Why does my sewing machine tension keep going wrong?

Tension is described as a setting, which is why people reach for the dial. It is better understood as a position: the point inside the fabric sandwich where the two threads lock together. Balanced tension puts that knot halfway through. Everything you can see on the surface is a symptom of the knot having been pulled too high or pushed too low, and the thread that is visible on the wrong face is always the loose one. That single rule diagnoses the whole problem.

What you'll need

• Two layers of the actual fabric you are sewing, as a test piece
• The same thread on top and in the bobbin
• A fresh needle of the right type for the fabric
• A seam ripper

Step by step

1. Test on the real fabric, in the real thread, on two layers.Tension correct for calico is often wrong for jersey or coating. Testing on a scrap of something else tells you nothing useful. Use contrasting top and bobbin colours while diagnosing — it makes the fault obvious at a glance.
2. Look at both faces of the test seam.A balanced stitch looks identical from the top and the underside: a plain line of top thread above, a plain line of bobbin thread below, and no sign of the other one on either face.
3. Bobbin thread showing on top means the top is too tight.The top thread is winning the tug of war and pulling the interlock up towards the surface. The seam usually feels hard and may pucker. Lower the top tension number half a step, retest, repeat. This is one of the few faults the dial genuinely fixes.
4. Top thread looping underneath means the top is too loose.The bobbin thread is pulling the interlock down below the underside. This is the nest of loops people call bird nesting. Do not raise the tension number — rethread first.
5. Rethread completely with the presser foot raised.The tension discs are clamped shut whenever the presser foot is down. Thread laid in with the foot down slides past the discs instead of between them, so the machine sews with effectively no top tension whatever the dial says. This is the single commonest cause of a loose top thread and it fixes itself the moment you rethread properly.
6. Check the thread is actually running through everything.The spool pin cap or thread net, the pre-tension guide near the top, the tension unit, the check spring, the take-up lever, and every guide down the needle bar. A thread that has jumped out of the take-up lever produces loops immediately.
7. Check the needle before adjusting anything else.Blunt, bent, fitted the wrong way round, not pushed fully up into the clamp, or too fine for the fabric. Any of those changes how the loop forms and shows up as an apparent tension fault.
8. Look for lint in the tension discs.Compacted fibre between the discs stops them closing on the thread. With the presser foot up, run a folded strip of thin cotton or a length of dental floss up and down between the discs to clear it. Do this before deciding the tension unit is broken.
9. Check the bobbin itself.The right bobbin type for the machine — a slightly wrong class of bobbin rattles in the case and gives erratic tension. Wound evenly and not overfull. Inserted so it turns in the direction the diagram beside the case shows. Thread pulled through the slot and under the spring finger.
10. Only then consider the bobbin case tension.The small screw on a removable bobbin case is factory set and is the last thing to touch, not the first. Mark or photograph its position before you move it, and turn it no more than an eighth of a turn at a time. On a drop-in bobbin with no accessible case, leave it alone entirely and take the machine to a service.
11. Recheck after any change of thread or fabric.Different thread weights, a metallic or a topstitch thread, or a jump from poplin to denim all shift the balance. Tension is per-job, not a setting you find once.

Tips

• Use two obviously different thread colours while you are diagnosing. Which colour appears where is the entire diagnosis and it takes two seconds.
• If both faces look wrong at once, rethread top and bobbin from scratch rather than trying to adjust your way out of it.
• Old, cheap or long-stored thread sheds badly and behaves inconsistently through the tension unit. If a machine that was fine last month has become erratic, suspect a bargain reel of thread before you suspect the machine.
• Write down the dial number that works for your everyday fabric. It gives you a known-good starting point to return to.

Common mistakes

• Turning the dial as the first response — Two of the three commonest tension faults — thread not in the discs, and lint packed between them — are unaffected by the dial. Adjusting it in those cases buries the real fault under a wrong setting you then have to unwind.
• Adjusting the bobbin screw early — It is factory set and easy to lose. Once the reference position is gone you are guessing on both sides of the stitch at once, which is much harder to recover from than a wrong top tension.
• Testing on one layer of fabric — A single layer gives a different result from the two layers you will actually sew, and hides puckering entirely.

If it doesn't work

Questions people ask