Understanding Fly Tying Thread Sizes

Understanding Fly Tying Thread Sizes
Fly tying thread comes varying sizes based on breaking strength and size, or denier. The thread size has a direct correlation with the size of the fly being tied. Streamers on size 2 and 4 hooks need larger 3/0, big fly or gel-spun thread, while dry flies on size 18 and 20 hooks need 8/0 or smaller thread to avoid overcrowding the hook. Although thread size is relative to breaking strength, most small threads have high breaking strength. Small threads have various functions; larger sizes are ideal for securing bulky materials to a hook.


Difficulty: Moderately Easy

Things You’ll Need:
  • Size 10 hook
  • Fly tying vise
  • 8/0, 6/0, 3/0 and big fly thread
  • Deer hair
  • Hair stacker
  • Razor or scissors
  • Dry fly hackle
  • Bucktail
Step 1
Place a size 10 hook in your fly tying vise. The hook will be used to demonstrate the strength of the fly tying thread while you become comfortable using different thread sizes for different materials.
Step 2
Wrap some 8/0 tying thread on the hook, and stack a bunch of deer hair in your hair stacker. Trim the butts of the deer hair, and pinch the hair between your thumb and index finger. Place the deer hair over the top of the hook, and make two loose wraps with the thread. Make a third wrap, and pull the thread tight to spin the deer hair. The thread is likely to break, thus demonstrating that 8/0 is not strong enough for heavy applications. Repeat the process with 6/0, 3/0 and big fly thread. You will find that the 6/0 has questionable strength, the 3/0 is functional and the big fly is bulky.
Step 3
Use scissors or a razor to strip the hook until it is bare. Wrap a section of 8/0 thread on the head of the hook shank. Tie dry fly hackle near the head of the fly. Wrap the hackle, and tie it off with the thread. You will find that the 8/0 is ideal for securing delicate hackle to the hook. Repeat the process with 6/0, 3/0 and big fly thread. The 6/0 will work fine on size 10 to 16 hooks but is too large for smaller hooks. The 3/0 can be used for large dry flies like hoppers and stoneflies but is too large for most applications, and the big fly thread is too bulky for working with hackle.
Step 4
Use scissors or a razor to strip the hook bare. Wrap a section of 8/0 thread over the hook shank. Attach a bunch of bucktail near the head of the fly in a streamer fashion. You will find that the 8/0 thread is functional but that its strength is questionable and numerous wraps are required to secure the material. Repeat the process with 6/0, 3/0 and big fly thread. The 6/0 requires numerous wraps but is functional, while the 3/0 only requires a few wraps and can handle more tension. The 3/0 may seem ideal for most applications, but for large size 2 and 4 flies the big fly thread will be better for securing large bunches of material.

Tips & Warnings

The more flies you tie, the easier it will become to recognize the necessary size of fly tying thread for each fly pattern. Before you begin tying a pattern, consider the size of the hook and the materials you will be using.
Thread most commonly breaks because it hits the hook point while the fly is being wrapped, so avoid contact between the thread and the hook point. Any contact made will compromise the strength of the thread.

Article Written By Zach Lazzari

Zach Lazzari is an outdoor writing specialist. He has experience in website writing as well as standard newspaper writing. He wrote an outdoor column for the Silver World in Lake City, Colo., and articles for Colorado-mountain-adventure.com. Lazzari is currently completing his bachelor's degree online through Arizona State University and lives in southwest Montana.

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