Thread Inserts for Soft-Metal Threads: Steel Armor from M2 to M39

An M6 threaded hole was tapped into an aluminium-alloy housing. On the assembly line the bolt went in three times — on the third pass there was a sharp crack, and the thread was gone.

The hole is still there; the thread is scrap. On a die casting, the whole housing is scrapped with it.

When threads are cut in soft metal, the enemy is not the bolt but the substrate, which is simply too soft. A very mature solution exists for this, yet it is often missed at the design and selection stage: fit a piece of steel armour into the threaded hole.

Three Failure Points of Soft-Metal Threads

The First Thread Carries Half the Load

In an ordinary tapped hole without an insert, the load is not evenly distributed — the first engaged thread typically takes about 50% of the total. That means the remaining threads have not contributed before the first is already near its limit. Light alloys are low in hardness, so this first thread is even easier to crush.

Stripping After a Few Assemblies

Threads in aluminium, magnesium and engineering plastics tolerate very few re-assemblies. Service repairs, line rework and repeated assembly during commissioning can quickly ruin the hole.

One Bad Hole Can Scrap the Whole Part

Die-cast housings and aluminium structural parts are usually high-value single pieces. Once a threaded hole strips, repair is expensive and scrapping the part is often the only option.

Wire Thread Inserts: a High-Strength Liner for the Threaded Hole

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

The installation route is straightforward: drill the core hole to the specified size in the substrate → tap the special thread → screw the insert in → break off the driving tang → done.

Once installed, the internal thread that the soft metal used to carry is carried by work-hardened stainless wire instead. The substrate only has to provide the external thread that retains the insert, and the load is transferred evenly across more threads.

A simple analogy: you do not thicken the wall, you embed a steel tube in the hole and let the bolt thread into that tube.

The Numbers: Hard Specifications of the Insert

Material: standard types use hardened 304 stainless steel; 316, phosphor bronze and Inconel nickel-based alloy are also available for high-corrosion, conductive and high-temperature duties.

Mechanical properties:

• Material hardness HRC 43–50
• Tensile strength not lower than 1370 MPa

Core benefits:

• Internal thread strength in light alloys (magnesium, aluminium) raised to Class 12
• Stress concentration eliminated; load shared by all threads instead of 50% on the first
• Reinforced light-alloy threaded holes can carry Grade 8.8 and higher high-strength bolts

Size range:

• Size: M2 – M39
• Length: 1D, 1.5D, 2D, 2.5D, 3D, 5D; non-standard lengths on request
• Function: free running and screw locking (self-locking)
• Installation: tanged and tangless (no tang to remove, suited to automated assembly and blind locations)
• Surface treatment: dry-film lubrication, tin / zinc / silver plating

More Than Reinforcement: Repair and Conversion

Beyond the strength gain in new designs, thread inserts offer two undervalued benefits.

Repair: a stripped or damaged threaded hole does not have to be scrapped. Drill it out, install an insert of the matching size and the thread comes back stronger than the original design. On high-value die castings and on threads in imported equipment, this is often the only viable repair.

Conversion: when inch threads must become metric, or fine threads coarse, an insert converts the thread standard without replacing the whole substrate part.

Vibration, wear and repeat assembly: on equipment that is serviced regularly and assembled often, inserts let bolts be run in and out without damaging the base material, while resisting wear and extending service life. Combined with screw-locking inserts and coating options, locking performance can be optimised further.

From Drilling to Final Inspection: Six Steps to a Correct Installation

The insert is a semi-finished part; how well it is installed determines the final performance. The standard process has six steps —

1. Drill: control core-hole diameter and depth per the specification table.
2. Tap: use the dedicated tap, with pitch matched strictly to the insert.
3. Thread inspection: confirm tapping quality with a thread plug gauge.
4. Install: mount the insert on the dedicated installation tool and screw it in perpendicular to the workpiece to the specified depth (usually one pitch, 1P, below the surface).
5. Remove the tang.
6. Final inspection.

The step most prone to problems is angular deviation: if the installation tool is not perpendicular to the workpiece, the insert sits crooked, which directly affects how the bolt engages and how load is carried. That is why we recommend the dedicated installation tool rather than forcing the insert in by hand.

Standards and Typical Applications

Standards:

• GB/T 24425.1-2009 (metric coarse wire thread inserts)
• GJB 119.4A-2001

The specification table also gives core-hole diameter, drilling depth, thread depth, number of free-state coils and installed length, so the data can be used directly for design selection and drawing callouts.

Typical applications:

• New energy vehicles: thread reinforcement in aluminium battery trays, e-drive housings and die-cast structural parts
• Consumer electronics: small-size threads (M2–M4) in thin, lightweight devices
• Industrial electronics and EMS: chassis and modular structures assembled frequently
• Aerospace and high-end equipment: high-vibration, heavy-load duties where threads must be reusable

About YF Zhichengjia: YF Zhichengjia specializes in precision fastener manufacturing and serves consumer electronics, new energy vehicles, industrial electronics, HPC AI servers, the low-altitude economy, embodied intelligence and light alloy die casting. We support the full cycle from prototype design and design consulting to teardown analysis and continuous process optimisation.

We believe something simple: the purchase price of a fastener is only 15% of its total lifecycle cost; the other 85% sits in design selection, assembly efficiency, quality rework and after-sales. So we do not just supply parts — we help you account for that 85% as well. Small parts, big responsibility.