I use /10/60 sharp if I am doing monofilament, my 50wt gutterman I use my 11/75 to piece. The rayons I think I used a 14/90 sharp. If thread is breaking your needles are too small. I have a brother and don't have a problem with needles coming out. Are you tightening the bolt down enough by hand. Your machine should be able to use all size sewing needles. I have used 10-16 with no problem. I think smetz sells a chart for a couple bucks that explains needle/thread sizes