $4.00/ea for large cones.