Loops underneath are almost always a top thread fault, not a bobbin fault. The instinct is to blame the bobbin because that is where you can see the mess, but the loops are excess top thread being pulled through without resistance.
The cause is nearly always that the top thread is not seated between the tension discs. Those discs only open when the presser foot is raised — thread the machine with the foot down and the thread lies outside them, and the machine sews with effectively no top tension at all.
What to do, in order. Raise the presser foot. Remove the thread completely and re-thread from the spool, making sure it drops into the tension channel rather than skating across the top of it. Check the take-up lever is at its highest point. Sew a test seam.
If that does not fix it, change the needle — a bent needle deflects and misses the hook, which produces a similar mess. Then check the bobbin is the right class for your machine and wound evenly.
Only after all of that should you touch a tension dial, and then in half-numbers with a test seam between each change. The bobbin case screw is the last resort, not the first, and turning it is how a small problem becomes a workshop visit.
Questions people actually ask
Is it the bobbin tension?
Very rarely. Bobbin tension is factory-set and drifts slowly if at all. Loops underneath mean the top thread had no resistance, which points at threading, not at the bobbin.
I re-threaded and it still loops.
Change the needle next, then check the bobbin class. A class 15 bobbin in a 15J machine sits at the wrong height and produces exactly this fault, and no tension adjustment will cure it.
Last reviewed 2026-08-11