Fly-Tying Threads

Diameter and breaking strain are separate properties

A thread can be thin and strong, thick and relatively soft, or anywhere between. Diameter controls head size and body bulk. Breaking strain controls how much tension the thread tolerates. The useful choice is the finest thread that still provides adequate grip and strength for the operation.

Why /0 and denier do not settle the comparison

The traditional /0 system developed without one binding dimensional standard, so 8/0 from two manufacturers may differ. Denier describes the mass of a given length of fibre, not the finished diameter or breaking strain. Fibre density, number of filaments, twist, bonding and flattening all affect how the thread ties.

Polyester and nylon

These are the main general-purpose thread families. Depending on construction, they may flatten into a smooth ribbon or remain more rounded. Stretch and surface grip vary between products. Use moderate tension first and learn the breaking point on a bare hook before tying valuable materials.

GSP and other high-strength threads

Gel-spun polyethylene and specialist power threads provide high strength for their diameter. They are useful for spinning or stacking deer hair, tying large streamers and applying firm pressure. Their smooth surface can slip, and high tension can cut soft material or even damage a hook finish. Use a secure foundation and controlled turns.

Silk, floss-like and monofilament threads

Natural silk is used in traditional patterns and changes character when waxed or varnished. Flat floss-like threads build smooth bodies as well as tying. Transparent monofilament reduces visible colour at a tie-in, but its smoothness and stiffness require careful locking turns.

Flat or round?

Counter-twist a multi-filament thread to let it spread flat for a smooth underbody and compact head. Add twist when a narrower, more cord-like thread gives better placement. Bonded threads remain more stable and do not flatten to the same extent.

Waxed and unwaxed thread

Wax increases grip and can help hold dubbing, but the amount and type vary. Additional wax is useful only when the technique needs it; excessive wax collects dirt, darkens pale material and makes fine work less precise.

Selecting thread by task

• Tiny dry fly or midge: very fine thread with enough strength for controlled low-bulk turns.
• General trout fly: a versatile thread that flattens and handles predictably.
• Deer-hair head: strong thread capable of applying pressure without cutting the hair.
• Large streamer or predator fly: high strength, reliable grip and resistance to abrasion.
• Smooth body: flat thread or floss-like construction over an even foundation.

Bobbin-holder compatibility

A damaged tube tip cuts thread repeatedly. Fine thread benefits from a clean polished, ceramic or similarly smooth path. Set spool tension only high enough to maintain control; excessive drag wastes available breaking strain.

Frequently asked questions

Which thread is strongest?

Strength must be compared at a useful diameter and in the intended knot or tying operation. High-strength GSP can outperform conventional fibres, but control and grip remain important.

Why does thread break at the same point?

Inspect the bobbin tip, hook eye and your fingernails for a burr. Repeated failure at one place usually indicates abrasion rather than insufficient thread size.

Should thread colour match the body?

Not always. It may be hidden, deliberately visible at the head, or influence a translucent body. Use colour as part of the construction.