Loops and nests of thread on the underside are almost always the top thread, not the bobbin, even though they appear under the fabric: the top thread is not under tension, so the hook pulls it through and leaves it in loops below. Rethread the top with the presser foot up so the thread seats between the tension discs, start sewing with the foot down and the threads held behind the foot, and nine times out of ten it stops. If it does not, the order of suspects is: the bobbin case and its tension spring, lint between the tension discs or under the needle plate, the wrong bobbin, a damaged needle or hook, and finally the tension assembly itself.
Causes, most likely first
- Top thread not in the tension discs. Rethread with the foot UP (the discs are open only then), pulling the thread firmly into the take-up lever and every guide. Check the spool is feeding freely, not catching on the notch of the spool.
- Starting with the foot up or the threads loose. Lower the foot before the first stitch and hold both thread tails behind the foot for the first three stitches.
- Bobbin case: inserted wrongly, dirty, or damaged. Remove it; brush lint from under the tension spring and inside the hook race; check the spring for nicks with a fingernail; check the bobbin unwinds in the direction the manual shows (clockwise on most front-loaders, counter-clockwise on most drop-ins). A dropped case with a bent spring or a dented edge cannot hold tension.
- Wrong bobbin or badly wound bobbin. Confirm the class, and rewind if the thread is loose, uneven or overfilled.
- Lint under the needle plate. Remove the plate and clean the feed dogs and hook area; a clump of lint under the bobbin case lifts it enough to throw tension.
- Needle: bent, blunt, wrong size for the thread, or not fully up in the clamp. Fit a new needle of the right size.
- Hook: a burr from a needle strike, or a hook out of time after a jam, shreds and loops thread. Run a fingernail round the hook tip; any catch is a burr.
- Tension assembly: discs grooved by years of thread, a weak check spring, or the release pin not resetting when the foot drops. If the stitch cannot be balanced by the dial after everything above, the unit is worn.
The part that fixes it
| What you found | Part | Where |
|---|---|---|
| Bobbin case spring nicked, case dented or latch loose | Bobbin case for your model | /shop/bobbins-cases/ |
| Spring alone damaged, case otherwise sound | Bobbin-case tension spring | <a href="https://ultimatesewingandvac.ultimatesewingandvac.com/shop/machine-parts/springs/”>/shop/machine-parts/springs/ |
| Check spring broken or limp, loops only at seam starts | Check spring or tension assembly | /shop/machine-parts/tension-assemblies/ |
| Discs grooved, tension drifts, dial has no effect | Tension assembly | /shop/machine-parts/tension-assemblies/ |
| Burr on the hook, shredding top thread | Hook assembly (and have timing checked) | /shop/machine-parts/hooks-races/ |
| Wrong bobbin class | Bobbins of the right class | /shop/bobbins-cases/ |
| Serger: loops on the underside | Lower looper (if burred) or lower looper thread not seated | /shop/machine-parts/loopers/ |
Filter any of those pages by your machine model and you will see only the parts confirmed for it.
When it is not a parts problem
Most of the time it is threading, and the honest advice is to rethread top and bottom from scratch with the foot up before buying anything. A fresh needle and a clean hook area fix most of the rest. Order a part when you can see the damage — a nicked spring, a burr, a dented case — or when the dial no longer changes the stitch at all. We would rather you spend five minutes rethreading than $30 on a bobbin case you did not need.
Still stuck?
Email a photo of the stitch, top and bottom, with your machine’s model number to support@ultimatesewingandvac.com. Tell us what you have already checked and we will tell you which part — if any — to order.
Frequently asked questions
Why is the bobbin thread looping on top of the fabric?
The opposite problem: bobbin tension is too loose or the bobbin case is dirty, or the top tension is too tight. Clean the case, check the bobbin winds evenly, and adjust the case screw a quarter turn at a time.
Is a thread nest the bobbin’s fault?
Usually not — it is the top thread with no tension. Rethread the top with the foot up first.
How do I check the bobbin case spring?
Remove the case, run a fingernail along the spring edge and across the groove; any catch or dent means replace it.
Can the wrong bobbin cause bunching?
Yes — a bobbin of the wrong class or height rubs the case and unwinds unevenly.
The machine jammed and now every stitch nests. What broke?
Possibly the hook timing, or a burr on the hook from the jam. Check the hook tip; if it is marked, replace it and have timing checked.
Does this apply to sergers?
Partly. Loops under a serged edge usually mean the lower looper thread is not seated in its tension or the lower looper is burred.