Sewing Machine Tension Troubleshooting: 9 Stitch Problems, 9 Exact Fixes

Most tension complaints on a factory floor are not tension at all. They are a needle that has been in the machine for three shifts, a thread that missed the take-up lever, or a bobbin case full of lint. The dial is the last thing to touch, not the first — and on most industrial heads the bobbin screw should not be touched at all until everything upstream has been checked.

What follows is the order a technician actually works in, then the nine faults that bring operators to the supervisor’s desk, each with the specific adjustment that fixes it.

What Is the Correct Order to Check Anything?

Work down this list and stop at the first thing that is wrong. Do not skip ahead.

1. Needle — correct system, correct size, straight, sharp, fully up in the clamp.

1. Thread path — every guide, both tension discs, the take-up lever, the needle eye from the correct side.

1. Bobbin — correct style, wound evenly, inserted in the right direction, case clean.

1. Tension — upper dial first, bobbin screw last.

1. Timing and hook — a technician job, and the only one on this list that needs a gauge and a manual.

This order exists because each step is cheaper than the next one and far more likely to be the cause. A bent needle produces the same symptom as a mis-timed hook, and one of them costs nothing to rule out.

What Does Correct Tension Actually Look Like?

On a 301 lockstitch, the upper and bobbin threads interlock inside the fabric, not on either surface. On two layers of 12 oz denim you should see a small, even dot at each needle penetration — not a loop, not a raised knot, not a line of bobbin thread pulled up into the top ply.

Two numbers make this checkable:

• Bobbin case tension: 200–300 mN, which is about 20–31 gf. That is the force needed to pull the thread out of a case held by its thread. Workshop gauges usually read in grams, so the same specification appears as 20–31 gf — 1 gf = 9.807 mN. If your gauge only reads grams, you are not reading a different specification; you are reading the same one in the unit your bench actually has.

• Stitch density: 2.5–3.5 stitches per cm on denim (roughly 6–9 per inch). Density that is too high packs thread into the same hole and makes every tension symptom worse.

And one thing that is not a number: the dial. Tension dials are spring-loaded and uncalibrated; “4” on one head is not “4” on the next. If a setting works, write down the material, the thread, the needle and the dial position together — the dial alone tells the next operator nothing.

1. Why Are There Loops Under the Fabric?

What you see: a nest of thread on the underside, sometimes so dense the fabric will not advance. Operators call it a bird’s nest.

What it is: the upper thread never entered the tension discs, so there is no upper tension to pull the knot up into the fabric. The bobbin thread is doing all the work and looping freely underneath.

The exact fix:

1. Raise the presser foot — this opens the discs — and re-thread from the spool, through every guide, into the take-up lever, down to the needle.

1. Lower the foot and pull the thread by hand. You should feel firm, even resistance. If it pulls freely, the thread is sitting on top of the discs rather than between them.

1. Sew 20 cm on the actual fabric. If loops remain, increase the upper tension one dial number at a time, re-testing after each.

Do not touch the bobbin screw for this symptom. Bird-nesting is an upper-thread fault in almost every case, and every quarter turn on the bobbin screw is a change you will have to undo later against a baseline you no longer have.

2. Why Are There Loops on Top of the Fabric?

What you see: loose bobbin thread pulled up through the top ply, or a line of small loops along the seam.

What it is: bobbin tension too low, or the bobbin is not feeding smoothly — lint under the tension spring, thread crossed on the bobbin, or the wrong bobbin style in the case.

The exact fix:

1. Remove the case and check the bobbin itself. Industrial lockstitch machines use L style (20.3 × 8.9 mm) or M style (24.9 × 10.7 mm) bobbins. A class 15 bobbin is a domestic size; it will sometimes seat and then damage the hook.

1. Blow or brush the lint out from under the tension spring — do not use a cloth, which leaves fibres behind.

1. Do the drop test: hold the thread end and let the case hang. It should slide down slowly and steadily, not free-fall and not hang motionless. Free-fall means too loose; hanging means too tight.

1. If adjustment is needed, turn the screw 1/8 turn at a time, and check with a gauge against 200–300 mN (20–31 gf).

3. Why Is the Seam Puckered?

What you see: the fabric gathers along the stitch line and does not flatten after pressing.

What it is: thread pulled tighter than the fabric can tolerate — usually excess upper tension, sometimes a needle that is too large for the cloth, or a thread that is too heavy for the seam.

The exact fix:

1. Reduce upper tension until the knot is still buried but the seam lies flat. That is the lowest usable setting, and it is where you want to be.

1. Check the needle against the fabric: light fabrics 70–90 (SES point), medium 100–110 (SUK point), heavy 120–140. A needle two sizes too large punches a hole the thread cannot fill and the seam puckers around it.

1. Test on 20 cm of the real material, then press it. Puckering that survives pressing is a tension or needle fault; puckering that disappears was handling.

4. Why Does the Machine Skip Stitches?

What you see: an occasional missing stitch, or a run of them, with the thread intact.

What it is: almost never tension. Skipping means the hook did not catch the loop of thread, and the usual causes are a blunt or bent needle, a needle that is too fine for the thread, a wrong needle system (the eye sits at the wrong height), or hook timing.

The exact fix:

1. Change the needle first — every time. On denim, needles are consumables at 4–6 hours of running; a needle that looks fine at arm’s length is often the whole problem.

1. Confirm the system: DB×1 on single-needle lockstitch heads, DC×27 on overlock, 135×17 / DP×17 on compound-feed and walking-foot machines. These are not interchangeable, and a needle that fits loosely is the one that skips.

1. Check the needle against the thread. Thread diameter should be about 40% of the needle blade diameter; too fine a needle leaves a loop too small for the hook to catch, which is why “use a thinner needle” is bad advice for a skipping machine.