tips and tricks for twin needle hemming

Have you ever tried making garments for your kids, or even for yourself, using knit fabrics? Recently, there's been a ton of inspiration for t-shirt sewing, and I've realized that many of the tutorials and pattern modifications I've shared in the past involve sewing with knits. Today, I want to share my personal journey with twin needle hemming. It's part sewing evolution story, part cautionary tale, and hopefully, a bit useful. Feel free to share any tips or tricks you might have in the comments below!

When I first started sewing knit garments for my kids, I used a zigzag stitch or experimented with a sewing machine lettuce hem. At the time, I was mostly satisfied. But deep down, I dreamed of achieving that professional-looking hem that resembled store-bought t-shirts.

I heard about twin needles and decided to investigate whether my sewing machine could handle them. After confirming that pretty much any sewing machine can use twin needles, I purchased one—just a single twin needle to start. Eager to try it out, I went home and immediately broke my brand-new twin needle because my sewing machine was still set to zigzag mode from my previous project. Lesson number one learned: always adjust your settings before diving into something new.

Twin needles come in various widths and types. While I probably only need stretch-specific twin needles, I have to admit I’m a bit casual about needle types. As a result, that universal needle somehow found its way into my collection. I usually opt for a 4mm twin needle for kids' garments and a 6mm for adult garments, simply because it feels right to me.

After purchasing another twin needle (and a spare), I finally got around to stitching my first double hems. From the front, they looked fantastic:

However, from the back, things weren’t quite perfect.

It wasn’t until after a few wears that I noticed the hems were starting to break. The bobbin thread would snap under strain, and all the needle threads would “pop.” Realizing that for the hem to stretch properly, the bobbin thread needed to zigzag more to allow for potential lengthening, I decided to increase my needle thread tension.

As you can see in the images below, increasing the needle thread tension affected the bobbin thread:

While this adjustment allowed the bobbin thread to zigzag nicely, the needle threads drew together, creating a tunnel-like effect. Here are the same three hems viewed in profile. The increased needle tension resulted in a lumpy seam finish, as shown on the right.

Despite these adjustments, the hems were still breaking. While this happens with store-bought garments too, I wasn’t satisfied with my one-wear hems. I turned to the forum for advice, and Liesl introduced me to Woolly Nylon.

Woolly Nylon is a strong, stretchy multi-filament nylon thread that’s available in a few colors. You can probably get away with just black and white, or maybe even gray. By using Woolly Nylon for the bobbin thread, you achieve a stronger, stretchier double hem. However, Woolly Nylon has more “drag” than regular thread, which made the tension issues worse.

I figured that since I couldn’t raise the needle thread tension any further (and I didn’t want to because of the tunneling issue), I’d have to lower the bobbin thread tension. How you do this depends on your machine, but I’m confident it’s possible for every machine. Some machines might have fancy bobbin tension dials. If you have one, dial down the 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 out the bobbin casing, you’ll notice a little screw that adjusts the tension.

I was hesitant to tamper 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 and dialed it all the way out to “super loose.” Thankfully, my new super loose bobbin casing doesn’t have the red arrow of my original, so I can easily distinguish between them. A bit of nail polish or correction fluid would’ve worked if they looked identical.

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

You can see here that I’m using cotton lycra, a fabric known for its unruly edges. It’s fantastic for kids’ clothes, but hemming unfinished edges can be a nightmare.

At this point in my sewing journey, I was completely hooked on knitting garments for my kids (no ironing required!) and had recently acquired an overlocker. Finishing the fabric edge with the overlocker made it lie flat, behave better, and added some stability to the fabric, helping to prevent tunneling.

I thought I had twin needle hems mastered until I started sewing more for myself. What worked on cotton lycra wasn’t working with lightweight jersey fabrics.

To prevent tunneling when sewing lightweight knit fabrics, you'll need to stabilize the fabric further. Overlocking helps, but a thin strip of knit interfacing, fusible webbing, or a wash-away hem stabilizer will work wonders. For situations where maintaining the fabric’s full stretch is crucial, the wash-away hem stabilizer is the best product for the job. Buy larger sheets and use a ruler and rotary cutter to cut it into 1/4" strips to make it cost-effective.

Here’s a rayon jersey that’s 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.

Swapping out my bobbin casings and switching to a walking foot only takes me a minute. When I remember to, I also switch to a walking foot. This helps prevent the fabric from stretching out as you sew and stops me from getting stuck on the side seam bumps. I’m not convinced it’s essential for hemming knits, but it certainly helps.

Sometimes the needle threads get a bit twisted and knotted together. If you're going to hem a dozen t-shirts in a row, you might have to stop and rethread your needles halfway through. I’ve definitely noticed that using cheap overlocker-type thread for the needle threads leads to a hot, knotty mess quite quickly. Always stick to good-quality threads for the needle threads.

Does it make any difference which direction the two needle threads unwind in? Honestly, I don’t think it does. But if things are getting knotty, you won’t hurt anything by flipping over the second thread spool. I’ve never remembered to make a note to track any statistically significant differences...

I’m happy to report that when I rummaged through 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 handed down, becoming someone else’s mending problem!

Shelley.

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