Due to the immense versatility and popularity of bolted connections, there is a also a large and constant demand for thread repair services and products. This demand can be observed in virtually any industry and can be broadly grouped into two categories: original equipment applications and repair applications.

1. OEM applications

In modern design, there is a trend towards lightening of materials while still achieving structural integrity of bolted or fastened joints. This requires stronger threads in soft materials such as aluminium, zinc or magnesium than can be achieved by direct taping the soft materials involved. Thread inserts are therefore used to make threads more durable, particularly in applications where comparatively small bolts have to bear high loads and where bolts undergo repeated torqueing and untorquing.

2. Repair applications

In repair applications, the focus lies in repairing threads that have been damaged by over-torqueing or by cross-threading, or that were designed with insufficient safety margin to start with and consequently failed in service.

A "blind" Full-Torque thread repair insert with counter bore. It has a standard metric thread on the inside and the patented spiral hook thread on the outside.
A "blind" Full-Torque thread repair insert with counter bore. It has a standard metric thread on the inside and the patented spiral hook thread on the outside.

Different designs

While it is sometimes possible to enlarge a damaged thread and to use a larger diameter bolt, the more elegant solution – and for design reasons often the only solution – is to restore the original thread and to install a new bolt or stud.

To restore damaged threads, coiled inserts or externally threaded inserts are most often used, though press-fitted solutions are also available for very low strength solutions. For OEM application, designers also have the option to use mould-in inserts, but they are only recommended for use in plastics and not metals.

Coiled inserts (also known as STIs)

Coiled thread repair inserts are similar to a coiled spring and are typically made of 18-8 stainless steel wire with a diamond-shaped cross-section. During installation of the insert, the coils are compressed and attempt to radially expand against the wall of the threaded housing. The resulting tension between the insert and the walls of the housing helps to retain the insert and keeps it from unthreading.

The main appeal of coiled inserts is that they are inexpensive and easy to install. But they lack strength and also create undesirable spreading forces – in fact often more so than externally threaded inserts. A further shortcoming of coiled inserts is their limited product range. They are usually available only in three different lengths (1D, 1.5D and 2D) and in sizes from about M2 to M39 (UNC size size 1 to 1.5″).

Externally threaded inserts

Unlike coiled inserts, externally threaded inserts are made from one solid piece. Like all repair inserts, they are easy to install. They are often self locking, for example by a pre-applied external coat of adhesives that activates upon installation. The adhesive ensures that they are pressure tight until about 41 MPa (6000 psi) up to a service temperature of about 150°C (300°F), at which point the adhesive will start to disintegrate.

Full-Torque inserts

A Full-Torque inserts is a special type of externally threaded insert. Its distinctive feature is its external Spiralhook thread which completely eliminates spreading forces and pulls the surrounding material into the thread, resulting in a very tight and strong fit. Because they do not  create spreading forces, they are ideal for high-load applications and for installation in soft or brittle material such as cast iron, aluminium and magnesium alloys. By virtue of their thread geometry, they are pressure tight up to 69 MPa (10,000 psi).

The secret to the superior strength of the Full-Torque inserts lies in the design of their outer thread.

A standard bolt and thread insert with a metric or unified standard (UNC / UNF) thread has a thread angle of 60°. During tightening of the bolt, the insert is pulled upwards and slightly compressed.

This results in a radial outward force on the surrounding material as the thread flank of the insert and the thread flank of the surrounding material are sliding off each other.

The patented Spiralhook outer threads on the Full-Torque insert pull the surrounding material into the threads of the inserts and add strength to the part being repaired.

A Full-Torque insert thus makes the bolt hole stronger than the bolt.

Attributes Coil style Key locking Self-tapping Full-Torque
Repairs stripped threads
Emergency repair solution
Stays solidly in place during lifetime

Does not create spreading force when installed
Does not create spreading force when installed
Does not create spreading force
Placement close to an edge is not a problem
Contains the spreading force of the bolt
Multi-application tooling
Adds strength to the surrounding material
96% external thread engagement
Solves the cause of cracked bolt holes
Allows multiple bolt torquing in aluminium
Prevents future failures
Repaired hole becomes stronger than bolt
Insert invisible as obvious repair
Creates pressure-tight seal on OD thread
Available in high-strength steel
Easy to install
Available as solid plug
Available with solid bottom
Available with a large shoulder

Full-Torque inserts are developed, manufactured and sold by Lock-N-Stitch Inc. in California, USA, since 2001. They have become known as the strongest thread repair system that exists today and are widely used the world over and they are protected by various US American and international patents.

At QuantiServ, we only use Full-Torque products for all our thread repair and we very much recommend to anyone to use them, whenever a thread needs to be reliably and permanently repaired or reinforced.

Full-Torque inserts are available with many different features that make them suitable for virtually any application.

Thread repair brochures

Click on the following images to download our thread repair brochures:

If you are looking for a style or size that is not listed in our catalogue, then contact us about it. We are able to quickly custom-produce any size and style with internal thread from M6 (¼”) – M125 (6″).

Contact us

Links

Metal Stitching

Very often, cracks originate from damaged bolt holes. If this is the case, then the crack must be repaired first.

Buy Full-Torque Inserts

All Full-Torque thread repair products are manufactured and sold by Lock-N-Stitch Inc. This link will bring you to their website.

Reference Cases

Full-Torque inserts are very widely used. See here some typical examples of their use in different industries.

Image Gallery

An M40 counter-sunk insert installed to repair a damaged cylinder head stud hole, shown here just prior to skimming of the landing surface.
An M40 counter-sunk insert installed to repair a damaged cylinder head stud hole, shown here just prior to skimming of the landing surface.
Two of 36 thread inserts installed in the bearing saddles of a reciprocal gas compressors. All saddles were treated this way to prevent them from cracking.
Two of 36 thread inserts installed in the bearing saddles of a reciprocal gas compressors. All saddles were treated this way to prevent them from cracking.
Another cylinder head stud hole repair. A counter-sunk insert was installed. Notice also the stitching pins that were installed to repair a radial crack.
Another cylinder head stud hole repair. A counter-sunk insert was installed. Notice also the stitching pins that were installed to repair a radial crack.