Last updated: August 21, 2026
There is a tiny screw on the side of the bobbin case, and the internet is full of people telling you to turn it. It is also the single easiest way to turn a machine that stitches slightly wrong into a machine that stitches badly at every setting, because once the factory value is gone there is no mark to go back to.
That does not mean the screw is off limits. It means it is the last step, not the first, and there is a specific set of symptoms that justify it. This guide covers how bobbin tension works, the checks that come before the screwdriver, and how to touch that screw in a way you can undo.
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Quick Answer
When should you adjust the bobbin tension on a sewing machine?
- Almost never. Singer says the bobbin tension is preset at the factory and usually never needs adjusting.
- Rule out the top side first. Brother says tension cannot be adjusted correctly if the upper or lower threading is wrong, and tells owners to reset the dial to 4.
- Check the obvious bobbin faults: wrong class of bobbin, uneven winding, thread caught under the tension spring, lint in the case.
- Only then, and only in small increments. Singer’s own wording is to adjust the screw on the side of the bobbin case in small increments.
Bottom line: photograph the screw slot before you turn it, move it a fraction at a time, and stop as soon as the stitch balances.
What the Bobbin Side Contributes
The bobbin thread runs out from under a thin metal leaf spring held down by that screw. The spring squeezes the thread just enough to give the top thread something to pull against, and the balance between the two decides where the stitch knot ends up in the fabric.
Because the top side is adjustable and the bobbin side is not, in ordinary use the bobbin is the fixed reference. That is the logic behind Singer’s position that the bobbin tension is preset at the factory and usually never needs adjusting. Move the reference and every top setting you knew becomes wrong at the same time.
Drop In and Front Loading Cases
A drop in bobbin sits in a race under a clear cover plate, and the tension spring is part of the machine rather than a removable case. A front loading machine uses a separate metal bobbin case that lifts out, with the screw on its side. The front loading design is the one you can safely test in your hand, which matters for the drop test further down.
The Checks That Come First
Four bobbin faults produce symptoms identical to a tension problem, and all four are free to fix. Work through them before the screwdriver comes out, because in most cases one of them is the actual cause and the screw was never the issue.
The Bobbin Class Is Wrong
Bobbins are not universal. A bobbin that is slightly the wrong height or made of the wrong material spins unevenly and gives inconsistent tension. Brother states plainly that a bobbin designed specifically for the machine must be used and that others will not work properly. Metal bobbins in a machine designed for plastic ones are a classic cause. Our guide to bobbin classes covers how to identify yours.
The Bobbin Is Badly Wound
Thread that piles up at one end, crosses over itself or was wound at a crawl comes off at varying speed and reads as fluctuating tension. Rewind it properly at a steady speed with the thread going through the winder guide, and do not fill it to overflowing. Brother lists an improperly wound bobbin among the causes of needle and thread trouble.
Thread Under the Spring, or Lint in the Case
Take the bobbin out and look at where the thread passes under the leaf spring. If it has slipped sideways out of the slot, or if a wad of lint has packed in behind the spring, the tension is effectively random. Brush it out with the small brush that came with the machine and rethread the case, following the arrow stamped on it.
The Drop Test
On a machine with a removable front loading case, there is a rough check worth knowing. Thread the case as normal, hold it by the thread so it hangs in the air, and give the thread a small sharp jerk. The case should drop an inch or two and stop, rather than plummeting or refusing to move.
A case that free falls has too little tension. A case that will not move at all has too much. It is a feel test rather than a measurement, and it is only meaningful against your own machine over time, so do it once while the machine is behaving well and remember what correct feels like. Drop in bobbin machines cannot be tested this way, which is another reason to leave those alone.
Turning the Screw Without Regretting It
If the top side is genuinely ruled out and the bobbin checks are clean, Singer’s guidance applies: remove the bobbin case and adjust the screw on the side in small increments. Small means a fraction of a turn, not a quarter, and one direction at a time with a test seam after each move.
Do three things before you start. Take a close photograph of the screw slot so you know the original angle. Work over a tray or a closed drawer, because that screw is small enough to vanish into a carpet forever. Use a screwdriver that actually fits the slot, since a loose fit chews the head and leaves you with a case you cannot adjust at all.
Which Way, and How Far
Clockwise tightens the spring and increases bobbin tension. Counterclockwise loosens it. Move a fraction, refit the case, sew a test line with contrasting colors, and look at both sides. If two small moves in one direction do not improve things, put it back to the photographed position and look again at the top side.
A spare bobbin case is worth having if your machine takes a removable one, so that you can keep the factory setting on one and experiment on the other. That is the standard approach for people who switch between ordinary thread and heavier thread regularly.
Symptoms and Where They Really Come From
The table sorts the bobbin related symptoms by their most likely origin, so you can see how few of them actually end at the screw. Work left to right and only reach the last column when the others have been cleared.
| Symptom | Most likely cause | Free fix | Screw justified? |
|---|---|---|---|
| Loose loops under the fabric | Upper threading | Rethread with the foot raised | No |
| Bobbin color pulled to the top | Upper tension too tight | Dial down one number | No |
| Tension varies within one seam | Uneven winding or wrong bobbin | Rewind, use the right class | No |
| Balanced top, bobbin still too loose after all checks | Bobbin spring genuinely off | Nothing left to try | Yes, in small increments |
If the fault is on the top side after all, our guide to sorting tension problems walks through the two color test that tells the two sides apart.
Frequently Asked Questions
Should I adjust my bobbin tension at all? Usually no. Singer says the bobbin tension is preset at the factory and usually never needs adjusting, and only suggests the screw if the bobbin is threaded properly and the problem still persists. Clear the top side and the bobbin basics first.
Which way do I turn the bobbin tension screw? Clockwise tightens, counterclockwise loosens. Move a fraction of a turn at a time and test after each move. Photograph the slot angle first so you can return to the factory position if the experiment goes nowhere.
What is the bobbin drop test? Hang a threaded front loading bobbin case by its thread and give a small jerk. It should drop an inch or two and stop. Free falling means too loose, refusing to move means too tight. It does not apply to drop in bobbin machines.
Can the wrong bobbin cause tension problems? Yes, and it is a common cause. Brother says a bobbin designed specifically for the machine must be used and that other bobbins will not work properly. Height and material differences between classes are enough to unbalance the stitch.
My bobbin tension screw fell out. Now what? Order a replacement bobbin case rather than substituting a screw from elsewhere, since the thread size and the spring pressure are matched to the case. Keep the case in a small box while you work on it so this cannot happen again.
Does bobbin thread type matter? It does. A heavier or fuzzier thread passes under the spring differently from a fine one, which shifts the balance without anything being broken. Test on a scrap when you change thread, and see our guide to sewing machine thread for how weights compare.
The Bottom Line
The bobbin screw is the last resort, not the first move. Rethread the top, reset the dial to the middle, check that the bobbin is the right class and evenly wound, look for thread out of the tension slot and lint in the case. If all of that is clean and the stitch is still unbalanced, photograph the screw and move it a fraction at a time. Read next: our full guide to reading the stitch to set tension and to which bobbin class fits your machine.