Something was keeping the thread from releasing from the tip of the hook way back at the Bobbin Case Cushion Spring. When it doesn’t release, it’s got to pile up near the Lint Pin area, since that’s where it contacts the feed dog assembly and can’t go any further.
One thing to watch for when thread breakage happens, is that there is not a loose thread either floating around in the hook, or hanging from the bottom of the fabric being sewn. If the hook can catch a thread from anywhere else along with the thread loop from the needle, it will act the same as the Bobbin Case Cushion Spring clearances being out of adjustment. If the needle hits a loose thread attached to the underside of the fabric on its way down, pushes it on down through the needle plate, and the hook catches it when it comes around, you can end up with a thread jam and broken upper thread when the hook takes the slack out of the loose thread.
I’ve been fighting with a Singer 401A, trying to get the clearances set right at the Bobbin Case Cushion Spring. I just can’t seem to find that “sweet spot” that it needs. I nicknamed him “Weenie” because he had a really loud motor whine when I got him, and when I’d increase and decrease the speed, he’d go “Weeee” and then “Neeee”. I’m close, because it will sew daily at the shop for several days before it hangs the thread again. This has been going on now for a very long time. My wife is ready to boot me and Weenie both out of the shop, so I’m going to get one of our spares serviced up to replace him and take him out of the rotation. I guess I’ll put him in my collection and just look at him from time to time. He’s the only one that we have that doesn’t have the bottom notch in the Stitch Length Plate, so he’s a little unusual in that respect. I hate to give up, but I’m about to wear my feeler gauge out on him too......
CD in Oklahoma