If you're stepping up your game with heavy-duty projects like leather gear or thick upholstery, you've likely realized that 138 bonded nylon sewing thread is the heavy hitter you need in your bobbin case. It's one of those materials that, once you start using it, you wonder how you ever got by with the thin stuff. Whether you're a hobbyist making custom knife sheaths or a professional trimming out a car interior, this specific thread size hits a "sweet spot" that balances extreme durability with a look that actually pops on the finished piece.
But what makes it so special, and why does everyone seem to recommend it for the tough stuff? Let's break down why this thread is a staple in workshops across the world.
What Does "Bonded" Actually Mean Anyway?
If you've ever tried to sew with cheap, unbonded thread at high speeds, you know the frustration of it "bird-nesting" or fraying into a million tiny fibers before it even hits the fabric. That's where the "bonded" part of 138 bonded nylon sewing thread comes into play.
Basically, the individual plies of the nylon are treated with a special resin coating. This coating acts like a sort of "glue" that holds the strands together, preventing them from untwisting or fraying as they pass through the tension discs and the needle eye. When your sewing machine is humming along at high RPMs, the needle gets hot—really hot. The bonding process helps the thread resist that heat and friction, so it doesn't snap right when you're in the middle of a perfect topstitch. It makes the thread feel a bit stiffer than your average garment thread, but that stiffness is exactly what gives you those crisp, clean stitches.
Understanding the Size: Why 138?
In the world of industrial sewing, the numbering systems can get a little confusing. You'll see terms like Tex, Denier, and Commercial sizes thrown around. In this case, "138" is the commercial size, which is roughly equivalent to a Tex 135.
To put it in perspective for someone used to home sewing, this thread is thick. It has a breaking strength of about 22 pounds. Think about that for a second—you could practically hang a small sledgehammer from a single strand of this stuff without it snapping. It's significantly beefier than size 69 (which is the standard for light upholstery) but not quite the "rope" that size 207 or 346 is.
Choosing the Right Needle
You can't just shove 138 bonded nylon sewing thread into a standard household needle and hope for the best. It's too thick. If you try, the thread will get stuck in the needle's groove, causing skipped stitches or shredded thread. For size 138, you're generally looking at a needle size between 21 and 23 (or 130 to 160 in metric).
Using the right needle ensures the hole in your material is big enough for the thread to pass through comfortably, while also allowing the "knot" of the stitch to sit securely inside the material rather than sitting on the surface.
The Best Projects for 138 Thread
So, where does this thread really shine? It's usually overkill for a shirt or a light cotton bag, but for anything that's going to take a beating, it's perfect.
Leatherworking
If you're into leathercraft, size 138 is a dream for wallets, belts, and holsters. It provides that chunky, rugged look that people associate with hand-crafted leather goods. It's also incredibly strong, which is vital for items like knife sheaths or tool pouches that have to withstand constant pressure and sharp edges.
Automotive and Marine Upholstery
Car seats and boat cushions take a lot of abuse. People are constantly sliding in and out of them, and the seams are under constant tension. 138 bonded nylon sewing thread is the industry standard here because it can handle the stress. However, a quick tip: if you're doing a boat cover that's going to sit in the direct sun 24/7, you might want to look at bonded polyester instead (nylon is strong, but UV rays can weaken it over several years). But for interior upholstery? Nylon is king.
Tactical Gear and Webbing
Making your own backpacks, slings, or dog collars? This thread is exactly what you want. It bites into nylon webbing perfectly and creates a bond that's often stronger than the fabric itself. When you're sewing multiple layers of heavy Cordura, you need a thread that won't give up under load.
The Pros and Cons of Working with Nylon
Everything has a trade-off, and 138 bonded nylon sewing thread is no exception. It's important to know what you're getting into before you load up your machine.
The Upside: Strength and Elasticity
The biggest "pro" is obviously the strength. But nylon also has a bit of "give" to it. It's slightly elastic, which is actually a good thing for seams that need to flex. If you're sewing a leather bag that gets stuffed to the brim, that little bit of stretch helps the thread absorb the shock instead of just snapping or tearing through the leather holes.
The Downside: The UV Issue
I mentioned this briefly, but it's worth repeating. Nylon doesn't love the sun. If you're sewing something that will live outdoors permanently—like a patio umbrella or a sail—nylon will eventually get brittle and fail after a few years of UV exposure. For those specific cases, bonded polyester is usually the better call. But for almost everything else, nylon's superior strength makes it the better choice.
Tips for Sewing with 138 Bonded Nylon
If you're new to using heavier threads, there's a bit of a learning curve. Don't get discouraged if your first few stitches look a bit wonky. Here are a few things I've learned the hard way:
- Check Your Machine: Honestly, most home sewing machines will struggle with 138 thread. They just aren't built for the tension or the needle size required. You really want an industrial walking foot machine to get the best results.
- Tension is Key: Because the thread is thick and bonded, you'll likely need to crank up your top tension. You might also need to adjust your bobbin case tension. Always do a few test scraps of your actual project material first.
- Watch the Spool: Industrial thread usually comes on large cones. If your machine isn't designed for these, don't just set it on the floor. Get a cheap standalone thread stand so the thread can pull off the top of the cone smoothly.
- The "Lighter Trick": Since it's nylon, the ends of the thread will melt. When you finish a seam, leave a little tail, pull it to the back, snip it, and carefully singe it with a lighter. Press the melted "bead" down with your finger (careful, it's hot!), and your seam will never unravel.
Why Quality Matters
It's tempting to grab the cheapest spool of 138 bonded nylon sewing thread you find on a random discount site, but that usually leads to headaches. High-quality thread has consistent bonding and a smooth finish. Cheap thread often has "thick and thin" spots or clumps of bonding resin that will clog up your needle and mess with your tension.
Investing in a reputable brand might cost a few dollars more, but it'll save you hours of re-threading your machine and picking out bad stitches. Plus, when you're puting hours of work into a project, the last thing you want is for the thread to be the weak link.
Final Thoughts
At the end of the day, 138 bonded nylon sewing thread is one of those tools that transforms the quality of your work. It takes your projects from looking "homemade" to looking "professional." It's tough, it's reliable, and it has a beautiful finish that highlights the hard work you put into your seams.
Whether you're repairing a heavy tarp or crafting a custom leather saddle, having a few spools of this thread in your kit is a total game-changer. Just make sure your machine can handle it, get your tension dialed in, and you'll be amazed at the results. Happy sewing!