As I understand it the FM bobbin is set at a lower tension or something. You can tell how non-technical I am. In UK it is about £25 (approx $40) but I bet it will be a lot less in US. The whole set will set me back £60 so will get the bobbin and see how it works with the foot that comes with my 4900. I love hand quilting, but it is taking its toll on my wrists and finger tips . Also it takes forever, and I haven't got forever. By the way my new machine arrived this morning, so I am now going to blitz the housework before I open it, or nothing will ever get done.