This is my first post here, and I just wanted to share how I've solved a problem with the thread jamming up on the bobbin case in a Singer 15-91 (actually, it's a 15-125, but the internal mechanics are identical). I don't have a sewing blog or any other outlet to post this, and since a lot of the information I got from the internet was on this board, I thought I'd post it here to share what I discovered, in case anyone else has a similar problem.
I bought my lovely green 15-125 at a thrift store recently, and after a good oiling and greasing, it was running great. The only problem was that it didn't come with a bobbin case. I quickly discovered that the class 15 cases they sell at JoAnn Fabric are not compatible (they're almost identical, but the finger points in the opposite direction), so I ordered a replacement off of ebay.
I loaded it up, and it sewed wonderfully - for about a few feet. It made a loud banging sound, and the needle thread was tangled up underneath the fabric. This appeared to be a case of thread nesting, so with a little research, I found Tammi @ ArchaicArcane's excellent posts about thread tensions and sussing out the cause of thread nesting:
https://archaicarcane.com/fly-little...d-nests-part-1
This was incredibly helpful, but none of the 7 reasons she mentioned seemed to be the cause.
Then I found Ed Lamoureux's blog post shuttle cushion spring, which if it's too close, can lead to jamming and tension issues.
http://sewing-machines.blogspot.com/...ger-15-75.html
But alas, there appeared to be plenty of space for the thread to slip through.
I was about to throw in the towel and give up on the machine, but then I found a post on the blog Rittles and Bittles that described the thread jamming on a 15-91 whenever a newer bobbin case was used. They even posted a picture comparing the two:
http://rittlesandbittles.blogspot.co...se-issues.html
Maybe this was it! I looked for original 15-91 bobbin cases on ebay, and while they were much less expensive than the featherweight bobbin cases, they were still more than I'd payed for the sewing machine (which to be fair, was only $20).
Before I ordered one, I looked again at the photo comparing the new and old cases. The curved part of the bobbin case's latch (singer part #125288) stuck out more on the new one. I slowly sewed a test strip, trying to get it to jam, and at the first hint of the machine struggling, I looked underneath and saw the thread neatly caught by that exact part of the latch!
[ATTACH=CONFIG]580822[/ATTACH]
I took the bobbin case apart and tried bending the curved little "finger" down. It was difficult (the metal is thick), and I wasn't sure if I'd made any difference, but when I put it back together & tried it again - no jams! It's been running steady for two days with no problems whatsever.
Here is the bobbin case before I bent it:
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And here it is after:
[ATTACH=CONFIG]580824[/ATTACH]
It's a pretty subtle difference - before I could just barely slide a piece of paper under that curved "finger", and afterwards I cannot.
Obviously, this is only helpful if you have a slightly misshapen newer bobbin case like I did, but I bought two of them off of eBay (long story), and they both had the same problem. I hope this helps someone out there who, like me, was at their wit's end with the thread tangling and jamming.