Bobbin’ Along – bobbin tension

You may have noticed in the previous post about tension that we didn’t even test sew the machine.

Yet.  We will test sew it, but not just yet.  You see what I’m trying to do is get the machine to a point where it’s in the ballpark first, then the rest is just small tweaks.  The goal is to help you rule out the big problems, the ones that require repair, or in drastic situations, possibly a trip to the sewing machine spa.

Consider this statement:

Tension too tight on the top can also be tension too loose on the bottom.
Top Tension too tight does not automatically mean bottom tension is too loose.
Tension too loose on the top can also be tension too tight on the bottom.
Top Tension too loose does not automatically mean bottom tension is too tight.

Very often we’re taught to only adjust the top tension and “DON’T TOUCH THAT!” for the bobbin tension.

Tension is about balance.  This is why it’s referred to as balancing tensions.  If there are two people on a see saw, if one is heavier, and the other is lighter, the lighter one is going to be in the air no matter what.  We need to balance the see saw so both can enjoy the ride.

When tensions are balanced, the stitch lays flat and looks nice.  It locks inside the fabric, and there is no – or minimal – top thread showing on the bottom, and no bottom thread on the top.

Note: with zig zag and decorative stitches, it’s fine for the top thread to show on the bottom.  This is normal.  What’s not normal is tunneling.  This is a puckering of the fabric in a “tube” that runs the length of the stitches.  This is a sign that though the tension may be “right” (i.e. top thread shows on the bottom some, or not at all, and the bottom thread doesn’t show on the top), it’s not balanced.  In most cases, it’s too tight in both the needle and bobbin tensions.

UGH.  You mean… the bobbin case needs to be adjusted? Yep.  It’s not as scary as it sounds, and no one’s going to yell at you.  I may applaud you though. 🙂

First, I want you to look to make sure that the bobbin case is not damaged.  Make sure there are no edges that look flattened (most common on a bobbin case you’d remove to change a bobbin – I’ll refer to these as class 15 style bobbin cases,  but it also refers to the featherweight / 301 bobbin case, the L type cases, the Pfaff cases, and many others you’ll encounter that are removed to load a bobbin into them) and that the bobbin case tension spring doesn’t look broken.   Additionally, make sure that there are no screws missing – there are usually up to 3 on a bobbin case:

  1. The one that holds the “lever” that snaps the bobbin case into the machine.  This is usually on the underside of the bobbin case (covered by the bobbin).  Also, if your bobbin case latch doesn’t snap back and forth correctly, it may be missing the spring that causes it to snap back and forth.  This is usually on the class 15 style cases.

    Here’s the hidden one that most of us miss.
  2. a screw to the far side of the tension spring.  This is simply to hold the spring to the bobbin case.  It will be tight.
  3. A screw to the middle of the bobbin case tension spring.  This one will be loose


There are a number of reasons to adjust bobbin tension:

  1. You’re using a significantly thicker or finer thread than the machine is set up to use.  Most often a sewing machine is set up to use 50wt mercerized cotton thread.
    • If you’re using a 25wt denim thread in your bobbin, there’s a good chance than the bobbin tension is going to be too tight, because the space allowed for a 50wt thread to slip through is too small for a 25wt thread.  The same would happen for a particularly coarse thread.  Clearly adjusting the top tension tighter isn’t going to help the thread in the bobbin pass through the tension spring any better.  This scenario would most likely result in tunneling.
    • Similarly, if you’re using a 70wt thread, you may find that it slips out of the bobbin case without any resistance at all.  The same can happen with very slippery threads. Providing proper resistance on the bobbin thread will result in a much better stitch than simply lowering the upper thread tension to match.  Adjusting the top tension would result in very loose stitches all around.
  2. The person who had the machine before you used a “non-standard” thread and adjusted the case for it.  You need to adjust it back to what you’re using.
  3. Maybe you’re using a 50wt thread, but it’s just too tight, or too loose in the case. This becomes a maintenance issue at this point and it’s one that you can do.  This can occur because thread weights aren’t really standardized.  50wt thread can be 2 strand or 3 strand.  It can be tri-lobed. It can have any number of features that will make it thicker or finer than whatever “standard 50wt mercerized thread” was used to calibrate the machine.

First, let’s cover how to tell if your bobbin thread tension is correct.

There are a few ways to do this without special measuring tools.

  • I alluded to one of ways in the previous article – the way I usually do this is with a 1oz weight. This method works best for the class 66 style bobbin cases (also known as a “drop in bobbin” case – the case must be removed from the machine for this test) but also for any of the class 15 type cases.
    • get a 1 oz weight, and tie it to the tail of a threaded bobbin
    • insert the bobbin into the bobbin case and thread it as though you’re going to sew (don’t put it in the machine at this point.  No needle in the world is going to draw that thread end up through the throat plate!)
    • hold the bobbin case so that the weight can try to draw the thread out of the case.  It should -just- hold it.
  • Another way I do this is using the “drop test”.  This works well for the 15 style bobbin cases.
    • With a bobbin properly threaded in the case, hold the thread end, and gently let go of the bobbin case.  It should drop about 3 -4″ then stop.  This is proper tension.
    • If it just keeps going, the tension is too loose.
    • If it doesn’t come out at all, it’s too tight.
  • The last way I do this is for class 15 style bobbin cases is to put the case in my hand, properly threaded, and pull up.  The case should stand up in your hand and almost leave it, but not quite.  The thread will start to come out first.
  • Finally, Dave McCallum uses a tool he built himself.

Ok, so now you understand how to tell if the tension on your bobbin case needs to be adjusted, so how do you adjust it?

It’s really simple.

On every bobbin case, there’s at least one screw on the tension spring.  The tension spring is the flat piece of metal that you slip the thread under to thread it properly.  You’ll notice that this screw isn’t tight.  This is the way it’s supposed to be.

The tighter this screw is, the tighter the tension on the thread.

If you turn the screw a tiny bit (about a 1/16 of a turn) at a time, it will start to adjust the tension on the thread. Remember “Lefty Loosey Righty Tighty.”  Turn the screw left to loosen it. Turn it right to tighten it.

If you reach full right and the tension is still not right, one of two things have happened:

  1. There’s lint under spring.  This holds it up, and makes the tension looser than it should be.  Take a pin, or a needle, and run it under the spring to push the lint out.  Run the pin from closest to the screw to the end of the spring.  This should be the same direction that the thread travels.
  2. The spring is damaged.  In most cases, this spring can be replaced without replacing the whole bobbin case.

Do be careful when adjusting the bobbin tension, the springs tend to be more brittle than in the past, and I have had 2 of them break in the last few months.  Additionally, if you loosen the screw too much – it may fall out and those little critters are a pain to find when they hop off the desk.  Typically those too can be replaced, but then you have to wait until the new one arrives before you can sew.  It’s just better not to lose them in the first place.

Pics to follow.  I just didn’t have the energy to shoot them tonight.

Does this clear up how bobbin tension works and how and when it should be adjusted? Let me know in the comments below.

8 thoughts on “Bobbin’ Along – bobbin tension”

    1. Hi Patricia! Without having the machine in front of me, it would be hard to detail what everything does. I’d suggest Googling for a free manual or even some bloggers who have the machine. It’s possible they will have shown some of its abilities in their posts. Good Luck!

      1. Marie! Thank you so much for that information! Not having that model in front of me makes it difficult to know how it was configured. I appreciate your input! 🙂

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