I know for a fact that if you put a Brother bobbin in your 6600 you will suffer greatly! I was at a class and my bobbin ran out. To save time, my friend loaned me a full bobbin. (She had a Brother machine.) Well, my machine started to have breaking thread, bird's nests, and finally just kept jamming up. I called my dealer and tried all kinds of things but neglected to mention the bobbin. As a result, I took my machine to the dealer the next day where they quickly discovered the problem. If you put a Janome bobbin next to a Brother bobbin, one bobbin is just a hair shorter. What a headache!