Size M bobbins are definitely Not all the same, although I *think* they are the same size. Some are steel, some are aluminum; some have holes in the sides, some don't; some have knurling to help the thread grip, some don't. Some longarms are happy with just about any bobbin, and others are fussier. If your machine is happy with the style bobbins you now have, it would probably be better to continue with that style than to try out something different.
Last edited by QuiltnNan; 01-13-2018 at 09:17 AM.
Reason: remove shouting/all caps