Wire Thread Insert Selection: Free-Running vs Screw-Lock Types
Executive Summary
The most common sticking point in wire thread insert selection is free-running vs screw-lock. Based on GB/T 24425.1 and GB/T 24425.3, this article explains the difference between one locking coil and a free-running helix, the M2-M39 size range, how to set length by nominal diameter multiples, and why STI special taps are required for the tapped hole.
Table of Contents
What Problem Does a Wire Thread Insert Solve
A wire thread insert (also called a helical thread insert or screw lock insert) is a precision diamond-section stainless steel wire wound into a helix. It is installed into a specially tapped hole (called an STI hole) and expands to form a new, standard internal thread inside the soft parent material. The problem it solves is simple: soft materials like aluminum, magnesium, zinc die castings, and engineering plastics cannot hold standard bolt threads reliably. Repeated assembly strips the threads, and even a single installation can tear the soft material if the bolt is torqued too high.
By inserting a stainless steel helix into the soft material, you create a steel internal thread that can withstand dozens of assembly cycles and full bolt torque. The insert also repairs damaged threads - if a tapped hole strips, you can drill it out, tap an STI hole, and install an insert to restore the original thread size. This is far cheaper than scrapping an expensive die casting or housing.
Free-Running vs Screw-Lock
The two main types are free-running and screw-lock. A free-running insert is a continuous smooth helix; once installed, it forms a standard internal thread that the bolt screws into freely, just like a normal tapped hole. It provides thread reinforcement and wear resistance but no additional locking. It is used when the joint does not see significant vibration, or when anti-loosening is handled by a separate device (prevailing-torque nut, thread adhesive, etc.).
A screw-lock insert has one or more polygonal or locking coils in the helix that grip the bolt thread radially when the bolt is installed. This creates a prevailing torque that resists vibration loosening. It is used in vibrating joints such as engine mounts, pump bodies, motor housings, and anywhere the bolt cannot be allowed to back out. The trade-off is that the screw-lock insert requires more torque to turn the bolt in and out, and it is slightly more expensive.
| Feature | Free-Running Insert | Screw-Lock Insert |
|---|---|---|
| Locking mechanism | None (smooth thread) | Radial gripping locking coil |
| Assembly torque | Normal (same as tapped hole) | Requires prevailing torque to turn bolt |
| Reusable? | Yes | Yes (limited number of reuses) |
| Typical application | Soft-material thread reinforcement, thread repair | Vibrating joints, engines, motors, pumps |
| Standard | GB/T 24425.1 | GB/T 24425.3 |
How to Determine Size and Length
Wire thread inserts are specified by the nominal thread size they create (e.g., M6, M8x1.25) and by length. The length is expressed in multiples of the nominal diameter: 1d, 1.5d, 2d, 2.5d. For example, an M8 insert at 1.5d length is 12 mm long. The length you choose depends on the parent material strength and the clamp load required. For aluminum, 1.5d to 2d is typical. For magnesium or soft plastics, 2d to 2.5d may be needed to spread the load. For stronger materials like cast iron, 1d may be sufficient.
The material is usually 304 stainless steel for general use, with Inconel for high-temperature or highly corrosive applications, and phosphor bronze for special electrical conductivity needs. The size range is M2 to M39 in metric, and #2 to 1-1/2 inch in UNC/UNF imperial sizes.
Tapped Hole and Installation Rules
The most important installation rule is that the tapped hole must be an STI (STI = Screw Thread Insert) hole, tapped with a special STI tap. You cannot drill out a stripped M8 hole and tap it directly to M8; you must drill to the STI oversize diameter, tap with an STI tap, then install the insert. The insert then forms a standard M8 thread inside. Using a standard tap instead of an STI tap will result in an incorrectly sized hole and the insert will not seat properly.
Installation requires a dedicated insert installation tool (a tap-like tool with a slotted drive that engages the insert's tang). The insert is screwed into the STI hole until it is slightly below the surface, then the tang (the small drive tab at the end) is broken off with a tang break-off tool. After installation, the hole should be checked with a GO/NO-GO gauge for the finished thread size.
Quick Selection Guide
- Aluminum die casting, non-vibrating joint: Free-running insert, 1.5d length, 304 stainless.
- Magnesium housing, vibrates: Screw-lock insert, 2d length, 304 stainless.
- Plastic enclosure, occasional assembly: Free-running insert, 1d to 1.5d length.
- High-temperature engine or exhaust: Free-running or screw-lock, Inconel material.
- Thread repair on a stripped hole: Free-running insert at the original nominal size, STI re-tap required.
Installation quality control. Wire thread insert failures usually come from poor installation, not from the insert itself. The STI tap must be sharp and correctly sized; a worn tap produces an undersized hole that the insert cannot expand into. The installation tool must match the insert size; using the wrong tool can cross-thread the insert. After installation, break off the tang and check the finished thread with a GO/NO-GO gauge. For production runs, inspect the first five inserts and then sample every hour. A properly installed insert should not spin or pull out, and the bolt should thread smoothly with normal run torque.
Cost and lead time for wire thread inserts. Wire thread inserts themselves are inexpensive parts, but the STI taps and installation tools add cost. For production use, the tooling is amortized over thousands of inserts. The insert material choice affects cost: 304 stainless is standard, Inconel is several times more expensive for high-temperature use, and phosphor bronze is for special conductivity. When designing with inserts, specify the insert length based on the parent material: 1.5d for aluminum, 2d for magnesium, 1d for cast iron. Over-specifying length wastes cost; under-specifying risks stripping. Work with your insert supplier to confirm the length calculation for your specific material and clamp load.
Insert material and finish options. The standard insert material is 304 stainless steel, which is adequate for most indoor and mildly corrosive environments. For high-temperature service above 425 C, use Inconel X-750 or similar nickel alloy inserts. For special electrical conductivity, phosphor bronze inserts are available. For medical or food equipment, 316 stainless is specified. The insert surface is usually plain, but dry-film lubricated inserts are available for reduced installation torque. When selecting the insert material, match it to the environment and the bolt material to avoid galvanic issues. A 304 insert in an aluminum housing is generally fine; a carbon steel insert in an aluminum housing would corrode.
Installation quality control. Wire thread insert failures usually come from poor installation, not from the insert itself. The STI tap must be sharp and correctly sized; a worn tap produces an undersized hole that the insert cannot expand into. The installation tool must match the insert size; using the wrong tool can cross-thread the insert. After installation, break off the tang and check the finished thread with a GO/NO-GO gauge. For production runs, inspect the first five inserts and then sample every hour. A properly installed insert should not spin or pull out, and the bolt should thread smoothly with normal run torque.
Cost and lead time for wire thread inserts. Wire thread inserts themselves are inexpensive parts, but the STI taps and installation tools add cost. For production use, the tooling is amortized over thousands of inserts. The insert material choice affects cost: 304 stainless is standard, Inconel is several times more expensive for high-temperature use, and phosphor bronze is for special conductivity. When designing with inserts, specify the insert length based on the parent material: 1.5d for aluminum, 2d for magnesium, 1d for cast iron. Over-specifying length wastes cost; under-specifying risks stripping. Work with your insert supplier to confirm the length calculation for your specific material and clamp load.
Insert material and finish options. The standard insert material is 304 stainless steel, which is adequate for most indoor and mildly corrosive environments. For high-temperature service above 425 C, use Inconel X-750 or similar nickel alloy inserts. For special electrical conductivity, phosphor bronze inserts are available. For medical or food equipment, 316 stainless is specified. The insert surface is usually plain, but dry-film lubricated inserts are available for reduced installation torque. When selecting the insert material, match it to the environment and the bolt material to avoid galvanic issues. A 304 insert in an aluminum housing is generally fine; a carbon steel insert in an aluminum housing would corrode.
Conclusion
Wire thread inserts turn soft-material tapped holes into steel threads. Choose free-running for reinforcement and repair; choose screw-lock when vibration loosening is a concern. Specify length by parent material strength (1d to 2.5d), use STI taps for the hole, and install with the proper tool. When sized and installed correctly, an insert outlasts the parent material itself.
