Wire Thread Inserts (Helicoil-Type)
Helically coiled stainless and superalloy inserts that strengthen or restore threaded holes in aluminum, magnesium and cast iron — free-running and screw-locking types, M2–M40, five material families, full lot traceability.
A wire thread insert is a precision coil of diamond-shaped wire that screws into a pre-tapped STI (Screw Thread Insert) hole and forms a high-accuracy standard internal thread. Installed in soft materials such as aluminum or magnesium, it shifts bolt loads from the parent metal onto the steel coil — increasing thread strength and wear resistance without changing the bolt size.
What Is a Wire Thread Insert?
Wire thread inserts look like springs, but every coil is a precisely rolled diamond section: sharp on the outside, flat on the inside. Rolled from round wire and wound into a helix, the insert is driven into a tapped STI thread with a mandrel tool. Once seated below the surface, its inner faces form a flawless standard thread for the mating bolt.
Helical Geometry and the STI Thread
The parent hole is tapped with a special STI tap, which is larger in diameter than the nominal bolt thread. The insert’s outer thread engages the STI thread; its inner thread matches the standard bolt. This is why inserts can be specified without any change to the mating fastener.
How Wire Thread Inserts Work
Because the coil flexes like a spring, bolt load is distributed along the full engaged length instead of concentrating on the first two parent threads. The result is a dramatic increase in load-bearing area — typical aluminum assemblies gain 3–4× thread loading capacity versus tapped parent threads — plus better resistance to vibration loosening and thermal cycling.
Helicoil-Type: the Generic Category Explained
“Helicoil” is a registered trademark; the generic international category is the helically coiled wire thread insert, standardized under DIN 8140, NASM21209/MS21209 and GB/T 24425. Ankolam manufactures helicoil-type inserts to these standards — equivalent in geometry and interchangeability, produced in our own Wuxi plant with full dimensional inspection.
Free-Running vs Screw-Locking Inserts
Ankolam supplies both functional types:
| Attribute | Free-Running (Standard) | Screw-Locking (Locking Coil) |
|---|---|---|
| Internal geometry | Uniform coils, smooth bolt passage | One or more polygon-shaped locking coils grip the bolt |
| Function | Strengthens and protects the parent thread | Same strength + frictional lock against bolt loosening |
| Best for | General strengthening, frequent assembly cycles, cost-sensitive designs | Vibration, impact and thermal cycling — eliminates secondary locking devices |
| Reusability | Bolt can be removed and reinserted freely | Bolt retained; locking torque retained over hundreds of cycles |
| Standards | DIN 8140-1, NASM21209, GB/T 24425 | DIN 8140-2 (form-fitting), locking variants per standard |
When to specify free-running: static loads, jigs and fixtures, and applications where the bolt must spin freely. When to specify screw-locking: anything that vibrates, transports or heats up — the locking coil removes the need for nylon patches, adhesives or lock washers, simplifying the BOM and saving assembly time.
Product Range: Sizes, Lengths and Materials
Metric and Inch Size Coverage
| Thread System | Size Range | Length Options (× nominal diameter) |
|---|---|---|
| Metric coarse | M2 – M40 | 0.5D – 3.5D (common: 1D / 1.5D / 2D / 2.5D) |
| Metric fine | M7×1 – M39×3 | |
| Inch (UNC/UNF) | 2-56 – 7/8-9 | 1D / 1.5D / 2D / 2.5D |
Materials: Stainless Steel to Superalloys
| Material | Temp. Range | Key Properties | Typical Applications |
|---|---|---|---|
| 304H / 07Cr19Ni10 stainless | −196°C to 800°C (long) | General purpose, high corrosion resistance | Standard industrial strengthening |
| 316 / 06Cr17Ni12Mo2 stainless | −200°C to 600°C (long) | Mo-alloyed, chloride resistance, food/medical safe | Food machinery, marine, pharma |
| Nitronic 60 | to −196°C (no ductile-brittle transition) | Excellent anti-galling, low friction, non-magnetic | Vacuum chambers, stainless bolted joints |
| Inconel X-750 | −196°C to 800°C; relaxation-resistant to 540°C | High-temperature creep strength | Gas turbines, engines, hot sections |
| Phosphor bronze | to 180°C (long) | High elasticity, self-lubricating, conductive, non-magnetic | RF/electrical grounding, instruments |
| Titanium alloy | to 550°C (long); >1,200 MPa tensile | Highest strength-to-weight | Aerospace, titanium structures |
Coatings
| Coating | Limit / Character | Primary Benefit |
|---|---|---|
| Dry-film lubricant | Lightly corrosive / hot environments | Easier installation, no grease at assembly |
| Silver plate | Lubricates to ~650°C | Anti-galling at high temperature; aerospace & telecom standard |
| Cadmium | Superb corrosion defense | Salt-fog and marine service (RoHS-exempt uses) |
| Tin / Zinc-nickel | Low galvanic potential vs aluminum | Prevents galvanic corrosion in Al/Mg housings |
Environmentally restricted coatings are offered alongside cadmium-free alternatives — tell us your compliance requirement and we will propose the matching option.
Where Wire Thread Inserts Are Used
Aluminum Die Castings & Thin-Wall Housings
Reinforce tapped holes in soft alloys without redesigning bosses — smaller flanges, thinner walls, same thread strength.
Jigs, Fixtures & Tooling Plates
Threads that survive thousands of clamp cycles; worn inserts swap out in minutes with an extraction tool.
Gearbox, Pump & Manifold Housings
Load-spreading coils resist stripping and leakage-path wear in cast iron and aluminum pressure parts.
Thread Repair
Restore stripped holes to original size with repair kits and tooling — no oversize bolts.
Tangless variants of the same coils remove FOD risk for aerospace assembly — see tangless inserts. For solid-body heavy-duty joints, compare key-locking inserts; for post-casting installation browse self-tapping inserts.
Standards and Dimensions
Ankolam wire thread inserts are produced to the standard your drawing calls out:
| Standard Family | Covers | Notes |
|---|---|---|
| NASM21209 / MS21209 | US aerospace wire thread inserts | Supplied to international and civil-aviation customers; see our NASM21209 page |
| DIN 8140 | Metric helical inserts (DIN 8140-1 free-running, -2 screw-locking) | European standard reference |
| GB/T 24425 | Chinese national standard | Automotive & industrial customers |
| HB 5513/5514/6200/6201 | Chinese aviation standards | Aviation-industry projects |
| DIN 65536 · LN 9039 · LN 9499 | Aerospace coil inserts | On request |
Installation tooling — STI taps, mandrels, tang breakers, gauges and extractors — is supplied from the same factory: see installation tools. Full dimension tables per size are collected in our dimensions reference.
Do Thread Inserts Conduct Electricity?
Yes — but poorly in stainless steel, which is why material choice matters for grounding points. 304/316 stainless inserts conduct at typical stainless rates and are unsuitable where low-resistance grounding is required. For RF connections and electrical grounding, specify phosphor bronze inserts: high conductivity, non-magnetic, with self-lubricating behavior that also prevents thread cold-welding. In a semiconductor-vacuum case below, phosphor bronze coils carried RF current while Nitronic 60 coils handled structural fastening in the same chamber.
Inconel X-750 Inserts with Silver Plate at 600°C+
A heavy gas turbine OEM fought preload loss and thread galling on combustion-section housings: thermal cycling relaxed standard stainless inserts, and bolts seized in high-temperature threads. Ankolam supplied Inconel X-750 wire thread inserts with silver plating — creep-resistant material holds preload through start-stop cycles, and the silver film lubricates to ~650°C, eliminating galling. The result: stable bolt preload over repeated thermal cycling and complete heat-number and batch traceability on every lot.
Wire Thread Insert FAQ
What sizes of wire thread inserts does Ankolam stock?
Metric coarse M2–M40 and metric fine M7×1–M39×3, plus inch series 2-56 to 7/8-9, in lengths from 0.5D to 3.5D. Semi-finished stock on common sizes supports short lead times; specials are made to drawing.
Are Ankolam inserts interchangeable with other helicoil-type brands?
Yes. Inserts produced to NASM21209, DIN 8140 or GB/T 24425 share identical envelope geometry — they fit the same tapped STI holes and accept the same bolts and tooling.
Which insert should I choose for a high-temperature bolted joint?
For continuous service above ~300°C, step up from 304H: Inconel X-750 with silver plate is the proven combination to 600°C+ in gas turbine service. Titanium serves to 550°C where weight matters. Tell us the temperature profile and we will match material and coating.
Can you supply certificates and traceability documentation?
Every lot ships with dimensional inspection records, and material heat numbers trace each batch to the wire source. Mill test certificates are available on request.
Request a Wire Thread Insert Quote
Send your thread spec, material and quantity — engineering confirmation within one business day, samples before volume orders.