A lockstitch is formed by two threads meeting in the middle of the cloth. The upper thread comes down through the needle, the hook wraps it around the bobbin thread, and the take-up lever pulls the knot into the fabric. Tension is simply how hard each thread pulls against the other. When they balance, the knot sits inside the cloth and neither thread shows on the other side.
What the dial does
The upper tension dial squeezes the upper thread between two discs. More pressure means the upper thread pulls harder, which pulls the knot down toward the bobbin side. Less pressure lets the bobbin thread win, and the knot rises to the top surface.
| What you see | Which thread is winning | What to change |
|---|---|---|
| Bobbin thread shows on the top | Bobbin is pulling harder | Increase upper tension |
| Upper thread loops underneath | Upper is too loose | Increase upper tension, or re-thread |
| Both threads show, stitch looks thin | Balanced but needle too small | Fit a larger needle |
| Fabric puckers along the seam | Too much tension on both | Reduce upper tension |
Check these before the dial
Six faults that look exactly like a tension problem and are not. Work down the list before you change a setting, because changing the tension to compensate for one of these makes the real problem harder to find.
- Is the presser foot raised when you thread? If it is down, the discs are closed and the thread never seats between them.
- Is the thread actually in the take-up lever? A thread that missed the lever produces loops and no stitch.
- Is the bobbin in the right way round? It has to unwind in the direction the case expects.
- Is the needle bent or blunt? A damaged needle causes skipped stitches and loops that look like tension faults.
- Is the needle the right type and size for the fabric? A universal needle in jersey will skip no matter what the dial says.
- Is there lint packed between the tension discs? Fluff stops the discs gripping the thread evenly.
The reset that fixes most of it
Raise the presser foot, unthread the machine completely, set the tension dial back to its middle position, and thread it again from the spool. This fixes the majority of "tension" problems without touching a setting.
How to set the tension properly
Start in the middle
Set the upper tension dial to its mid-point, usually 4 or 5. The bobbin tension is set at the factory and rarely needs changing.
Test on a scrap of the real fabric
Sew a seam about 15 cm long with two different thread colours, so you can see which is which.
Look at both sides
On the right side you should see only the upper thread colour. On the wrong side you should see only the bobbin colour. If a colour shows on the wrong side, that thread is pulling too hard.
Adjust in small steps
A quarter turn at a time, then re-test. Large jumps overshoot and you end up chasing the setting.
Re-test on a double layer
A seam through two layers behaves differently from one through six. Test the thickness the garment will actually use.
The bobbin tension
The bobbin case has its own tension screw, and on most domestic machines it is set once and left alone. The test is the drop test: hold the bobbin case by its thread and let it hang. It should stay put with a light bounce, not fall freely and not stay rigid. If you adjust it, do it in eighth-turn steps and note the starting position.
Do not chase a fault with the dial
If a re-thread and a new needle do not fix it, the cause is usually mechanical: timing, a burr on the hook, or lint in the bobbin area. Turning the tension dial will not fix a mechanical fault and will make the machine harder to reset.
The practical rule: change the needle, re-thread, then test. Only after those three does the tension dial become the right thing to adjust.
