Fly Tying Thread: Materials, Sizes, and Applications
Understanding thread is critical to your success at the vise. Yet, with so many options available today, choosing the right thread can be confusing. This guide will break down the basics of fly tying thread - from sizing systems to materials to practical applications.
Understanding Thread Sizing Systems
Thread Sizing
There are two main sizing systems used in fly tying: the "aught" system and the denier system.
The Aught System
The aught (or “0”) system originated from the way surgical silk was historically sized. The more zeros there are, the smaller the thread diameter. For example, an 8/0 thread is finer than a 6/0 thread of the same brand and thread type. Danville was one of the first manufacturers of fly-tying specific threads, and it used the aught system, making it familiar to many. While other thread manufacturers have adopted this system, there is little consistency in the actual size between brands, making comparison difficult.
The Denier System
More recently, companies like UTC popularized the denier system in fly tying, which provides a more standardized measurement. Denier measures the weight in grams of 9,000 meters of thread. For example, 70 denier thread weighs 70 grams per 9,000 meters. This provides a standard unit for comparing threads from different manufacturers.
However, different materials (nylon vs. polyester) can have the same denier but feel and perform differently due to their inherent properties. A 70-denier nylon thread and a 70-denier polyester thread will not perform the same. Therefore, it is essential to consider the material when comparing threads using the denier system.
Thread Materials and Properties
Nylon Threads
• Generally lay flat like a ribbon
• Have about 25-30% stretch
• Build bulk horizontally on the hook
• Examples: Danville, UTC
• Great for thread bodies and smooth underbodies
Polyester Threads
• Generally have a rounder profile (though some are flat)
• Only about 15% stretch
• Build bulk vertically on the hook
• Slightly coarser than nylon of the same denier
• Examples: Uni-Thread (round), Semperfli Classic Waxed Thread (flat)
Specialty Threads
GSP (Gel Spun Polyethylene)
• Ultra-strong for its size
• Slick
• Doesn't hold dye well
• Example: Semperfli Nano Silk
• Great for small flies and applications requiring strength
Kevlar
• Very strong
• Flat profile
• Limited in size options (200 denier)
• Better grip than GSP for deer hair
• Natural yellow color holds up best with head cement
• Ideal for bass bugs and large patterns
Monofilament Thread
• Clear tippet material
• Single stranded
• Prone to twisting
• Excellent for resin/epoxy heads where transparency is desired
• Primarily used in saltwater patterns
Practical Tips and Considerations
Thread Twist
Right-handed tyers add twists as they wrap. Flat threads show less obvious twist issues than round threads. Manage twist by spinning your bobbin when needed.
Waxed vs. Unwaxed
Most fly-tying threads come waxed, but this isn't for dubbing purposes. The wax helps prevent the thread from fraying and coming unwound when broken or cut, but you'll still want to use dubbing wax with dubbing.
Choosing the Right Thread
Keep in mind that threads from different manufacturers are difficult to compare. A 6/0 thread from one manufacturer may differ significantly in diameter from another's 6/0. On the other hand, even within the same brand, designations of different sizes might share the same denier rating.
When selecting thread, consider the following:
1. The size of the flies you're tying
2. How much thread work is involved
3. Whether you need strength (e.g., spinning deer hair) or delicacy (small dries)
4. If you’re doing thread bodies or just structural wraps
5. Your tying style and personal preferences
Most fly-tying threads can handle a variety of tasks well, but each has its strengths and weaknesses. Experiment with different brands, materials, and sizes to find what works best for you. The key is finding what works best for your style of tying and the patterns you often tie.
