Sewing Machine Bobbin Tension: Check, Then Adjust Only If Allowed

Check in this order

1. Rethread the bobbin. Pull the thread until it slips into the tension spring of the case. Brother’s SE700 manual: “the thread will enter the tension spring of the bobbin case to apply the appropriate tension to the bobbin thread during sewing.”¹⁴ Brother also says: “If the thread is not correctly inserted through the tension-adjusting spring of the bobbin case, it may cause incorrect thread tension.”¹⁵
2. Check the direction. “If the thread unrolls in the wrong direction, it may cause the thread tension to be incorrect or break the needle,” says Brother.¹⁵ The direction depends on your bobbin system (see the FAQ below).
3. Check the winding. “Not winding the bobbin properly may cause the thread tension to loosen, and may break the needle.”¹⁵
4. Adjust the upper tension. Kentucky: “Assume the lower tension to be correct until proved wrong, and make adjustments on the upper tension as much as possible.”⁷
5. Clean the spring. New Mexico State lists “Lint, dirt, or thread has accumulated under the tension spring of the bobbin case” among the causes of problems.¹⁶
6. Only then, the screw, and only if your maker allows it (table below).

Brother has a quick check after sewing: remove the bobbin cover and look at the thread; if it doesn’t run as the manual shows, “the thread is not inserted through the tension-adjusting spring of the bobbin case correctly.”¹⁷

Can you turn the screw? What each maker says

Bobbin tension screw: who may adjust it

Maker	What it says
SINGER manuals (4452, M3500)	“set correctly at the factory, so you do not need to adjust it”1,11
SINGER help center, front-load cases only	Usually never needs adjusting; if the bobbin is threaded properly and loops persist, “adjust the screw on the side of the bobbin in small increments”8
Brother (NV1250D, SE1800 and others)	“set to factory specification and does not require adjustment”2
Brother green-screw case	“This bobbin case should not be adjusted”9
Brother embroidery case, no colour on the screw	Adjustable: slotted screw only, about 30 to 45 degrees at a time6
BERNINA 335	Lower thread too tight or too loose: “Contact your BERNINA dealer.”18

New Mexico State adds: “Some new model machines have factory set bobbins that should not be tampered with. Check your machine’s manual.”¹⁶

If your maker allows it: how to adjust

Brother embroidery case (no colour on the screw). Take the bobbin out of the case first. If the bobbin thread shows slightly on the right side of the fabric, it is too loose: turn the slotted screw clockwise about 30 to 45 degrees, “being careful not to over-tighten the screw”. If it is too tight, turn it counterclockwise by the same amount.⁴ Brother warns: “DO NOT adjust the position of the phillips (+) screw on the bobbin case as this may result in damage to the bobbin case, rendering it unusable.”⁶ And: “If the slotted screw (-) is difficult to turn, do not use force.”⁶

Cases with a screw-held spring (Kentucky’s method). “If two screws fasten the lower tension spring to the bobbin case, adjust by turning the screw nearest the center of the spring—not the screw on the end.” An eighth of a turn at a time.⁷ Kentucky’s guide is a 2006 university guide, not a maker’s manual: check yours before turning anything.

When the case itself is the problem

A worn spring is not a tension setting. New Mexico State lists a “Spring on bobbin case is worn to a sharp groove”; JUKI’s TL manual says to replace the case when “The rubbing of the bobbin case spring has produced a slot”.^5,16 Brother: “Never use a bobbin case that is scratched”.¹⁵ See our bobbin case guide for replacements.