Serger thread is 50 wt., just like Mettler thread. I can only use 50 wt. cones and was told that if I want colored thread to try Serger thread. Not a thing wrong with using Serger thread! My Bernina loves it!! Gutermann is 40 wt and my machine refuses to sew with it. Am going to the dealer and finding out why. I paid enough that it should sew with 40 wt and should sew with spools, not just cones. Amy Gibson, famous for her sewing, uses 50 wt. Presencia thread, like I use now. Don't let anyone tell you it's not strong enough. It is! If they buy spools of Mettler, they are buying 50 wt.