⚡ Key takeaways
1. A wire thread insert grows a steel thread inside a soft material. Aluminium, magnesium, zinc alloy, plastics and cast iron all have weak threads that strip after a few assemblies. Fit a stainless insert and the thread pair becomes steel on steel, with three to five times the load capacity.
2. Free-running and screw-locking are two different products. Free-running has a continuous thread and can be assembled repeatedly. Screw-locking has a polygonal crimp that grips the bolt — but it only resists rotation, not transverse vibration.
3. Length is the parameter most often got wrong. Length is specified in diameters: 1D for thin walls, 1.5D as the general aluminium default, 2D–3D for high loads. Too short loses strength; too long means a deeper hole and more cost.
4. The pilot drill is not the nominal diameter. An M6 insert uses a 6.3 mm pilot hole (M6×1.0 free-running) and an ST6×1.0 tap. Using a 6.0 mm drill is the most common installation failure on the shop floor.
5. Break the tang with a punch, never with pliers. Twisting it out damages the insert. The remaining tang does not affect function, but aerospace and medical work requires it removed.
6. 304 stainless is the general answer. 80 % of applications use 304; above 300 °C use Inconel 718; severe corrosion calls for 316; zinc-plated steel inserts are only for carbon steel bodies.
7. Inserts are not universal. For walls under 1.5D, for plastics above M4 with assembly torque over 3 N·m, a self-tapping insert or a heat-set insert is the better answer.
1. What a wire thread insert is and what it fixes
2. How it strengthens a thread
3. Free-running versus screw-locking
4. Choosing the length: 1D / 1.5D / 2D / 3D
5. Metric size table (2026)
6. Material selection
7. Installation in five steps
8. When not to use a wire thread insert
9. Common mistakes
10. FAQ: ten real engineering questions
11. Standards and references
1. What a wire thread insert is and what it fixes
A wire thread insert — commonly called a CoilSert, after the Helical Coil Corporation trade name — is a helical bushing wound from precision stainless wire with a diamond-shaped cross-section. It looks like a spring, but both its inner and outer faces carry a standard 60° thread.
Screwed into a hole tapped with a special ST tap, it expands radially against the bore wall so that its outside locks into the host material and its inside presents a standard internal thread. The bolt then runs in steel rather than in aluminium or plastic.
1.1 Three problems it solves
- Reinforcing threads in low-strength material. 6061 aluminium yields at 276 MPa; a direct M6 thread strips after about five assemblies. With an insert the pair is 304 stainless against a class 8.8 bolt and will survive 50 or more.
- Repairing damaged threads. A stripped or cross-threaded hole can be drilled out, tapped with an ST tap and fitted with an insert to restore the original nominal size — the standard repair for cylinder head and gearbox housing threads.
- Unifying thread standards. An imperial insert in an aluminium housing avoids mixing fastener systems, and a reducing insert turns a large hole into a small thread in thick plate.
1.2 Where they are used
- Aerospace: airframe, accessory gearboxes, landing gear — aluminium structures, high vibration, weight critical. Covered by the HB series.
- Automotive: gearbox housings, cylinder heads, oil pans — aluminium die castings.
- Electronics and telecom: laptop chassis, 5G base station housings, optical module shells — magnesium-aluminium, thin walls.
- Robotics: joint housings and reducer mounting faces in 7075 aluminium.
- Medical: surgical instruments and CT gantries, where 316 or Inconel is required.
2. How it strengthens a thread
The first question engineers ask is how a spring can carry load. The answer is the cross-section.
2.1 The diamond wire section
The wire is not round. It is a flattened diamond with one edge outward and one inward. Wound into a helix, the outer edge beds into the bore wall and the inner edge forms a standard 60° internal thread.
When a bolt is driven in, its flank pushes the insert radially outward so the whole 360° of the outer surface presses on the bore. That spring-fit removes any relative motion between insert and bore: load travels around the full circumference instead of concentrating in the first few aluminium threads.
2.2 How the stress distribution changes
In a directly tapped aluminium hole the load concentrates in the first three threads — about 40 % in the first, 80 % in the first three. The aluminium crushes there before load moves further along, which is exactly why aluminium threads strip.
An insert is an elastic element that distributes load across every engaged thread. Test data shows three to five times the fatigue life and two to three times the pull-out force compared with a direct thread.
2.3 What the tang does
A free-running insert has a small radial tang at one end. The installation tool engages it to wind the insert in; once seated, the tang is broken off with a punch. Screw-locking inserts have no tang because the crimp occupies that position.
3. Free-running versus screw-locking
| Free-running | Screw-locking | |
|---|---|---|
| Internal thread | Continuous standard 60° | One or two polygonal crimps (5–10 % diameter reduction) |
| Assembly feel | Smooth, like a normal nut | Clear resistance at the crimp (prevailing torque) |
| Locking principle | None — relies on the spring fit in the bore | Crimp grips the bolt and resists rotation |
| Re-assembly | Unlimited | 10–15 times before the crimp fatigues |
| Typical use | Access covers and service points | Joints rarely opened but subject to vibration |
| Temperature | 304 to 300 °C, Inconel to 600 °C | Same, same material |
| Cost | 1× | About 1.3–1.5× |
4. Choosing the length: 1D / 1.5D / 2D / 3D
Length is quoted as a multiple of nominal diameter. M6×1.5D means 9 mm of effective length.
| Length | Host material | Typical use | Capacity in aluminium |
|---|---|---|---|
| 1D | Wall thickness over 8 mm | Light covers, dowel locations | About 60 % of bolt tensile |
| 1.5D | Normal 4–8 mm aluminium walls | The default for 80 % of work | About 85 % of bolt tensile |
| 2D | Highly loaded aluminium, magnesium | Engine brackets, joint faces | 100 % — the bolt breaks first |
| 3D | Thin walls or cast iron | Full thread through thin plate | 100 %, but a deep hole |
5. Metric size table (2026)
Free-running metric inserts we stock, with pilot drill, ST tap and recommended length:
| Thread | Pitch | Pilot drill (mm) | ST tap | 1.5D length (mm) | Load class |
|---|---|---|---|---|---|
| M3×0.5 | 0.5 | 3.30 | ST3×0.5 | 4.5 | Light |
| M4×0.7 | 0.7 | 4.50 | ST4×0.7 | 6.0 | General |
| M5×0.8 | 0.8 | 5.60 | ST5×0.8 | 7.5 | General |
| M6×1.0 | 1.0 | 6.30 | ST6×1.0 | 9.0 | General |
| M8×1.25 | 1.25 | 8.30 | ST8×1.25 | 12.0 | High |
| M10×1.5 | 1.5 | 10.40 | ST10×1.5 | 15.0 | High |
| M12×1.75 | 1.75 | 12.50 | ST12×1.75 | 18.0 | High |
6. Material selection
| Material | Max temperature | Corrosion | Typical use | Relative cost |
|---|---|---|---|---|
| 304 stainless | 300 °C | General | The default for 80 % of work | 1× |
| 316 stainless | 300 °C | Marine, chloride | Shipboard, coastal equipment, implants | ≈ 1.5× |
| Inconel 718 | 600 °C | Hot and aggressive | Aero engines, exhaust systems | ≈ 10× |
| Zinc-plated 1070 steel | 250 °C | Moderate | Carbon steel hosts, no stainless requirement | ≈ 0.6× |
7. Installation in five steps
Sequence decides whether the insert works at all.
Step 1 — Drill
Use the ST pilot diameter from the table. Drill depth is insert length plus at least one pitch, so the insert does not bottom out. An M6×1.5D (9 mm) insert needs at least 10.5 mm of depth.
Step 2 — Tap
Tap with the dedicated ST tap, not an ordinary M6. Use cutting fluid, reverse a quarter turn for every half turn to clear chips, and blow the hole clean with compressed air afterwards.
Step 3 — Wind in
Mount the insert on the installation tool, align with the bore and turn clockwise with steady pressure. Stop when the top face sits 0.2–0.4 mm below the surface — never flush and never proud.
Step 4 — Break the tang
Place a punch on the tang root and tap it off with a small hammer; it snaps at the notch. Remove the fragment with a magnet or tweezers. Screw-locking inserts skip this step.
Step 5 — Inspect
Check with a GO/NO-GO gauge: the GO gauge should run through, the NO-GO gauge should not enter more than two turns.
8. When not to use a wire thread insert
8.1 Thin plastic walls
Drilling and tapping ABS, PC or PA66 will split the boss. Use a heat-set or ultrasonic insert that is melted into the plastic instead.
8.2 Where you want to remove a process step
A wire insert needs drill, tap, wind in and break the tang — four steps. At high volume, a self-tapping insert screwed straight into a moulded pilot hole saves about half the labour.
8.3 High radial load or a need for location
A wire insert is a thin-walled bushing and cannot take shear. Where the bolt is also a pivot pin, use a key-locking insert with radial keys that resist rotation and shear.
8.4 Permanent assemblies
For a joint that is never opened, a threadlocker on a directly tapped thread is cheaper than an insert.
9. Common mistakes
It should sit 0.2–0.4 mm below. Proud, the bolt head stands off and preload is lost; flush, the bolt bottoms on the insert's top face.
Functionally it can. But in aerospace, medical or high-vibration equipment it can break free over time and become loose debris inside the assembly. Remove it where those rules apply.
It does not. It resists rotation only, not transverse vibration. In a Junker-type joint it loosens exactly like a plain nut.
No. 0.2 mm oversize and the outer surface no longer grips; the entire insert rotates out with the bolt. Hold the bore tolerance within ±0.05 mm.
Watch galvanic corrosion. Stainless in direct contact with aluminium accelerates attack on the aluminium in wet environments. For coastal, outdoor or wash-down applications, seal the insert or specify an aluminium insert.
10. FAQ: ten real engineering questions
Q1. M6 in aluminium: 1.5D or 2D?
1.5D (9 mm) by default. Go to 2D (12 mm) only for class 8.8 and above in continuous vibration.
Q2. Can an insert be reused?
Free-running: fit a new insert; the old one has a slightly deformed diamond section. Screw-locking: the crimp relaxes each time, so do not reuse.
Q3. How do I break the tang on an M3 insert?
The tang is very fine and a punch tends to pull the whole insert out. Use a dedicated tang-breaker, or nip the tang off before installation (which can affect location).
Q4. What if the tapped hole is not deep enough?
The insert should finish at least 0.5 mm above the hole bottom. If it bottoms out, the top stands proud. Deepen the hole or use the next shorter length.
Q5. Can I use an insert in titanium?
Yes, but watch galvanic effects — titanium against stainless is acceptable, titanium against aluminium is not. Titanium is usually strong enough not to need an insert unless the wall is thin.
Q6. How is prevailing torque measured?
To ISO 16047: wind a conforming bolt into the insert and record the running torque with a torque wrench. For M6 it should fall between 0.3 and 0.8 N·m. Lower means the crimp has failed; higher means the wrong size.
Q7. What is used to repair cylinder head threads?
Automotive practice uses key-locking inserts rather than plain wire inserts, because an aluminium head at temperature under vibration will spin a wire insert out, while the radial keys lock it positively.
Q8. Can an insert go into steel plate?
Technically yes, but there is no point: steel threads are already strong and an insert reduces the thread section. Wire inserts exist for low-strength materials.
Q9. How do I inspect a batch?
Sample and check: GO/NO-GO gauging; a 5 % pull-out torque test (the torque needed to rotate the whole insert out of the bore, which should exceed 5 N·m for M6); and a visual check for corrosion or deformation.
Q10. Is a branded insert worth the premium?
Branded product offers tighter tolerance and better batch consistency at three to five times the price. Domestic inserts made to DIN 8140 / GB/T 24425 cover 80 % of industrial work. Use branded or Recoil for critical aerospace parts; domestic is fine elsewhere.
11. Standards and references
Chinese standards
- GB/T 24425.1 Wire thread inserts — types, dimensions and designation
- GB/T 24425.2 Wire thread inserts — technical conditions
- GJB 51 Specification for wire thread inserts for aerospace
International standards
- DIN 8140 Wire thread inserts for ISO metric screw threads
- ISO 16047 Fasteners — torque/clamp force testing
- NASM 122076 Inserts, helical coil, screw thread
Manufacturer literature
- Helical Coil Corporation (HeliCoil®) catalogue, 2024
- Recoil thread repair system technical manual
- Stanley Engineered Fastening — insert selection guide
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