Just go with what your machine calls for. I have 2 Singers. I have bobbin case for each machine and each case is labeled. My needles are in separate cases for each machine. I have 16 bobbins that work with my 401 because they were made for my 401. I have that many bobbins for my 2662 because they were made for it. So far I've not had any problems with bobbins that weren't my fault.