Bobbin thread visible on the top surface means the top tension is pulling harder than the bobbin, so the lock is being dragged up above the fabric.
This is the mirror image of loops underneath, and unlike that fault, this one usually is a tension setting.
What to do.
Loosen the top tension by half a number and sew a test seam. Repeat until the two threads lock in the middle of the fabric layers and neither colour shows on the other side.
Test with contrasting threads. Top in one colour, bobbin in a strongly contrasting one, on a doubled scrap of your actual fabric. On matching thread this fault is nearly invisible until the garment is finished.
But check these first, because they mimic it.
- Threading. Re-thread with the presser foot up. A partially seated thread behaves unpredictably rather than consistently.
- Fabric weight change. A tension set for denim is too tight for lawn. If the fault appeared when you changed fabric, that is your answer.
- Bobbin wound unevenly or too loosely, which lets it pay out faster than it should.
Leave the bobbin case screw alone. It is factory-set, a quarter turn changes a great deal, and it is the fastest way to turn a five-minute adjustment into a workshop visit.
Questions people actually ask
How much should I loosen it?
Half a number at a time, with a test seam after each. Large jumps overshoot and you end up hunting.
It changed on its own.
Usually lint in the tension discs, which prevents them closing properly. Clean between them with a folded strip of fabric.
Last reviewed 2026-08-11