Sewing machine tension problems: read the stitch, then fix it
A balanced straight stitch links the top and bobbin threads inside the fabric layers: both faces show smooth stitches, no dots of the other color, no loops, no puckering. Once you can read which side the imbalance shows on, the fix is mechanical.
The two-color diagnostic test
- Thread the top with one color and the bobbin with a clearly different one.
- Sew a 15 cm straight seam on a doubled scrap of your project fabric.
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Read the result:
- Bobbin color visible on top → top tension too tight (it’s hauling the bobbin thread up). Lower the top dial.
- Top color visible underneath → top tension too loose. Raise the top dial.
- Links buried between layers on both faces → balanced. Stop.
Adjusting the top dial properly
Change one number at a time, sew a fresh test seam after each change, and keep notes. Higher numbers grip tighter, lower numbers release. Adjust with the presser foot down (the discs are open with the foot up, so a setting made foot-up doesn’t take effect until later, which is how people overshoot). If you reach either extreme of the dial before the stitch balances, the problem isn’t the setting — go back to threading, lint, and needle.
Puckered seams vs. loose loops
Loops mean too little tension somewhere; a seam that gathers the fabric like a drawstring means too much overall tension for that fabric — common on lightweight and sheer fabric. Reduce top tension, shorten the stitch slightly, use a finer needle (70/10), and support the fabric with tissue or stabilizer. On knits, a wavy stretched-out seam is the reverse problem: the fabric stretched while sewing — lighten presser-foot pressure or use a walking foot.
When the bobbin side is genuinely at fault
Suspect the bobbin case only after the top checks out: lint under the case’s flat tension spring, thread not actually inside that spring (it must click into the tension slot when you load the bobbin), a nicked or warped bobbin, or an overfilled one that drags in the case. Cleaning and reloading fixes almost all of it; the adjustment screw is the last resort (see FAQ).
Computerized machines with “automatic tension” still have all the same mechanics — auto mode just picks the starting value. If auto stitches look wrong, the machine is asking for cleaning or rethreading, not a firmware miracle.
Frequently asked questions
What number should my tension dial be on?
For all-purpose polyester thread on medium woven fabric, the middle of the range — typically 3 to 5, and many dials mark the default with a box or “AUTO.” But the number is only a starting point: the correct setting is whatever produces links that meet inside the fabric layers on the actual fabric-and-thread combination you are sewing. Test on a doubled scrap before every project that changes fabric or thread.
When should I touch the bobbin tension screw?
Rarely — and only after top tension adjustments have failed on a clean, correctly threaded machine. The screw on the bobbin case is set at the factory; a quarter turn is a big change (righty-tighty). If you sew with unusual threads regularly, technicians suggest buying a second bobbin case to adjust, keeping the original factory-set.
Why is my tension fine on cotton but terrible on knits or fleece?
Different fabrics need different loop geometry — thickness and stretch change where the thread link settles. That is normal, not a broken machine: re-balance on a scrap of the new fabric, use the right needle type, and for very stretchy seams switch to a stretch stitch, which is engineered to leave slack in the seam.
Sources & further reading
Manufacturer documentation last checked on 2026-07-03.