...at least in the Singer 6233. My Mom and my friend have this model and both complained that there was something wrong with their machne. It was collecting lots of thread underneath. I said I'd clean and oil them and take a look. Now, I'm no pro at fixing machines. I just love cleaning them and bringing them back to life! Well, Mom had a metal bobbin in hers that sat just a tad too high in the bobbin case. (drop in style) I changed it to a slightly slimmer plastic bobbin and it sews fine now. Tried the metal bobbin again and the problem started again. Back to the class 66 plastic bobbin and no problem. Yesterday my friend asked me to look at hers. It was the same model as Mom's so the bobbin was the first thing I looked at. She had a plastic bobbin but it was just like the metal one (in my Mom's) in that it sat too high in the bobbin case. I changed it to a slimmer plastic bobbin that said Singer right on it. It sewed just fine then. There are differences in bobbins. Some are slimmer than others. This makes a difference. Check out your bobbin when you're having problems where the thread collects under the fabric or the top thread leaves large loops underneath. Hope this keeps someone at home sewing instead of at the repair shop!