Bobbin thread showing on top of the fabric means the upper thread is pulling harder than the bobbin thread, so the knot of each stitch is dragged up to the surface instead of sitting between the layers. Either the top tension is too tight, or the bobbin thread is running too freely. Most of the time the cause is on top: the thread is catching somewhere between the spool and the needle, or the dial is set too high. Rethread the top from the spool, set the dial to its default (usually 4 on a 0 to 9 dial), and lower it half a number at a time on a doubled scrap until the bobbin thread disappears; then check that the bobbin is wound firmly and threaded under its tension spring.
What should a balanced stitch look like?
In a lockstitch the top thread and the bobbin thread interlock, and when the two tensions are matched the knot sits in the middle of the fabric. From above you see only the top thread as a flat, even line; from below, only the bobbin thread. The thread tension entry has a diagram.
The quickest test uses two colors. Wind a bobbin in a contrasting color, thread the top in the color of the fabric, and sew 10 cm (4 in) of straight stitch on two layers of the project fabric. Turn it over and look at both sides. Dots of the bobbin color on top mean the top is too tight (or the bobbin too loose); dots of the top color underneath mean the opposite. Keep this scrap: it is the reference for every adjustment that follows.
Symptom, cause and fix at a glance
| What you see | Most likely cause | Fix |
|---|---|---|
| Bobbin thread dots on top, top thread lying straight along the seam | Top tension too high | Lower the dial half a number at a time |
| Bobbin dots on top and the top thread breaks or shreds | Thread snagged on the spool notch, a wrong spool cap or a guide | Rethread the top, change the spool cap, use a thread stand for cones |
| Started after changing thread | Heavier or stiffer thread on top than in the bobbin | Match the threads, or lower the top tension for the heavier thread |
| Fine on medium fabric, bobbin dots on light fabric | Tension set for the previous fabric; needle too big | Lower the tension; fit a 70/10 needle |
| Bobbin dots on top and loose, loopy bobbin thread underneath too | Bobbin thread not under the bobbin case spring | Rethread the bobbin so it clicks under the spring |
| Comes and goes along the seam | Bobbin wound unevenly or overfilled; damaged bobbin | Rewind at a steady speed, replace a chipped bobbin |
| After sewing with metallic, elastic or topstitching thread | Tension changed for that thread and not reset | Return the dial to its default |
How do you diagnose bobbin thread on top, step by step?
- Rethread the top completely. Raise the presser foot so the tension discs open, pull the thread out, and thread again from the spool following every guide and the take-up lever. While you are there, look at the spool: the thread must leave it freely. A spool cap that is too small lets the spool jump and the thread catch in the notch cut in the spool's rim; a horizontal spool pin suits ordinary spools, while cross-wound cones need to stand on a thread stand behind the machine. Anything that grips the thread on the way adds tension the dial does not show.
- Set and test the top tension. Return the dial to its default, often marked or shaded, and sew the two-color test. If the bobbin color still shows on top, lower the dial by half a number and sew again, until the surface is clean and the underside is not looping. Machines with automatic tension still have an adjustment in the menu. Sewing machine tension problems goes through the whole range of tension faults and how far it is safe to go.
- Check the bobbin. Take it out and look: it should be wound evenly, at a steady speed, with the thread going through the bobbin winder's tension guide, and not filled above the rim. Put it back the way the manual shows and draw the thread into the slot and under the spring until you feel a click; pulling the thread should give a light, even drag. A bobbin thread lying over the spring instead of under it has no tension at all, and the top thread wins every stitch. On a front-loading machine the case must be threaded the same way; drop-in vs front-loading bobbin shows both.
- Match thread and needle. Pre-wound bobbins are often a finer thread than the all-purpose polyester on top, and a heavy top thread paired with a fine bobbin thread needs lower top tension. A needle whose eye is too small for the thread makes the thread drag through it, which has the same effect as raising the dial. The needles and thread guide pairs thread weights with needle sizes; as a rule, if the thread does not slide freely through the eye when you hold the needle up, go one size larger.
- Clean between the tension discs. Lint and a stray strand of thread can wedge between the discs and hold the thread tighter than the setting. With the foot raised, fold a scrap of fine cotton over a length of thread and draw it back and forth through the discs, then blow out the dust.
- Consider the fabric. On chiffon, voile and lawn the default tension is often too high, because there is no fabric weight to resist the thread; lower it a little further and use a 70/10 or 60/8 needle. On very thick fabric the opposite can happen and the top thread shows underneath. Test on each new fabric before the first seam. If you are unsure which thread or needle suits a fabric, this is the kind of question Sewly's sewing guide answers about your own pattern.
- The bobbin tension screw, last. If every step above is correct and the stitch is still unbalanced with the top dial already low, the bobbin case tension may be too loose. On a front-loading case, the screw on the spring adjusts it; on a drop-in machine the case lifts out and has the same screw. Note the screw's position, turn it clockwise by no more than a quarter turn to tighten, and test again. Most sewists never need to touch it, and some manuals ask you not to.
Why does bobbin thread show only on one side of a zigzag?
On a zigzag or an overcasting stitch, the needle swings sideways and pulls the top thread across the fabric to each side, so the knots form at the edges of the zig and the zag rather than in the middle. Slightly less top tension than for a straight stitch keeps the bobbin thread out of sight on a zigzag. The same applies to decorative stitches, which is why many computerized machines lower the tension automatically when you select them.
How do you stop it from coming back?
Write the tension setting that works for your usual fabric and thread on a sticky note on the machine, and return to it after sewing with any special thread. Keep bobbins wound from the same thread you sew with, change to a finer needle when you change to a finer fabric, and run the two-color test on a scrap of each new project before the first seam. A minute of testing saves unpicking a visible line of dots along a hem.