Three Technical Routes for Thread Inserts: Wire Thread Inserts, Internal-External Thread Sleeves and Rivet Inserts
Executive Summary
There are three technical routes for strengthening threaded holes in soft metal: wire thread inserts, internal-external thread sleeves and rivet inserts. They differ in principle, installation, load capacity and suitable base material, and are often confused. This article uses comparison tables to clarify the mechanical mechanism, selection parameters and failure modes of all three routes, helping you choose correctly the first time in aluminum/magnesium die-casting, PCB and sheet-metal scenarios.
Table of Contents
Why the Three Routes Are Often Confused
When making threaded joints on aluminum/magnesium die castings, thin-wall sheet metal and PCBs, engineering drawings often say "add a thread insert here," but exactly which kind of insert is usually not questioned until the RFQ stage. The result is that for the same position, supplier A quotes a wire thread insert, supplier B quotes a rivet nut, and supplier C quotes an internal-external thread sleeve—three completely different structures, prices differing several-fold, and installation methods that are not interchangeable.
More troublesome is that choosing wrong is not just "a bit more expensive." A rivet insert needs the sheet to have enough plasticity to flow and engulf it; fitted on a die casting, it will crack directly. A wire thread insert requires a dedicated installation internal thread to be tapped first; on 1 mm sheet metal it simply cannot be tapped.
The applicable boundaries of the three routes are actually quite clear; the problem is the lack of a table that compares them side by side.
Route 1: Wire Thread Inserts
A wire thread insert is a helical coil wound from hardened stainless-steel wire of diamond cross-section. It is screwed into a dedicated internal thread pre-tapped in the base material, and its inner side forms the new load-bearing thread.
Mechanical Mechanism
The outer crests of the diamond cross-section fully conform to the base thread, distributing the load transmitted by the bolt across more turns of the base thread, thereby solving two problems at once: the strength of the load-bearing thread and the uniformity of load distribution. In an ordinary soft-metal threaded hole, the first thread often carries about half the load; after fitting an insert, this proportion drops significantly.
Key Parameters
| Item | Typical Value | Notes |
|---|---|---|
| Material | 304 stainless steel (316 and others optional) | Hardness HRC 43–50 |
| Nominal length | 1d / 1.5d / 2d / 3d | Commonly 1.5d–2d |
| Load level | Compatible with grade 8.8 and higher bolts | The key benefit for soft-metal base material |
| Installation | Dedicated tap + screw-in | Requires dedicated tap-drill diameter and tap |
| Reusability | Replaceable when thread wears | Turns an irreversible feature into a replaceable element |
Route 2: Internal-External Thread Sleeves
An internal-external thread sleeve is a metal sleeve with both internal and external threads: the external thread screws into the base material, and the internal thread receives the upper part. It splits the joint into two stages—the first fixed in the base material, the second used for assembly.
Why It Suits Thin Parts
Because the load is carried by the sleeve itself rather than relying on the thread strength of the base material. The base material only needs to provide one thread that can withstand the screwing torque, so the wall-thickness requirement is lower than for a wire thread insert. On ultra-thin PCBs of 1.2 mm and 1.4 mm, the internal-external thread sleeve is a way to "outsource" the joint capability to outside the board.
| Comparison | Wire Thread Insert | Internal-External Thread Sleeve |
|---|---|---|
| Structure | Helical coil, no solid wall | Solid sleeve, with internal and external thread sections |
| Base wall-thickness requirement | Higher | Low, suits thin parts |
| Load path | Distributed into base thread | Mainly transmitted through the sleeve itself |
| Protrusion above base surface | Basically flush | Usually some protrusion |
| Typical use | Die-cast thread strengthening | PCB, thin sheets, ultra-thin structures |
Route 3: Rivet Inserts
A rivet insert (including rivet nuts and rivet studs) is fixed by plastic deformation and mechanical interlock. During installation, the insert is pressed into a pre-punched hole in the sheet; the knurls, ribs or steps on the insert squeeze into the sheet, forming a mechanical lock against rotation and pull-out.
Installation Characteristics
- No tapping required: the sheet only needs a punched or drilled hole, eliminating the tapping operation.
- Single-side or double-side pressing: depending on the structural type; some models can be completed from one side.
- Torque resistance by knurl interlock: knurl depth must match sheet hardness; soft, thin sheets have insufficient grip.
- Relies on sheet plastic flow: the sheet must have some ductility and thickness, otherwise it cracks during pressing.
Cross-Comparison of the Three Routes
| Dimension | Wire Thread Insert | Internal-External Thread Sleeve | Rivet Insert |
|---|---|---|---|
| Fixing principle | Elastic rebound + thread conformance | External thread screwed in | Plastic deformation interlock |
| Suitable base | Al/Mg die casting, plastic, steel | Thin sheet, PCB, thin-wall parts | Sheet metal, cold-rolled sheet |
| Tapping required | Yes (dedicated tap) | Yes (external-thread base hole) | No |
| Base wall-thickness requirement | High | Low | Medium (must be thick enough to grip) |
| Reusability | Replaceable insert | Can be unscrewed | Basically non-removable after pressing |
| Effect on sheet surface | None | Local protrusion | Bulge may appear on the back side |
| Load capacity | High | Medium to high | Medium |
| Typical cost level | Low | High | Medium |
The way to use this table is simple: first look at the base material, then at wall thickness and whether there is enough space to tap, finally at whether reusability is needed. These three conditions basically determine the route.
Selection Decision Path
- Is the base a die casting or plastic part? Choose a wire thread insert. Die castings have poor plasticity and crack when riveted; for plastic parts, prefer inserts with self-tapping structures or heat-set brass nuts.
- Is the base sheet metal with no space to tap? Choose a rivet insert, provided sheet thickness and ductility are enough to form an effective grip.
- Is the base a thin sheet or PCB needing threads outside the board? Choose an internal-external thread sleeve. It makes the joint capability independent of the base material, so wall thickness is no longer a bottleneck.
- Repeated disassembly is needed and the thread will wear? Prefer a wire thread insert; when worn, just replace the insert instead of the entire clamped part.
- High vibration and anti-loosening are required? For wire thread inserts, choose the screw-locking type to solve thread strengthening and anti-loosening in one step.
The common value of the three routes: they turn the thread from a non-replaceable part feature into a connection element that is designable, replaceable and verifiable. The strength of a threaded hole in soft metal no longer has to be determined by the base material.
Conclusion: Choose the Route by Base Material
Selecting a thread insert is not about which is more advanced, but about which best matches the base material. A die casting needs a wire thread insert that distributes the load; a thin sheet needs a rivet insert that avoids tapping; an ultra-thin structure needs an internal-external thread sleeve that makes the load-bearing independent.
YF Zhichengjia offers matching products and installation solutions across all three routes—wire thread inserts, internal-external thread sleeves and rivet inserts—and can provide a full set of selection advice based on base material, wall thickness and load requirements, involving itself at the design stage to confirm whether the tap-drill size and wall thickness are viable, avoiding the discovery that the insert cannot be fitted only after production starts.
