The Importance of Thread in a Fly
If the hook is the canvas upon which every fly is crafted, the thread is certainly the glue that holds it all together. Mixed metaphors yes, but accurate none the less. Every bit as important as selecting the proper hook, feather, fur and dubbing is the selection of thread. A fly tying t hread can make or break the construction, durability and appearance of a fly.
There are several factors to consider when selecting a thread for tying flies, such as diameter, strength, material, twist and wax. Once you understand these characteristics, selecting the proper thread to tie a particular fly is easy.
Thread diameter is most often determined by the size of fly being tied. Because each material added to a fly requires thread wraps, the thread size must coincide with the size of the fly. The addition of extra bulk to a small fly by using oversized thread can easily destroy the proportion, appearance and performance of a finished fly.
There are two accepted standards for measuring the diameter of fly tying thread. The original standard is the _/0 system (3/0, 6/0, 8/0, 12/0, 14/0). This system adds a zero for each level of fineness of the thread, 3/0 being somewhat heavy and 14/0 being very fine. Because there has always been some va riation in size between manufacturers the scale has never been completely accurate. The common range of threads is 14/0 to 3/0.
The second standard is the denier (den’yer) scale. The denier rating of a thread is equal to the number of grams by weight per 9,000 meters of thread. Originally established by the garment industry, this standard gives a much more accurate description of thread size. Because the denier scale of measuring thread is more accurate all major manufacturers of tying thread are making the conversion. When the denier weight is shown on a spool of thread sold under the /0 scale, the customer can make a more accurate size estimation. The common range of fly tying thread is 40 denier to 280 denier.
The simple answer is to use the finest thread possible that is strong enough to hold the materials securely to the hook.
Factors Affecting Thread Strength
Not only does the diameter of a thread affect its strength, the material makes a considerable difference. The most common materials used to manufacture tying thread are nylon, polyester, silk, gel-spun polyethylene and Kevlar.
Nylon and polyester make up the majority of modern fly tying threads. They have relatively high break strength, grip materials well, whip-finish well and are inexpensive to manufacture. Each has slightly different properties but both are suitable for most fly tying applications. Many tyers have a personal preference, developed for use in specific applications.
Silk thread has been around for centuries and has excellent qualities as a fly tying thread. Still used by many tyers, silk has all the positive features we look for in thread. High strength to size ratio, excellent flattening/twisting qualities, great gripping power and perfect knotting make the relatively high cost justifiable.
Gel-spun polyethylene thread is very strong with break strength near that of Kevlar thread. However both of these products present unique difficulties for the tyer due to their ultra-slick surface. Both are difficult to start on the hook, tend to cut spinning hair like butter and they do not hold a whip-finish well. Adding wax to the thread helps but not enough to completely overcome these issues for use in all applications.
Flat or Twist?
All these threads are manufactured in multi-filament strands and can be described as flat, semi-twist and twisted. These terms refer to how they are laid on the spool during manufacture. As the terms suggest, flat has no twist, semi-twist has minimal twist and twisted has been manufactured with a fairly tight twist in the filaments. Each has its own application.
Flat thread will obviously lie thinner on the hook and is excellent for building smooth heads on flies. It doesn’t grip some materials as well as twisted thread but does allow for a smother tie-in. Break strength can also be slightly less than that of twisted threads under use.
Twisted thread s are made similar to rope with multiple groups of filaments twisted together during manufacturing. This twist creates a spiral surface on the thread that grabs materials better than any other. However these threads don’t lie as smoothly on the hook which can be a help or hindrance depending on the bulk desired.
Semi-twist thread is the most versatile in that it can be manipulated to be either flat or tightly twisted by simply spinning the bobbin during tying. Manufactured with a minimal amount of twist as a single group of filaments, the tyer can use it to perform multiple tasks.
Wax or No Wax?
The use of wax on tying thread does have advantages. Wax lubricates the thread and helps to prevent abrasion in the bobbin tube. Wax helps to grab materials better as opposed to the slick surface of un-waxed thread. Wax also helps to prevent deterioration of thread over time.
The disadvantages are that some manufactures use a heavy wax that can clog bobbin tu bes, and it prevents some water-based head cements from penetrating well. To offset these problems use lightly waxed threads and alcohol-based head cement.
Adding wax to a non-waxed thread can be done by pulling it along a cube of wax. Beeswax and paraffin wax are very good for this use. Dubbing wax is less desirable due to its sticky consistency.
Knowing which thread is best for any given application is best learned with experience and experimentation though this information should give an inexperienced tyer a good place to start.