Why Does My Bobbin Thread Keep Breaking?

Bobbin thread breakage causes dropped stitches, tangled thread in the bobbin case, and seams that fall apart. Here are ten common causes and how to fix each one.

If your upper thread is the one breaking, see the companion post on upper thread breakage.

1. The Machine Is Not Threaded Correctly

Incorrect upper threading affects the bobbin thread too. When the upper thread does not have the right amount of tension, it pulls unevenly on the bobbin thread and can snap it.

Unthread completely and rethread from scratch. Make sure the take-up lever is at its highest position before threading. Follow your user manual exactly for the thread path, and check that the bobbin thread is also correctly routed through the bobbin case tension slot.

2. The Thread Is Poor Quality

Weak, thin, or inconsistently twisted thread breaks under normal machine tension. This is especially common with bulk thread packs, which are typically produced to lower quality standards than brand-name thread.

Switch to a quality thread brand for machine sewing. You can test thread quality by holding a short length and pulling firmly with both hands. Quality thread stretches slightly and holds; poor thread snaps with little resistance.

3. The Bobbin Thread Tension Is Too Tight

Unlike upper thread tension, bobbin tension is adjusted via a tiny screw on the bobbin case rather than a dial. When bobbin tension is too tight, the thread cannot feed freely through the case and snaps.

Test the bobbin tension by holding the bobbin thread and letting the bobbin case hang. It should drop slowly when you give the thread a gentle shake. If it does not drop at all, the tension is too tight. Make very small adjustments to the tension screw, turning counterclockwise to loosen. A quarter turn is significant. If you are not comfortable adjusting the bobbin case screw, a technician can recalibrate it.

Also confirm you are using the same weight thread in the upper spool and the bobbin. Mismatched weights create an imbalance that can cause either thread to break.

4. There Is a Burr in the Bobbin Case

Metal parts develop burrs over time as thread repeatedly rubs against them. A burr in the bobbin case, on the tension spring, or on the underside of the throat plate will snag the thread and snap it on every pass.

Inspect the inside of the bobbin case closely, running a fingertip or a cotton swab around the interior. Rough spots will snag the cotton fibers. Check the throat plate underside and the tension spring as well. Small burrs can sometimes be smoothed with very fine sandpaper or a fingernail file. More significant burrs may require a replacement bobbin case.

5. The Bobbin Case Is Full of Lint

Lint packed around the hook race restricts the hook's movement and creates friction against the bobbin thread. Some bobbin cases also have an anti-backlash spring, and lint under this spring is a specific cause of thread snagging and breaking that is easy to miss.

Remove the bobbin and clean the bobbin case, hook race, and the area around and under any springs in the case. Use the brush that came with your machine and sewing tweezers for reaching tight spots. Clean after every few projects.

6. The Bobbin Is Inserted Backward

Each machine requires the bobbin thread to unwind in a specific direction, either clockwise or counterclockwise. Inserting the bobbin backward threads the case in the wrong direction. The thread will usually sew for a few stitches before the incorrect path causes it to snag and break.

Check the diagram printed on or near the bobbin case for the correct orientation. Re-insert the bobbin and make sure the thread passes through the tension slot correctly and pulls smoothly when you tug it gently.

7. The Bobbin Is Not Wound Correctly

An unevenly wound bobbin feeds thread inconsistently. A knot in the bobbin thread, a bobbin wound too tightly or too full, or thread that slips off an uneven winding all cause breakage.

After winding, check that the thread is layered evenly across the bobbin with no gaps or bulges. Never wind past the bobbin edge. Wind by machine rather than by hand whenever possible since hand-wound bobbins are prone to uneven tension and knots. If your machine's built-in winder is producing uneven bobbins, the rubber winding ring may need replacing.

8. You Are Using the Wrong Bobbin

Bobbins that are too small bounce in the bobbin case and cause thread to snag. Bobbins that are too large do not spin freely and create drag that breaks the thread.

Always use bobbins designed for your specific machine model. Check your user manual for the correct bobbin class. When in doubt, bring your machine to a sewing machine dealer to confirm the right bobbin.

9. The Bobbin Is Warped or Damaged

Plastic bobbins warp from heat exposure and no longer sit correctly in the bobbin case, causing uneven thread feed and breakage. Metal bobbins can occasionally develop small burrs or surface damage.

If the bobbin does not sit flat and stable on the winding pin or in the bobbin case, discard it and wind a fresh one. Store plastic bobbins away from heat sources and direct sunlight. Metal bobbins are more heat-resistant and worth considering if warped bobbins are a recurring problem.

10. The Hook Timing Is Off

Hook timing is the synchronization between the needle's downstroke and the hook's rotation. When the timing is off, the hook does not catch the upper thread loop correctly, which puts irregular stress on the bobbin thread and can snap it.

You can do a basic check by slowly turning the handwheel toward you and watching through the needle plate opening. As the needle rises slightly from its lowest point, the hook tip should pass directly behind the needle at the level of the needle's eye. If the hook is early, late, or not close to the needle, the timing is off.

Hook timing adjustment is best left to a sewing machine technician. It is checked during every professional service. Always turn the handwheel toward you only — turning it the wrong direction can worsen the misalignment.