Machine Problems

Sewing Machine Bobbin Thread Tension

Sewing machine bobbin thread tension problems are easy to mistake for top tension problems. Both can make stitches look uneven. Learning to tell them apart stops you from changing the wrong thing.

Spotting a bobbin-side problem specifically

Sew a test seam with a different thread color in the bobbin than on top. Then look at both sides.

• Bobbin thread shows as dots on the top of the fabric. Either the top tension is too tight or the bobbin thread tension is too loose.
• Top thread lies flat along the underside. Either the top tension is too loose or the bobbin thread tension is too tight.
• Bobbin thread lies loose on the underside and pulls out easily. The bobbin side is the likely cause: a loose wind, a loose case, or the thread missing the tension spring.

The first two can come from either side, so fix the top thread first. If the stitch is still wrong after a full rethread at the default top setting, move on to the bobbin.

Checking the bobbin winding first

Before you decide the bobbin case screw needs a turn, check how the bobbin was wound. A badly wound bobbin can cause tension trouble that has nothing to do with the case. The thread may pile up on one side, or the bobbin may be wound too loose.

Rewind the bobbin with care. Guide the thread evenly by hand if your machine's winder doesn't do it well. Then test again before you change anything else.

Testing and adjusting bobbin tension

Once you rule out the winding, use the drop test. Hang the threaded bobbin case by its thread and give it a gentle bounce. It should drop an inch or two and hold.

This tells you whether the screw is really the problem. If it is, turn the small screw in tiny steps. Test with the drop test and a real seam after each change, since this screw has a narrow working range.

Common bobbin thread tension mistakes

• Assuming every underside problem is bobbin tension, when uneven winding or incorrect insertion are more common causes.
• Mixing bobbin thread types or weights between the top and bobbin, which can throw off the balance even with both individually well-tensioned.
• Skipping the drop test and jumping straight to adjusting the screw based on how a seam looks alone.
• Using a bobbin wound on a different machine, which may behave slightly differently on yours even if it looks fine.

When to leave it alone

Say your bobbin tension has stayed steady over many projects. Then a new problem shows up just after you change fabric or thread. The more likely cause is the new fabric or thread acting differently with the same setting.

The bobbin tension itself probably hasn't drifted. Try the fabric and thread checks first: a fresh needle, the right needle type and good, even thread. Don't disturb a bobbin case setting that has been working well.

Bobbin case type matters too

Some machines use a removable bobbin case with a screw you can reach. Others use a drop-in bobbin built into the machine. On those, tension works differently, or you are not meant to adjust it at all.

Check your manual to see which system you have. Don't assume the drop test and screw steps apply to every machine. Trying to adjust a part that isn't built for it can cause more problems than it solves.

Keeping a spare, pre-tensioned bobbin case

If your machine uses a removable bobbin case, keep a spare case at the factory setting and never touch its screw. It gives you a reliable reference whenever you wonder if your working case has drifted. Swap in the spare and test. If it behaves differently, you know the working case's screw is the cause. It is not the thread, the fabric or the top tension.

Sewing machine bobbin thread tension by bobbin type

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On a drop-in system, the most common bobbin-side mistake is missing the small tension slot when you drop the bobbin in. Most drop-in holders have a groove the thread must slide into. Arrows on the cover plate often show it. Pull the thread through that groove until it stops. Then check the stitch again.

A quick bobbin thread tension checklist

1. Bobbin is the type your manual lists, not only one that fits.
2. Thread is wound evenly and firmly, with no loose outer layer.
3. Bobbin unwinds in the direction the case or cover plate shows.
4. Thread sits in the tension slot or under the spring.
5. Case area is free of lint.

Work down this list before you touch any screw.

Related reading

Bobbin Tension on a Sewing Machine covers the drop test and the screw in more detail. Sewing Machine Thread Tension explains how top and bobbin tension work together.