The dial tightens or loosens two metal discs that the top thread passes between on its way to the needle. Nothing else. It is not a general quality control, and it does not touch the bobbin.
What the number means. A higher number closes the discs harder, so the thread is held back and less of it feeds into each stitch. The bobbin thread then wins the tug of war and gets pulled up towards the top surface. A lower number opens the discs, more top thread feeds, and the top thread is drawn down to the underside.
Why the middle of the range is marked. Most machines have a normal setting, often around 4, chosen so that a mid weight woven and a general purpose polyester thread lock in the centre of the fabric. It is a starting point, not a rule.
The part people miss. The discs only grip if the thread is actually seated between them, and they only open when the presser foot is raised. Threading with the foot down lays the thread across closed discs, where it sits outside the tension path entirely. The dial then does nothing at all, whatever number you set.
When the dial is the wrong tool. Skipped stitches, thread nests under the plate, and a machine that will not stitch at all are threading, needle or hook faults. Turning the dial in response builds a second problem on top of the first.
Write down your normal number before you experiment, so you can get back to it.
Questions people actually ask
Which way is tighter?
Higher numbers are tighter on almost every machine. If in doubt, sew a test line, move two whole numbers, and look at what changed.
Does the dial affect the bobbin?
No. The bobbin has its own separate tension, set by a small screw on the case, and it is adjusted far less often.
Last reviewed 2026-08-11