Understanding Thread Sizing, Construction, and Materials

Understanding Thread Sizing, Construction, and Materials

There is a lot of good information about fly tying out there. Between books, magazines, and the Internet, there is no shortage of adequate instruction and advice on almost all aspects of fly tying. Except for the apparent stepchild of fly tying: thread. Much of what's been written about this important tool is false or misleading.

Note that I used the word "tool" rather than material. In my mind, thread is a tool chosen to perform a specific task. I own no less than 1,000 spools of thread in a huge range of sizes, colors, and configurations, and while every one of them is good for something, none are good for everything. A large selection lets me choose the right tool for the job in every instance. You don't need 1,000 spools of thread, but hopefully I can help clear up the details of why threads are different, and what those differences mean to you.

Keeping it Together

Before I get started on thread differences, let's talk wax. Almost all tying thread comes from the factory waxed, and if you want unwaxed thread you'll have to use silk, GSP, Kevlar, or a specialty thread like unwaxed 3/0 Danville's Monocord. Waxed thread is not a substitute for dubbing wax, and is applied to the thread in the manufacturing process merely to keep the thread from fraying and unwinding wildly when it breaks.

Size is another major source of confusion for many consumers. Historically, fly-tying thread was sized using the archaic aught system that came originally from sizing silk surgical sutures, and is still used today for that purpose. Using a zero as a baseline and additional zeros to denote smaller sizes, the aught number described relative size based on a range of diameters and worked nicely in an age of organic materials where exactness was not always possible. Many tying threads are still measured this way today, as an example, 000000 or 6/0 (six aught) being smaller than 000 or 3/0 (three aught) thread.


The Danville Chenille Company used this system for tying thread for 50 years with no issues, but in the 1990s, UNI Products entered the tying market. While UNI also used the aught system to distinguish its larger and smaller threads, the company used a different baseline diameter, and that's where everything went to hell. Using the aught system, tiers assumed that the new 8/0 UNI-Thread was smaller than 6/0 Danville's. And 6/0 UNI-Thread seemed much stronger than 6/0 Danville's. Despite the numbers on the spool, those comparisons weren't fair because the two threads aren't the same size at all.

Wapsi Fly Company moved away from the confusing and archaic aught system when it introduced UTC Ultra Thread, and used the thread industry denier standard of measurement. Denier is the weight, in grams, of 9,000 meters of thread. It's a physical actual weight for a length of thread. It says nothing of the thread configuration, material, or strength. UTC Ultra Thread comes in 70, 140, 210, and 280 deniers, and simplified thread sizing to a significant degree. It's pretty simple math to figure out that 140-denier thread is twice as heavy as 70-denier thread, 210 is three times as heavy, and so on. Using the denier system retroactively tells us that 8/0 UNI-Thread is 72 denier, 6/0 Danville's Flymaster is exactly 70 denier, and surprisingly, 6/0 UNI-Thread is 135 denierno wonder it seemed so much stronger; it's almost twice as heavy!

There is no constant baseline used by all manufacturers to clearly and accurately label their threads. As tiers, we should all know that 70-denier UTC Ultra Thread, 6/0 Danville's Flymaster, and 8/0 UNI-Thread are all about the same weight, but we should also know that these three threads are still very different because of both the materials used to make them, and the configuration (round or flat when viewed in cross section).

Most fly-tying threads are made of nylon or polyester. Polyester is slightly heavier than nylon and has a higher denier for a length of the same diameter. Polyester is also a bit stronger. There are also threads made of other materials like gel-spun polyethylene (GSP), Kevlar, silk, and monofilament, but those are for very specific usesmost general-purpose threads are nylon or polyester.

Danville's and UTC Ultra Thread are both made of nylon. Nylon has a fair amount of stretch, (25 to 30 percent), naturally lies flat, and has a silky smooth finish and a glossy sheen. Nylon threads allow dubbing to slide more than polyester threads and while still perfectly usable for dubbing, you should expect poorer adhesion to a slicker thread.

Both of these threads are flat and wrap on the hook like a ribbon. They are configured with multiple strands held together with a binder strand twisted around the center strands to hold the thread together. UTC Ultra Thread lies flatter on the hook because it is held together with a binder strand that twists around the main core at a rate of only one revolution per inch. This open twist allows the thread to spread out and lie flat on the hook shank. Danville's has a binder strand with more revolutions than UTC Ultra Thread so it doesn't lie quite as flat.


These flat threads are particularly useful for thread-body flies or flies requiring a smooth underbody, as they create silky, seamless tapers and bases. I like both of these threads for Black Beauties, Copper Johns, and Humpys. In the case of the Black Beauty and Copper John, this is because the smooth texture and low bulk allow me to build smooth bodies and underbodies without ridges. Conversely, my friend Jay Zimmerman prefers 8/0 UNI-Thread for the Black Beauty, as he believes the ridged thread body holds the wire rib in place more securely.

In the case of the Humpy, I find these threads to be perfect for building both the upright divided wings with flattened thread to keep them from flaring, and then building the thread hump to create a vibrantly colored, smooth underbody. I twist both of these threads into a cord (by spinning the bobbin) when I first attach the hair to a hook, as I find round thread grabs, cinches, and compresses hard hair better than flat thread.

I like larger thread—140-denier UTC Ultra Thread, 6/0 UNI-Thread, and good old 3/0 Danville's Monocord (rated at 140, 135, and 116 denier, respectively)for streamers and saltwater flies. I find the slightly narrower nature of 3/0 Danville's to be particularly useful for streamers like the Baby Gonga and Dirty Hippy, but again, the colors here are somewhat limited and muddy.

UNI-Thread 6/0 is similar to Danville's 3/0 and I use it in many of the same instances. It is one of my favorite threads for flies like Crazy Charlies, as the round shape bites into the D-Rib overbody as well as the hard calftail wing for a more secure tie-down. Medium saltwater flies like my Ragin' Craven and Flip Flop use 140-denier UTC Ultra Thread as there is space for extra bulk on larger patterns, and this flatter thread creates a smooth, clean profile around the weighted eyes.

I only use the largest sized UTC threads, 210 and 280 denier, on flies that require quick and heavy thread coverage like the Pigsticker, or for the hot head on Cliff Watts's Kilowatt. In these cases, thread that builds and covers quickly and smoothly is paramount, but otherwise these threads are, quite frankly, overkill.

Polyester UNI-Thread has less stretch (15 percent), is more roughly textured, and sits more rounded or corded on the hook. The rougher texture holds dubbing particularly well. Colors are typically a bit more muted and subtle in polyester than they are in nylon, with more of a matte and less shiny finish.

UNI-Thread can be flattened slightly by diligently unwinding it or wrapping the thread left-handed. Yes, you read that right. Most threads are twisted such that as a right-hand tier wraps them around the hook they contribute to the factory twist in the thread. Left-hand tiers enjoy an unintended advantage in that as we wrap these threads we unwind them . . . causing them to lie much flatter than when wound by our right-handed friends.


UNI-Thread is a bonded thread that is reinforced with multiple binder strands that twist around the core strands at a much higher rate to produce the rounded shape. Because of that shape, UNI-Thread tends to build bulk vertically on the hook shank rather than horizontally as nylon threads do. The round cross section coupled with less stretch and a more textured surfaceof this polyester thread allows for a bit more "bite" and "grab" to the thread in tying applications. Round thread bites into and flares hard hairs like deer, elk, and moose better than flattened thread does.

The rougher texture of polyester threads also holds dubbing better than the slicker nylon. Because of this and my inherent left-handedness, I like 8/0 UNI-Thread and use it on many of my trout patterns. Its ability to be flattened or twisted into a cord and the harder nature of polyester versus nylon make it a great choice for smaller flared hairwing flies like a Compara-dun or X-Caddis or anything made predominantly out of dubbing. My biggest gripe with UNI-Thread is the colors are not as vibrant as the nylon threads, but that's simply a matter of my preferences . . . the fish don't seem to care. I should mention here that 14/0 Gordon Griffith's Sheer (72 denier) thread is made of polyester as well, but is constructed more like 6/0 Danville's, striking a nice compromise between a hard, round polyester thread and a flatter nylon.

Veevus is a new brand that has recently come on the scene and become quite popular. It is the only modern era thread that is unwaxed. All of the Veevus middle-sized threads (8/0, 10 /0, 12/0 and 14/0) are built in a conventional fashion with a bit of twist but with the ability to be flattened beautifully by unwinding, while both their largest and smallest sizes (6/0 and 16/0) are built of just two intertwined strands in a counterclockwise twist. These threads also flatten or cord up nicely and in the case of the 16/0, create negligible bulk when tying. Both of these two sizes lend themselves wonderfully to split-thread dubbing, in case you're into that sort of thing. Oddly, a little bit of online research shows that Veevus 6/0, 8/0 and 10 /0 threads are all rated at 110 denier, and the 12/0 and 14/0 are rated at 70 denier with the 16/0 rated at a fine 50 denier. I cannot for the life of me figure out how they have managed to make distinctly different sized threads out of a material of the same denier, but they have. The 12/0 and 14/0 threads are clearly different sizes. They lie flat or cord up at the tier's whim, they have surprisingly good strength, and the colors are more than adequate for all the tying I do. 

I just wish I could give you a straight answer as to how they make the stuff but, after an extensively long e-mail string with the head of the company, they either don't know, or don't want us to know! Their advice was not to concentrate so much on the technical aspects of the thread but to instead buy a few different sizes and see what works best for your own tying. This isn't bad advice, although I must say it wasn't very helpful in writing an article about these technical differences.

Specialty Threads

GSP (gel-spun polyethylene) threads are super slick, strong for their size, have very little stretch (3 percent), and lie flat on the hook. They are typically used for spinning deer hair or for synthetic materials that call for an abnormally strong yet small-diameter thread. GSP thread holds up well to toothy fish and creates durable flies. The slickness can be a liability in other applications however, particularly dubbing retention, and the dye used to color them is not as stable on GSP thread as it is on other materials, so I only use white.

Kevlar was sort of a precursor to GSP. It lies quite flat, is incredibly strong, and comes in just one size (200 denier). It is most often used in larger spun deer-hair patterns like bass bugs. Dye is not stable in this material either, and can run when you apply head cement, so I only use this thread in its natural color of pale yellow. There's nothing worse than tying a perfect white bass bug, and watching the thread leach dye into it when you add a final drop of head cement.

These two specialty threads are basically interchangeable. GSP thread is much slicker, and both can be hard to cut with your tying scissors. I've heard horror stories about the abrasiveness of both these threads eating bobbin tubes, but after almost 40 years of tying I've never seen it. GSP thread does come in a wider range of sizes, from 30 denier up to 200.

Monofilament threads are just what they sound like: fine, round, single-strand nylon monofilamentjust like tippet material. Use mono threads when you want the thread to disappear under a coat of epoxy or head cement when you are tying Surf Candies or EP Minnows. The single-strand construction leads to excessive twisting in the tying process and they must be managed to control and unwind this twist while you use them. Mono threads are not particularly strong either.

Silk was the first thread used to tie flies and is still wonderful stuff. Two twisted strands are used to create this thread and silk can easily be untwisted to flatten or twisted as you see fit. It has fallen by the wayside for many reasons, mostly because it's not very strong and doesn't hold its true color when wet. It's still useful in historic applications and I use it for soft-hackled patterns as well as ribbing in some cases, but as a general tying thread it has been surpassed.

After all this guidance, it's still useful to buy several different kinds of threads and sizes and see what suits you and your tying best. While a more consistent and understandable and consistent sizing system would be a lovely advancement, it's not likely to happen.

If you're now a true "thread nerd" and want an even deeper understanding of thread sizing, materials, and breaking strength, check out Martin Joergensen's fantastic thread table on the Global Flyfisher website (globalflyfisher.com/tie-better/fly-tying-thread-table). I use this chart for a reference to the denier sizes and diameters of all thread brands and find it invaluable.

Charlie Craven co-owns Charlie's Fly Box in Arvada, Colorado, and is the featured tier in two Fly Fisherman DVDs: Warmwater Fly Tying and Saltwater Fly Tying. His latest book is Tying Nymphs: Essential Flies and Techniques for the Top Patterns, available from Stackpole Books/Headwater Books (2016).

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