I have had this same problem with the plastic bobbins that came with my Pfaff 4.2 Quilt Expression. Two broke while I was winding the bobbin. The whole side just popped off the first time. I heard it crack and stopped winding the second one. I have never been happy with the bobbin winding on this machine. The lever that is supposed to "feel" that the bobbin is full has far too much play in it. It wiggles back and forth. No amount of tightening works. The shop where I purchased it has been no help. They must say to keep retightening. I just keep an eye on the bobbin when winding and stop when it seems full enough.