tips and tricks for twin needle hemming

I've always enjoyed sewing knit garments, whether for my kids or for myself. Recently, there's been a resurgence of interest in t-shirt sewing, and I realized how many of the patterns and tutorials I've shared previously involve working with knits. As someone who's been through the learning curve of mastering twin needle hemming, I thought I'd share my journey—part evolution, part cautionary tale, and hopefully helpful for those who are diving into this technique.

When I first started sewing knit garments for my kids, I was content with using a zigzag stitch or experimenting with a lettuce hem. While these methods worked well enough, I often found myself yearning for that professional-looking hem you see on store-bought t-shirts. That’s when I heard about twin needles and decided to investigate whether my sewing machine could handle them—and where to buy one.

I discovered that almost any sewing machine can accommodate twin needles, so I purchased my first one. Unfortunately, I made the rookie mistake of forgetting to adjust my machine after completing my initial t-shirt project, which was still set to zigzag mode. My new twin needle promptly broke. Lesson number one learned.

Twin needles come in various widths and types, and while I mostly stick to stretch twin needles, I must admit I’ve been somewhat cavalier about the specific needle types. Hence, my universal needle somehow found its way into the collection. For kids’ garments, I typically use a 4mm twin needle, while for adult garments, I opt for a 6mm needle, simply because it looks proportionate to me.

After purchasing another twin needle (and a spare), I stitched my first double hem. From the front, it looked fantastic:

However, the backside didn't look quite right:

It wasn’t until a few wears later that I noticed the kids' hems were breaking. The bobbin thread snapped under stress, and the needle threads all gave way. Realizing that for the hem to stretch properly, the bobbin thread needed to zigzag more to allow for potential lengthening, I adjusted the needle thread tension accordingly.

As you can see from the images below (left to right), increasing the needle thread tension resulted in a more zigzagging bobbin thread:

While this approach created a nice zigzag effect on the bobbin thread, the needle threads drew together, leading to a lumpy seam finish. Here are the same three hems viewed in profile. The increased needle tension resulted in the lumpy seam finish on the right:

Even with these adjustments, the hems continued to break.

It’s worth noting that this happens with store-bought garments as well, but I wasn’t satisfied with my hem that only lasted one wear. I posted a query on the forum, and Liesl introduced me to Woolly Nylon.

Woolly Nylon is a strong, stretchy multi-filament nylon thread. It’s available in a few colors, but you can likely get by with just black and white—or maybe even gray. By using Woolly Nylon for the bobbin thread, you achieve a stronger, more stretchy double hem. However, Woolly Nylon has more “drag” than regular thread, which worsened the tension issues.

Figuring I couldn’t raise the needle thread tension any further (and I didn’t want to due to the tunneling effect), I decided to lower the bobbin thread tension instead.

How you do this will vary depending on your machine, but I’m confident it’s possible with any machine. Some machines might even have a bobbin tension dial. If you have one, dial down your tension, sew your hem, then dial it back up for regular sewing.

My basic machine has a bobbin casing like this:

Once you remove the footplate and take the bobbin casing out, you’ll notice a little screw that adjusts the tension.

I was a bit hesitant to tinker too much with the screw, fearing I might not be able to reset it correctly for regular sewing. So, I invested in a second casing. I dialed this one all the way out to “super loose.”

Thankfully, my new super loose bobbin casing lacks the red arrow of my original, so I can easily distinguish between the two. A dab of nail polish or correction fluid would’ve done the trick if they had looked identical.

With this setup, I could finally sew a smooth, flat, strong, and stretchy twin needle hem.

As you can see here, I’m working with cotton lycra, a notoriously tricky fabric with its curling edges. It’s great for kids’ clothing, but hemming the unfinished edge can be a real challenge.

By this point, I was thoroughly hooked on sewing knit garments for my kids (no ironing required!) and had acquired an overlocker. Using the overlocker to finish the fabric edges made them lie flat, behave better, and added some stability to prevent tunneling.

I thought I had twin needle hemming mastered, but then I began sewing more for myself. What worked on cotton lycra didn’t translate well to lightweight jersey fabrics.

To avoid tunneling when working with lightweight knits, you’ll need to stabilize the fabric further. Overlocking helps, but adding a thin strip of knit interfacing, fusible webbing, or a wash-away hem stabilizer can make a big difference. For fabrics where maintaining full stretch is crucial, the wash-away hem stabilizer is the best option. Purchase larger sheets and cut them into 1/4-inch strips to make it cost-effective.

Here’s a rayon jersey hemmed with my perfected twin needle hem on the left, and again on the right, but this time with a thin strip of stabilizer underneath.

Switching out bobbin casings and sometimes using a walking foot only takes me about a minute. When I remember to, I swap to a walking foot, which helps prevent the fabric from stretching as you sew and avoids getting stuck on seam bumps. I’m not convinced it’s absolutely necessary for hemming knits, but it certainly makes the process smoother.

Sometimes the needle threads get twisted and knotted together. If you’re planning to hem a dozen t-shirts in a row, you might need to stop halfway through to rethread the needles. I’ve definitely noticed that using cheaper overlocker-type thread for the needle threads results in a tangled mess quite quickly. Always opt for good-quality threads for the upper thread.

Does the direction in which the two needle threads unwind matter? Honestly, I don’t think it does. But if things start getting knotty, flipping over the second thread spool won’t hurt. I just haven’t kept track to see if there’s any statistically significant difference...

I’m pleased to say that when I scoured my kids’ wardrobes looking for a t-shirt with a popped hem to photograph, I couldn’t find any. They’ve all been outgrown and passed on to become someone else’s mending project!

Shelley.

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