I use the thinnest thread I can find for the bobbin. Either a 70wt or 80 wt. Aurifil has a good 80 wt. on large cones. Connecting Threads 70wt is good too. There are many brands of thin thread for bobbins. Poly is the most used because it's strong for thin thread. For the top I use 50 wt 2 ply or 60 wt. The right size needle is very important. 90/14 needle is too big for 50wt 2 ply but fine for 50wt 3 ply. You can't just to buy a spool of thread anymore. They are all different!