By Matt Dowd

What is a Wire Thread Insert?

A wire thread insert, is a helically shaped metal coil that can be inserted into a pre-tapped hole to create a stronger female thread for a male threaded bolt.

Wire Thread Inserts are sometimes also called helical inserts, helically wound inserts, Recoils or Helicoils.

What do Wire Thread Inserts do?

Wire thread inserts are typically used for one of two reasons:

  1. To repair a thread that has been stripped
  2. To create a stronger thread in weaker materials such as Aluminium

They are most frequently used to strengthen threads in OEM castings and machined parts made from lighter weight but inherently weaker materials; Aluminium being by far the most common.

They are also sold in the form of a thread repair kit along with an installation tool, drill and tap providing an all-in-one repair solution which recreates the thread to accept the original bolt. These tend to be very popular for thread repairs in the automotive aftermarket or machinery and avoid costly replacement of components .

How does a Wire Thread Insert Work?

Once the wire is wound into a helical coil and installed into a tapped hole, it provides a permanent and wear resistant thread in the parent material that is generally 20% stronger than the original thread.

The inserts are designed to be greater in diameter than the tapped hole and compress as they are installed. This allows maximum surface contact area with the tapped thread, safely and permanently anchoring the inserts into place. The insert’s compensatory action shares the load over the entire bolt and hole, increasing pull out and torque out strength. With a Recoil insert in place, load and stress are more evenly distributed over the assembly.

Stress concentration problems which typically occur in the parent material when using solid inserts are therefore eliminated. A Recoil insert will dimensionally adjust both radially and axially, to any expansion or contraction within the parent material.

Typically, threads created directly in material can result in:

  • Limited contact point
  • Poor flank contact between bolt and parent thread
  • Unequal distribution of bolt load over engaged threads
  • Failure of threaded components when loaded (especially true in aluminium and other weaker metals)

The diagram below depicts graphically the advantages a Recoil wire thread insert has over a conventional thread. In conventional threaded joints over 75% of the load is placed on the first three threads of the assembly. The Recoil insert on the left shows how the spring-like design of the insert allows the shear loading to be transformed into a preferable “hoop stress” or radial loading over the entire length of the insert. This provides a much stronger thread than can be obtained by conventional drilling or tapping.

This improved strength allows designers to select a fastener based on the minimum strength of the bolt, enabling manufacturers to use smaller diameters and shorter thread lengths with confidence even in low strength materials such as magnesium or aluminium alloys.

How is a Wire Thread Insert installed?

There are 4 simple steps to install a wire thread insert.

  1. The hole (new or repair) is drilled out to the correct diameter.
  2. The drilled hole is tapped with an STI tap to create the correct thread to fit the external diameter of the insert.
  3. The insert is wound into the thread, creating compression as it is installed.
  4. Once the tang is broken off using a tang-break tool the internal diameter of the insert is now ready for the original bolt.

What are typical applications for Wire Thread Inserts?

  • Automotive
  • Industrial Electronics
  • Consumer Electronics
  • Aerospace – Avionics, Engines, Airframe
  • Ship Building
  • Defense
  • Power Generation
  • Transport
  • Manufacturing Equipment