Understanding Fly Tying Thread Sizes and Types A Guide for Fly Tyers
Monday 9th September 2024.webp)
Fly tying is a craft that requires precision, creativity, and a solid understanding of materials. One of the most important—and often most confusing—elements of this craft is choosing the right thread. The thread you select plays a crucial role in the durability, appearance, and effectiveness of your flies. Given the many sizes and types of thread available, it’s no surprise that many anglers are confused.
I frequently get asked about thread sizes and how they vary between brands and materials. To complicate things further, the industry lacks a universal standard, and the two main systems of measurement—the Aught and Denier—can be frustrating to navigate. In this guide, I’ll break down the key aspects of fly tying threads to help you make informed decisions and improve your fly-tying game.
The Importance of Choosing the Right Thread
Strength vs. Fineness: When selecting a thread, you’ll need to balance strength and fineness. Thicker threads offer more strength but can make your fly bulkier, which might not suit certain types of flies. Finer threads give you more control over the delicate details but require a lighter touch to avoid breakage. The key is matching the thread to the specific fly and materials you’re working with.
Durability: The longevity of your flies depends on the durability of the thread. If your thread frays or breaks easily, your fly will fall apart after only a few uses. Using strong, high-quality thread ensures your flies stay intact through multiple casts and catches.
Understanding Thread Sizes
The Two Main Systems
Fly tying threads are sized primarily through two systems: the Aught System and the Denier System. Each has its own set of rules, and unfortunately, neither system is foolproof when applied across different brands or materials. Let’s explore both in detail.
The Aught System
The Aught system (e.g., 6/0, 8/0) is the more traditional sizing method. At first glance, it seems simple, but it can be misleading. Contrary to what you might think, lower numbers mean thicker threads—so 3/0 is much thicker than 8/0.
Think of the Aught system as a baseline: 0 represents the base thread size, and additional zeros indicate thinner threads. Thus, 3/0 thread is one-third the thickness of this mythical zero, and 6/0 is half as thin. The more zeros, the finer the thread.
The problem? There’s no industry-wide baseline for what this "zero" should be, leading to variations between brands. For example, UNI’s 8/0 thread (which is waxed polyester) lays fairly flat on the hook but can still cause a small bump. It has a breaking strain of 16 ounces. Meanwhile, Veevus 8/0 thread is a two-strand polyester with a breaking strain of 32 ounces—double that of UNI’s 8/0 thread. Clearly, both are labeled as 8/0, but their strengths and characteristics differ significantly.
The Denier System
The Denier system offers more consistency. It measures the weight of 9,000 meters of thread in grams, so lower denier numbers indicate finer threads. For example, 70 Denier thread is much finer than 210 Denier thread. While it’s a more reliable indicator of weight and thickness, it still varies depending on the material. For instance, 70 Denier silk is much finer than 70 Denier nylon.
To give you a rough idea, common thread sizes in the Denier system are:
- 70 Denier: Suitable for small dry flies and midges (roughly equivalent to 8/0).
- 140 Denier: A versatile option for nymphs and dry flies (similar to 6/0).
- 210 Denier: Best for larger flies like streamers (equivalent to 3/0).
However, even these conversions aren’t exact. For instance, UNI’s 8/0 thread is rated at 72 Denier, giving it slightly more strength than UTC’s 70 Denier thread.
