When You Actually Need a Wire Thread Insert

A wire thread insert is essentially a "spring-like" coil of rhombic steel wire. In the free state its outside diameter is larger than the tapped hole; after it is drawn down and screwed into an STI (special threaded-insert) tapped hole by the installation tool, it expands outward under the wire's own elasticity, permanently anchoring itself in the parent material, while its inner bore forms a precise 60° standard internal thread. The Heli-Coil catalogue states that the mirror-like surface roughness can reach 8–16 microinches, and it is compatible with ordinary metric bolts (per GB/T 197 / ISO 965) as well as inch bolts (MIL-S-7742, UNJ controlled root radius).

According to the explicit statement in Chapter 1 "Scope" of GB/T 24425.1-2009 Free-Running Wire Thread Inserts, wire thread inserts are suitable for: increasing the strength of threaded holes in aluminum alloy, magnesium alloy, copper alloy, cast iron and non-metallic (plastic) bodies; increasing the fatigue strength of screws; and repairing damaged threaded holes. This statement almost exactly covers the three types of real pain points engineers encounter most often on the shop floor:

• Thread reinforcement in soft materials: On aluminum/magnesium alloy die castings and glass-fiber-reinforced plastic housings, directly tapping M6/M8 threads and repeatedly assembling and disassembling them a few times causes thread stripping. A wire thread insert replaces the mating pair with a hard stainless-steel internal thread, so the bolt breaks before the parent-material thread is damaged.
• Anti-loosening under vibration and shock: On continuously vibrating parts such as engines, motors, pump bodies and chassis, ordinary threads easily loosen; a screw-lock wire thread insert provides a sustained, reversible prevailing torque.
• Repair of already damaged threaded holes: When a bolt breaks inside the hole, the thread is scored, or the threads strip, the whole part does not have to be scrapped; after oversizing the hole, a wire thread insert is fitted to restore the original specification thread.

The Heli-Coil catalogue also points out that, with a wire thread insert, a bolt can achieve the same clamping force on a softer, lighter, smaller boss—the boss diameter only needs to be about twice the bolt nominal diameter. This is critical for "weight-saving and space-saving" scenarios such as NEV battery packs and electronic housings.

Free-Running vs Screw-Lock: The First Selection Question

The most basic classification of wire thread inserts is "Free-Running (Standard)" versus "Screw-Lock". The two look similar; the only difference is that the screw-lock type has one or more coils that are not a complete circular coil but are flattened into two parallel "chords". When a bolt is screwed in, these two chords expand outward and exert elastic pressure on the bolt, thereby generating a sustained prevailing torque.

Beyond these two mainstream types, Heli-Coil also offers Hi-Torque (a higher-prevailing-torque type, with roughly 40% higher prevailing torque than the screw-lock type, #10–3/8" UNF), Stud-Lock (the highest prevailing torque, used for threaded studs/threaded-in parts), and Tangless types (which eliminate the tang-break-off operation, can be installed in either direction, comply with NASM 1130, and suit blind holes and high-volume automatic assembly).

Material and Finish: Don't Default to 304 Stainless

In engineering, more than 90% of wire thread inserts default to 304 stainless steel (corresponding to Heli-Coil AS7245). This is the standard, universal, best-stocked material, with a service temperature up to about 427°C (800°F), tensile strength of 200–250 ksi, and hardness RHc 43–50. But under special conditions such as high temperature, seawater, vacuum and low magnetism, the material must be selected according to the service condition:

The finish also affects assembly and life: Dry Film Lubricant (AS5272) lowers prevailing torque and is recommended for use with screw-lock types; Primer-Free coating blocks galvanic corrosion between the insert and the aluminum/magnesium parent, eliminates the zinc-primer operation, and is a common choice for NEV housings; silver plating is used for high-temperature anti-galling at about 649°C (1200°F); cadmium plating is no longer recommended for new designs due to toxicity. In Heli-Coil part numbers the material code and finish code are marked separately—for example, CN=stainless steel, BN=phosphor bronze, TN=Inconel X, GN=titanium; Y=cadmium plated, W=dry film lubricated, Blank=uncoated, PF=primer-free.

Installation Process: The Hard Rules for Drilling, Countersinking and Tapping

Wire thread inserts themselves are not expensive, but a wrong installation process can scrap a whole batch of parts. The standard process given in the Heli-Coil catalogue has four steps: drill → countersink → tap → inspect (gage), after which the insert is screwed in with the installation tool and the tang is broken off (tangless types skip the break-off).

1. Drill: The drill diameter must be looked up in a dedicated table according to the parent material. For the same M6 tapped hole, aluminum/magnesium parts use a 3.4 mm drill, while steel/magnesium/plastic parts use a 3.3 mm—choosing the wrong drill directly results in an oversized or undersized hole. The drilling depth F must leave a residual depth at the bottom after the full insert length is screwed in; it is generally estimated as "insert nominal length Q + 0.5×bolt diameter + 4P (cone reserve) + 1P (countersink reserve)".
2. Countersink: Machine a 90°–120°×45° countersink at the hole mouth to prevent chipping of the thread hole edge during assembly; the countersink angle matches the tap's lead cone angle (a 120° countersink corresponds to a 60° thread).
3. Tap: An STI special tap must be used (an ordinary tap will not do!). Use a plug tap (4-thread taper) for through holes and a bottoming tap (2-thread taper) for blind holes; for deep holes or sticky soft materials, spiral-point taps or high-spiral-flute taps are recommended for chip evacuation. The tapping depth H = insert nominal length Q + 1P; blind holes must leave 1–2 threads of incomplete thread and 1P of tap tail clearance.
4. Inspect (gage): Inspect the tapped hole with an STI special go/no-go gage; only after it passes should the insert be installed, to avoid sealing an out-of-spec tapped hole directly into the product.

Installation tools fall into manual inserting tools, electric and pneumatic high-volume tools, and prewinders used for batch pre-winding. After installation, free-running types need a tang breaker to punch the tang off at the break-notch and remove it; tangless types have no such operation and can be repeatedly disassembled and adjusted.

How to Read Metric/Inch Sizes and Part Numbers

The size range of wire thread inserts is very broad: Heli-Coil metric coarse runs from M2×0.4 up to M39×4, and metric fine from M8×1 up to M39×3; inch UNC runs from #2-56 to 1-1/2-6, and UNF from #0-80 to 1-1/2-12; UNEF, UNS, 8UN, 12UN, 16UN, spark-plug threads and pipe threads are also offered. The national standard GB/T 24425.1/24425.3 covers M2–M39 (free-running starts at M2, screw-lock starts at M3).

Metric lengths are marked as "multiples of the nominal diameter d", with the common grades 1d, 1.5d, 2d, 2.5d and 3d. The selected length is determined by the thickness of the connected part and the bolt engagement length—Heli-Coil recommends at least 1d engagement in a steel parent and 1.5d–2d in an aluminum/magnesium soft parent. The Heli-Coil part number is an "assembled code"; reading it is equivalent to reading a complete specification sheet—for example, 1185-4CNY375S:

Domestic drawings are marked according to the GB/T 1237 designation method, for example "M8×1.25-1.5d free-running wire thread insert GB/T 24425.1"; the tapped hole is machined per GB/T 24425.5 Internal Thread for Wire Thread Inserts, and the technical conditions follow GB/T 24425.6. When selecting, you can refer to the YF Zhichengjia wire thread insert product page for metric/inch sizes and material options.

Selection Recommendations and Common Misconceptions

Condensing the previous sections into a "selection decision table", you can simply tick the boxes against the service condition on site:

• Static load, steel/cast-iron thick wall, ordinary assembly: free-running 304 stainless, 1d–1.5d length is enough.
• Aluminum/magnesium die castings, plastic housings, repeated assembly/disassembly: free-running 304, length 1.5d–2d, boss diameter ≥2×bolt diameter.
• Continuous vibration/shock, anti-loosening needed: screw-lock 304, tapped hole machined to 4H5H/5H, paired with dry-film-lubricant finish.
• High temperature (>427°C): Inconel X750; seawater/salt spray: phosphor bronze or titanium; vacuum/low magnetism: Nitronic 60 or titanium.
• Galvanic-corrosion sensitive (aluminum housing + steel bolt): choose the primer-free type, or add an isolating coating.
• Threaded hole already stripped / broken bolt: first oversize the hole to STI specification, tap with the corresponding STI tap, then install an insert of the same size to restore the original thread.