GSP fly tying thread – what is it and how to choose the right denier?

GSP fly tying thread is chosen when a standard fly tying thread is either not strong enough or too bulky for the fly. Many fly tyers associate GSP mainly with large pike flies, but that is only one part of its use. Fine sizes such as 20D and 30D are also very useful for tiny dry flies, micro nymphs and the smallest grayling patterns tied on size 20 hooks and smaller.

This article is not about finding one “best” thread for everything. The better approach is to match the thread size to the fly pattern: a small grayling dry fly needs a different thread than a Muddler-style trout streamer, and a large pike fly tied with synthetic fibres and hair needs something different again.

What is GSP fly tying thread?

GSP stands for Gel Spun Polyethylene. In practice, it means a very strong thread made from polyethylene fibres. Its main advantage is high breaking strength combined with a very small diameter.

This allows a fine GSP thread to secure materials firmly while keeping thread build-up low. On small flies it helps reduce the number of wraps and keep the head small and the body slim. On larger flies it helps apply more pressure when tying in hair or synthetic materials.

It is worth remembering that denier helps compare thread thickness, but it does not describe everything. GSP threads are flat, but they can be twisted or untwisted with the bobbin holder while tying, which changes how the wraps sit on the hook and how much control you have over the material.

GSP vs standard fly tying thread

Not every fly needs GSP. A standard polyester or nylon fly tying thread is often more comfortable for everyday nymphs, dry flies and many other patterns. It gives good control, and standard tying threads are available in a much wider colour range.

GSP is the better choice when you need more strength with less bulk. It makes sense on very small flies where you do not want to make the head too large, and on bigger patterns where larger bunches of material must be tied in firmly.

For that reason, GSP and standard tying threads should be treated as different materials, not as simple replacements for each other.

What does denier mean in GSP threads?

Denier, usually marked with the letter D, helps describe thread thickness. In simple terms, the higher the denier number, the thicker and stronger the thread. A 20D thread is therefore much finer than 150D or 200D.

Fly tying threads are also often labelled with sizes such as 18/0, 12/0, 8/0 or 3/0. This system is not always easy to compare between manufacturers. With GSP threads, denier and practical use are usually the clearest way to understand the size.

How to choose the right GSP thread for a fly pattern?

The best starting point is a simple question: how large is the fly, and what material do you need to tie in? Small patterns need precision and fewer wraps. Large flies usually need stronger material control.

• 20D–30D – tiny dry flies, micro nymphs, emergers and very small grayling patterns.
• 40D–50D – medium nymphs, wet flies, small streamers and more general fly tying use.
• 75D – trout streamers and Muddler-style heads where deer hair needs to be tightened firmly.
• 100D – larger streamers, hair, synthetic materials and patterns that need stronger thread pressure.
• 150D and stronger – large pike flies, bigger streamers and larger bunches of material.
• 200D–300D – heavy-duty use: large pike flies, predator streamers and saltwater flies.

This list is not a strict rule. If you tie a light pike fly with only a small amount of material, you do not always need the strongest thread. If you tie a very small nymph, a GSP thread that is too thick can quickly enlarge the head and spoil the proportions of the finished fly.

When do fine GSP threads 20D and 30D make the most sense?

Fine GSP threads are often underestimated because many fly tyers think of GSP only as a thread for large and powerful patterns. In reality, 20D and 30D are very practical for the smallest flies.

On a tiny dry fly, micro nymph or small grayling pattern, every thread wrap matters. A fine thread helps keep the head small, the body even and the proportions clean. At the same time, GSP offers better break resistance than many standard threads of a similar diameter.

With fine GSP threads, the way the thread lies on the hook is also important. When controlled well, the thread can sit flat, so even several extra wraps do not build up the head as much as a thicker thread would. This is a major advantage on small hook sizes, where every wrap immediately affects the proportions of the finished fly.

This does not mean that fine GSP solves every problem by itself. You still need to control thread tension and the number of wraps. It does, however, give you a larger safety margin, especially with delicate materials and small hooks.

75D for Muddlers and 150D+ for pike flies

The 75D size is very useful for trout streamers with Muddler-style heads. In my opinion, it is one of the best thread sizes for trout streamers. In this type of fly, the thread must tighten deer hair firmly and hold it securely on the hook. 75D already gives clear strength, but it is not as strong as the typical threads used for large pike flies.

For large pike flies, stronger sizes usually work better, most often 150D, 200D or heavier. They allow you to tie in large bunches of synthetic materials or hair with much more control.

Thread tension still matters. A very strong thread can cut delicate material if it is pulled too aggressively. This is one of the main differences between GSP and standard tying thread: GSP gives more power, but it also requires more conscious control. Be especially careful with 50D and finer threads when tying flies with foam or hair.

Differences between GSP thread brands

Among GSP threads, you will often find series such as Veevus GSP, Semperfli Nano Silk, Hends NanoThread and Sybai Nano Strong Thread.

All of these threads are based on GSP fibres, but they differ mainly in size range, colour range and price.

The most practical approach is to choose the thread by application. For micro nymphs and small dry flies, look for a fine thread with a small diameter. For Muddlers and deer hair work, choose a size that lets you tighten the material firmly. For large pike flies, choose a thread with more strength in reserve.

Ceramic bobbin holder and sharp scissors

With GSP threads, a good bobbin holder is not a minor detail. The thread is thin, strong and often used under high tension, so it should pass through a smooth tube or insert without sharp edges.

A poor or damaged bobbin holder can fray the thread or, in extreme cases, cut it. The safest choice is a good ceramic bobbin holder, especially if you regularly use fine GSP threads or tighten hair and synthetic materials strongly.

The same applies to scissors. GSP has a multi-fibre structure and cuts much better with sharp scissors.

When is GSP not the best choice?

GSP is very strong, and in terms of breaking strength it is clearly different from the standard tying threads many fly tyers have used for years. For many fly tyers, it is a big step up in strength, which is why they now use GSP as one of their main tying threads.

For simple flies where colour choice, softer thread control or easy finishing matters more than maximum strength, a standard fly tying thread can still be the better choice. The best approach is simple: use GSP where its strength really helps. Not just because it is the strongest option, but because it gives a specific advantage in a given fly pattern.

Summary: choose the right denier, not only the strongest thread

GSP threads are very strong, but their real value is the ability to choose the right thickness for a specific fly. 20D and 30D work well for tiny dry flies, micro nymphs and delicate grayling patterns. Around 75D is a very good choice for trout streamers with Muddler-style heads. 150D, 200D and stronger sizes make sense for large pike flies and larger material bunches tied on bigger hooks.

When choosing a thread, look mainly at the type of fly and the materials used. For tying with GSP, also choose a good ceramic bobbin holder and sharp scissors. Then GSP is not just a “strong thread”, but a carefully chosen material for a specific job at the tying bench.

If you want to choose the right size for your own patterns, you will find fine and stronger options from different brands in the GSP fly tying thread category.

Author:
Krystian Niemy – KN Baits & Flies.