After 15 years of walking into shops where someone’s been fighting a machine for three days over “a tension problem” that was actually a burr on the hook, I learned one thing: you need to know what a correct stitch looks like before you can diagnose a bad one. Here’s the cheat sheet I keep pinned on my bench.
**Correct stitch — threads lock in the middle of the fabric.** Top and bottom threads interlock right between the layers, not pulled to either side. If you peel the seam open gently, you see tiny dots of the opposite color centered in the material. Leave it alone.
**Loops on the bottom (birdnesting) — top tension is too loose.** The upper thread is hanging slack, so the bobbin case pulls too much of it under the fabric. Tighten the top tension dial one number at a time until the loops disappear. Also check the top thread path — it might have slipped out of a take-up lever or a guide.
**Loops on top — bottom tension is too loose (or top tension is too tight).** Thread is being pulled up from the bobbin. Give the bobbin case screw a quarter-turn clockwise. If that doesn’t fix it, back the top tension off a number or two.
**Puckered fabric — both tensions are too tight.** The fabric is being gathered instead of sewn. Loosen both top and bobbin tensions equally. If the fabric still puckers, try a smaller needle or a shorter stitch length.
**Zigzag pattern on the bottom — timing issue, not tension.** The hook is slightly out of time, catching the needle thread late. This one gets misdiagnosed constantly. Don’t touch the tension dials — check your timing instead.
For the full diagnosis guide, see the
Sewing Machine Tension Troubleshooting guide.
Related Guides
Reference:
Tension Systems Guide