Why Light Alloy Threads Strip, and How to Specify Wire Thread Inserts

1. Three attempts and the thread is gone

An M6 tapped hole in an aluminium housing. On the assembly line the bolt went in and out three times, and on the third pass there was a click. The thread was gone.

The hole was still there. The thread was scrap. On a die casting, that means the whole housing is scrap.

Almost every industry building structures from light alloys meets this: EV battery trays and e-drive housings, thin consumer electronics enclosures, modular industrial chassis, low-altitude aircraft and robot frames. With a soft metal thread, the enemy is not the bolt — it is the parent material being too soft.

There is a mature answer, and it is simply not considered early enough in design: put a steel armour sleeve inside the thread.

2. Four mechanical root causes

Stripping is not an operator leaning on a wrench. It is the sum of four mechanical facts, and understanding them explains why the fix is to rebuild the hole rather than to buy a better bolt.

Cause one: the first thread takes about 50 % of the load

In a plain tapped hole without an insert, the load is not evenly distributed. Because the pitch of bolt and parent body shift relative to each other under load, the first engaged thread from the load direction sees the greatest pitch compensation and carries roughly 50 % of the total load, the second about 25 %, and the rest falls away quickly.

The consequence: before the remaining threads have done any work, the first is already at its limit. Light alloys have low hardness and low shear strength, so that first thread crushes easily. Once it yields plastically the load shifts forward and the failure propagates thread by thread until the whole length strips.

Cause two: aluminium has one third the elastic modulus of steel

This is the most overlooked cause and the most fundamental. Aluminium alloy runs about 70,000 N/mm² against 210,000 N/mm² for steel.

The result is that at the same preload the aluminium thread deflects far more, contact area between the flanks falls and peak contact stress rises. The bolt is still elastic while the parent thread is already being crushed. Put plainly, a light alloy thread usually fails on stiffness, not on strength.

Cause three: strength and hardness are simply lower

Even high-strength aluminium alloys are far from steel. Reference properties for common aluminium fastener materials, density about 2.8 g/cm³, elastic modulus about 70,000 N/mm², thermal expansion about 23.6 × 10⁻⁶/K:

For comparison, steel has three times the elastic modulus, and even a medium-strength alloy steel fastener has a far stronger shank than any of these. In a steel bolt into an aluminium thread, the aluminium side is always the one that pays. That is why the trade generally advises reinforcing the thread before using a high-strength bolt in light alloy.

Cause four: differential expansion causes preload drift

Aluminium expands at about 23.6 × 10⁻⁶/K, roughly twice steel. Through thermal cycling — battery pack warm-up, electronics thermal cycling, day-night swing outdoors — the aluminium body and steel bolt grow by different amounts and the preload drifts. Repeated cycling also produces micro-motion on the flanks, accelerating wear and galling. In aluminium, thread galling or cold welding is another frequent failure: not "will not tighten" but "will not come apart".

The same causes in cost language

On the 15/85 rule, the direct purchase price of a fastener is about 15 % of what the joint costs, and the other 85 % hides in design, supply chain, assembly, inspection and after-sales. Light alloy thread failure lands squarely in the expensive part of that 85 %:

• Line rework: one stripping station can stop the whole takt.
• Field repair: aluminium, magnesium and engineering plastic threads tolerate very few assembly cycles, so commissioning alone can ruin a hole.
• Scrapping the part: die cast housings and aluminium structures are high-value single pieces. A stripped thread is expensive to repair and often means scrapping the whole component.

So the correct answer is not a softer bolt. It is replacing the weak link — turning a soft parent thread into a hardened stainless thread.

3. What a wire thread insert is

A wire thread insert is a helical internal thread sleeve wound precisely from high-strength stainless wire.

Installation is direct: drill the tapping hole to the specified size, cut the special STI thread, wind the insert in, remove the tang, done.

Afterwards the internal thread that was carried by soft metal is carried by hardened stainless wire. The parent body only has to hold the insert's external thread, and the load passes through the insert into many more threads.

The image worth keeping: you are not thickening the wall, you are embedding a steel tube in the hole so the bolt tightens into the tube.

Quantified benefits:

• Light alloy internal thread strength raised to class 12 equivalent — the reinforced thread can carry what a class 12 thread pair would.
• Stress concentration removed: load moves from "the first thread takes 50 %" to being shared across the full engagement.
• A reinforced light alloy hole can carry class 8.8 and above, a combination not recommended in an unreinforced aluminium thread.

4. Why the insert shares the load

Most people picture a wire insert as a harder liner. The real engineering content is in its elasticity.

Mechanism one: radial preload from installation. A free insert has a larger outside diameter than it does once installed. It is compressed as it winds into the special parent thread, so after installation the outer wall presses continuously on the parent thread and the inner wall on the bolt thread. That preload brings every coil into contact with the bolt flank, instead of the few threads that touch in an ordinary thread.

Mechanism two: the helix redistributes load axially. The insert is essentially an elastic wire coiled into a helix. As the bolt is loaded, force passes through the coils in stages, and the insert's own axial elasticity redistributes deformation along the engagement length, spreading what was concentrated in the first thread across the whole length. That is precisely the answer to the 50 % first-thread problem.

Mechanism three: stiffness correction. Return to cause two. The insert moves the bearing surface off low-stiffness aluminium and onto high-stiffness, high-hardness stainless wire. The parent body only carries the external thread load needed to retain the insert, so its working stress drops sharply and the failure sequence that started with the parent body is rewritten.

Together these deliver more than strength: vibration resistance, wear resistance and repeated disassembly without damaging the parent material. On equipment that is serviced regularly, the insert lets bolts come out and go back in many times, and it resists wear to extend service life. With locking inserts and coating options, anti-loosening performance improves further.

5. Step 1: size, length and thread count

Selection starts with size, and size is more than diameter. Insert length sets the number of engaged coils, and engaged coils set load capacity.

Length is expressed in multiples of D: 1D, 1.5D, 2D, 2.5D, 3D, 5D, with non-standard lengths available. The rule of thumb is that the softer the parent material and the stronger the bolt, the longer the multiple you need, because a longer engagement is what spreads the stress. Conversely, a harder parent or a gentler load is usually served by 1D to 1.5D.

Before drilling, three dimensions have to be fixed: tapping hole diameter, drilled depth and thread depth. The table below gives conventional metric drilling and tapping reference sizes for plain threads, to establish the principle that drilled depth must exceed thread depth:

⚠ A critical distinction: when fitting a wire insert, you are not drilling and tapping an ordinary thread. The insert has wall thickness, so to finish with a standard internal thread — M6, for example — the hole and thread must both be enlarged to a wire insert thread (STI) cut with a dedicated tap. This is a separate thread system defined in GB/T 24425.5. The 5.0 mm shown for M6 above is the plain thread tapping size and must never be used to install an insert. Actual insert hole and tapping sizes must come from GB/T 24425.5 or the supplier's table, which is the d_y column below. The table above exists only to establish that drilled depth exceeds thread depth.

Once length is fixed, look up the tapping hole, free-state diameter, free-state coil count and installed length in GB/T 24425.1-2009 for the standard type. Extract for common sizes at 1D length:

Selection note: "installed length" is the actual length after fitting, which is shorter than the free length because the insert is compressed radially and shortens slightly. Thread depth must be designed from the installed length, not the free length, or the hole depth will be wrong.

6. Step 2: function type

Two inserts can share a size and behave completely differently. This is the step most often skipped. Four dimensions, combined in pairs, give the specific part number:

The dimension that needs the most judgement is tanged versus tangless. The rule: blind hole, invisible after assembly, or an already automated line — choose tangless. Manual assembly, cost sensitive, through hole — tanged is entirely adequate. Be aware that tang break-off debris is a hidden failure source: a metal tang left inside a housing can short a circuit or jam a mechanism after the equipment is running, and automotive electronics and consumer electronics are especially sensitive to it.

7. Step 3: material and surface finish

The standard insert is hardened 304 stainless, which covers most industrial situations. Upgrade only when the environment goes beyond that:

Surface finish is not decoration either; it changes assembly behaviour directly:

Important: a coating changes the friction coefficient, and friction decides how much preload a given torque produces. So on one machine, do not mix inserts in different coating states — the same torque value will then give different clamp loads and process consistency becomes impossible. If preload consistency matters, put the coating state on the drawing at design stage.

8. Step 4: parent body design

The insert is a semi-finished component, and much of its performance is decided by the parent body. Four items must be settled before the drawing is frozen:

One functional dimension is easily missed: after installation the top of the insert normally sits about one pitch (1P) below the workpiece surface, so it cannot protrude into the mating face. Put that position tolerance on the drawing, or assembly will produce either a proud insert or one sunk too deep.

9. Installation: six operations, most problems at the fourth

The insert's performance is only realised through installation quality. The standard sequence has six steps:

The most common failure is angular deviation: if the installation tool is not square to the workpiece, the insert goes in crooked, which directly compromises bolt entry and load capacity. That is why hand-forcing is not recommended and a dedicated tool is required. It is equally important to use the STI tap matched to the insert pitch; a mismatched thread accepts the insert and produces a hidden reject.

Common installation failure modes, for quick fault-finding on the line:

10. Beyond reinforcement: repair and conversion

Two further values show up mainly in firefighting situations, and both are decisive when they do.

Repair: make a stripped hole usable again. A stripped or damaged thread does not have to mean a scrapped part. Open the hole out and fit the matching insert and it recovers to better than the original design strength. For high-value die castings and threads in imported equipment — especially equipment that is out of production with no spare parts — this is often the only viable repair.

Conversion: change the thread standard without changing the part. Where an inch thread must become metric, or a fine thread become coarse, an insert converts the standard without replacing the parent component. In import substitution, localisation of multinational projects and reuse of legacy drawings, that avoids re-cutting a whole mould or remaking a batch.

Repeated assembly: on equipment that is serviced regularly, the insert lets bolts come out and go back in without damaging the parent material, while resisting wear and extending life. That matters for serviceability: it turns thread life from a consumable into a fixed feature of the structure.

11. Typical applications

12. Quick selection checklist

Condensed into something you can put straight into a design review deck:

• ① Do you need one? Parent body is aluminium, magnesium, zinc alloy or engineering plastic, and the thread will be assembled repeatedly or carry meaningful load. Rule of thumb: if the parent is softer than the screw or has less than half the elastic modulus of steel, evaluate it seriously.
• ② What size? M2 to M39 by nominal diameter; length 1D to 3D depending on parent hardness and load, with non-standard lengths available.
• ③ How long? Soft parent with a high-strength bolt — go longer, 2D to 2.5D. And confirm the installed length, not the free-state length.
• ④ Locking? Vibration, or no adhesive allowed — screw-locking type per GB/T 24425.3.
• ⑤ Tanged or tangless? Blind hole, automated assembly or a location you cannot see — tangless. Manual, through hole, cost first — tanged.
• ⑥ Blind or through? Blind — GB/T 24425.2 for the standard type or GB/T 24425.4 for the locking type.
• ⑦ What material? General 304; marine or chloride 316; electrical and galvanic-critical phosphor bronze; hot or aggressive Inconel.
• ⑧ What coating? Tight torque consistency — dry-film lubricant. Electrical connection — tin or silver. Never mix coating states across one machine.
• ⑨ Has the parent got room? Hole diameter, drilled depth, thread depth, wall and edge margin, installation position tolerance at 1P below the surface — dimension all five.
• ⑩ Will it be installed properly? Dedicated STI tap, dedicated installation tool, squareness control, plug gauge re-check. Miss one and the rest do not help.

13. Sources and standards

• Product Handbook — Thread Inserts and Wire Thread Inserts (Toprecision): specification and standard tables, installation sequence and precautions, material and finish selection, industry applications and the 15/85 cost model.
• Toprecision Wire Thread Insert Product Brief: the three root causes of soft metal thread failure, hardening figures (HRC 43–50, at least 1,370 MPa), strength raised to class 12 equivalent, class 8.8 capability, and the repair and conversion value.
• Fastener Technology Handbook: mechanical property references for aluminium and copper alloy fasteners (density 2.8 kg/dm³, elastic modulus 70,000 N/mm², thermal expansion 23.6 × 10⁻⁶/K), stainless fastener groups and corrosion selection.
• Standards: GB/T 24425.1-2009 (standard wire thread inserts), GB/T 24425.2-2009 (standard, blind hole), GB/T 24425.3-2009 (locking type), GB/T 24425.4-2009 (locking, blind hole), GB/T 24425.5-2009 (internal thread for wire thread inserts), GB/T 24425.6-2009 (technical conditions), GJB 119.4A-2001.