The Essential Guide
to Coil Thread Inserts

What is the difference between a coil thread insert and a Heli-Coil? Actually, there is no difference. Just as Kleenex is commonly used to refer to facial tissue and Xerox has become synonymous with photocopies, “helicoil” has become a commonly recognized term for screw thread inserts. Heli-Coil® is a registered trademark of Stanley Engineered Fastening, a division of Stanley Black & Decker. Its design has become the industry standard for coil thread inserts.

Coil thread inserts, also known as helically wound inserts, are coiled wire inserts placed inside the drill hole to create a strong threaded surface to secure a bolt or other threaded fastener. When a drill hole has been stripped or the material is too soft to secure a drilled fastener, as can be the case with magnesium, wood, aluminum, zinc, or plastics, the coil provides an extremely strong and lasting bond by transferring the pressure exerted from the bolt to the sides of the parent material.

The helically wired coil is screwed into place and creates a threaded track for the screw threads of a bolt or fastener. The diameter of the coil is wider than the tapped hole, so as the bolt is screwed in, the coil compresses against the side of the tapped hole. This compression creates a safe and highly secure connection—much stronger and with more torque than similar assemblies. In fact, when used for repairs, coil thread inserts create a stronger bond than the original tapped hole.

With ordinary bolt-and-hole inserts, the top two screw threads carry 65-75 percent of the load. Helically wound inserts distribute the load so each coil shares part of the load across the length of the coil. The result is a stronger connection and longer thread life, even after assembling and disassembling multiple times.

Coil thread inserts are especially useful in applications that have to withstand stress, severe vibrations, and extreme temperatures. Even under extreme conditions, these inserts minimize wear of the threaded hole and increase the integrity and useful lifespan of the connection. That’s why coil thread inserts have become a popular fastener solution for critical applications such as automotive manufacturing, aerospace, railways, factory machinery, and transportation systems.

Coil thread inserts are quite versatile and available in a wide range of thread diameters and thread sizes. They also come in a variety of materials. Stainless steel coil thread inserts, for example, are resistant to corrosion and extend the useful life of the fastener. They also can withstand temperatures ranging from -320 degrees to +800 degrees Fahrenheit because the coils can expand and contract with temperature changes.

Tanged Coil Thread Inserts

Tanged coil thread inserts have a straightened piece of coil at one end that extends across the radius of the coil. This piece is called a “tang” and is used as an anchor during installation. A special helical insert tool grips the tang and uses it to screw the coil into the tap hole. Once the coil is inserted and secured, the tang is broken off and removed.

CoilThread tanged inserts are extremely reliable and long-lasting. CoilThread is made from cold-rolled 304 stainless steel wire (AS7345), and work-hardened to a rating of Rc 43-50 and a tensile strength that exceeds 200,000 psi. More importantly, the finished surface is extremely smooth (8-16 microinches), which virtually eliminates thread erosion caused by friction. CoilThread is also available in other materials, such as phosphor bronze, Inconel X-750, Nitronic 60®, Nimonic 90®, and titanium, as well as stainless steel. They also come with special coatings for specific applications.

Tanged coil thread inserts fall into two basic subcategories:

• Tanged free-running coils are designed so each coil adjusts independently, which means each bolt or fastener thread adjusts itself for maximum efficiency. More surface contact between the individual threads and the parent materials makes for a stronger bond because the load is distributed evenly across the entire coil. Free-running coil thread inserts also are subject to less stress because there aren’t specific stress points, which means no fatigue concentrations and fewer failures. Stainless steel free-running coil thread inserts resist corrosion and reduce galvanization, eliminating corrosion caused by a chemical interaction between the bolt or parent material and the coil.

• Tanged screw-lock coil thread inserts also use a tang for coil insertion, but they additionally feature a series of straight segments as part of the coil. Once they are inserted in the tapped hole, the entry of the bolt flexes the straight segments outward for a stronger bond. This places more pressure on both the bolt threads and the parent material, maximizing the contact area. Locking inserts are designed to meet the rigorous specifications of the NASM8846 aerospace standard for military applications, which requires the inserts to pass a 15-cycle torque test.

Both free-running and screw-lock tanged helical inserts come in UNF sizes (measured in inches) and metric versions.

Tang-free Coil Thread Inserts

Tang-free coil thread inserts, as the name implies, do not have a tang at the bottom of the coil. Instead, there is a driving notch strategically located in the coil that serves as a grip point for the helical coil insert tool. Tang-free coils are identical to tanged coil thread inserts, other than lacking a tang, and are available in free-running and screw-lock configurations.

Increasingly, manufacturers are moving to tang-free coil thread inserts because they are just as easy to install and they eliminate the need to remove the tang. This removes the risk of loose tangs or pieces of metal getting into the machinery, and it speeds up installation and inspection time. Some helical coil makers estimate you can save between $0.15-$0.50 per hole by standardizing on tang-free coil thread inserts. These helically wound inserts are especially popular with electronics manufacturers and high-tech manufacturing—wherever smaller, high-precision inserts are used.

Tang-free coil thread inserts are available with the same free-running and screw-lock designs as tanged coil thread inserts, with all of the same advantages. They are made out of work-hardened steel and other materials for maximum strength with minimum friction. They also are available with custom coatings. All tang-free coil thread inserts are available in UNF (inches) and metric sizes.

Additionally, all coil thread inserts come in either bulk or strip-fed versions, where the inserts are positioned in a plastic ribbon which is passed through an assembly nozzle. Strip-fed coil thread inserts are used in high-volume production applications with tooling that accepts strip-feed inserts. All types can be inserted by hand or using a strip feed. With tangless coil thread inserts, ribbon-fed inserts can increase the number of installation cycles with less operator fatigue.

Kato Fastening Systems

Kato Fastening Systems has made coil thread inserts since the 1970s. Kato coil inserts are widely used in the aerospace industry and their ColiThread inserts are made of cold-rolled stainless steel with a tensile strength of 200,000 psi.

Kato inserts also are available in a variety of materials for different applications:

• Inconel X-750 is a nickel-chromium alloy hardened by adding aluminum and titanium for high-temperature applications up to 1,000 degrees Fahrenheit (ideal for nuclear reactors).
• Phosphor bronze inserts are ideal for saltwater applications because they resist corrosion. They have a tensile strength of 140,000 psi and a hardness of HRB 95 with low magnetic and electrical conductivity, making them ideal for naval vessels, aircraft instrumentation, mining and drilling, and similar applications.
• Nitronic 60^® is designed to minimize friction and galling with stainless steel screws thanks to the addition of silicon and manganese to the alloy. No additional coatings are required, so they are useful in a vacuum because there is no outgassing. They are non-magnetic and corrosion-resistant, and are typically used in aerospace and semiconductor manufacturing.
• Nimonic 90^® inserts are designed for extreme pressure and extreme heat. This is a nickel-chromium-cobalt alloy that is useful for jet engine parts, exhaust nozzles, and turbine components, among other applications.
• Titanium coil inserts offer a superior strength-to-weight ratio. They have a slightly lower tensile strength than steel coils but are much lighter, so they are typically used in biomedical applications such as implants and prosthetics.

Recoil from Arconic Fastening Systems

Arconic’s line of Recoil coil thread inserts are manufactured in Australia, although the company also has warehouses in North America, Asia, and Europe. Recoil fasteners are available tanged and tang-free in a wide range of metric and standard sizes and in free-running and screw-lock configurations.

Recoil coil inserts also come in a variety of materials, including stainless steel (grades 304 and 316), Inconel X-750, phosphor bronze, Nimonic 90, and Nitronic 60. This versatility makes Recoil inserts useful for various applications and industries, such as automotive, ship building, power generation, manufacturing equipment, consumer electronics, aerospace, transportation, and industrial electronics, to name a few.

A variety of lubricants and coatings also are available:

• Silver plating reduces galling of screw threads in high-temperature applications, such as aerospace engines in which service temperature can reach 1,200 degrees Fahrenheit.
• Cadmium plating is useful as an anti-corrosive for marine applications, providing protection against pitting and thread seizure.
• Dry film lubricants are used in mildly corrosive and high-temperature applications. Dry film lubricants have small particles of solid lubricants applied as a thin film to grease-free metals. Film lubricants can be formulated for various industrial applications.  
• Tin plating is specified in ISO2093 for moderately corrosive applications, such as auto manufacturing,
• Red dye or other color coatings can be applied for identification purposes.

In addition to helically wound inserts for manufacturing, Recoil also offers thread repair kits, complete with an STI (screw thread insert) tap, installation tool, tang removal tool, drill, stainless steel inserts, and instructions.

Here are just some of the common applications for helically wound coils in various materials:

• Aluminum – Aluminum is a soft metal with about one-third the density of steel or copper, so it doesn’t handle screwed fasteners well, especially steel bolts. However, as a light metal, aluminum is ideal for aircraft manufacturing and other applications where weight is a factor, and coil thread inserts ensure that fastenings are secure and set to last.
• Plastic – Plastics have become a universal material for manufacturing, and coil thread inserts overcome the problem of drilling connections in malleable plastic. Specifically, non-conductive and non-corrosive materials are commonly used for plastic helically wound inserts.
• Wood – When working with wood, especially softer woods such as pine or composite board, screws and bolts tend to strip. Helically wound inserts ensure a secure and lasting joint that won’t loosen over time.
• Fiberglass – Fiberglass has become a popular material for various applications that require flexible, strong material, such as boats and hot tubs. Helically wound inserts are useful for secure, blind bolt installations and to ensure lasting, watertight connections.
• Steel – We have touched on a variety of coil thread insert applications for steel. Here is a common example from the automotive industry: Coil thread inserts are extremely useful for repairing cylinder heads when the thread socket has been stripped because of unintentional over-torquing or spark plug cross-threading. Helically wound inserts also are commonly used in exhaust systems.
• Concrete – For construction applications, building frames and other materials have to be securely fastened to concrete foundations and structures. The only way to ensure a lasting bond that won’t deteriorate as concrete weakens with time is to use helical coil thread inserts.

Because helically wound inserts were first used in aerospace and military applications, they were made to exacting manufacturing standards using precision engineering and the most stringent quality controls. All the inserts offered by Bay Supply either meet or exceed military and aerospace specifications, and they have full lot traceability, so you can be sure of their quality.

A complete line of free-running and locking inserts for all applications is available, either in bulk or in strip feed reels. Bay Supply also offers the required installation tooling for any manufacturing environment, including a full line of taps, gauges, hand tools, air tools, and electric tools.

Here is an overview of some of the additional requirements you should understand when installing coil thread inserts:

Coil Thread Insert Tools – Helically wound coils can be inserted by hand or using any number of power tools. Coil thread insert tools are available in pneumatic, hydraulic, and electric-powered units from manufacturers such as Atlas Tools, Advel, Gesipa, and Recoil. Kato and Advel also offer manual tools, which are ideal for simple repair jobs.

• Pneumatic tools typically are used for high-volume, industrial applications, such as in a factory, using strip-feed coil thread inserts.
• Electrical coil thread insert tools are becoming increasingly popular. For heavy-duty, high-volume applications, electrical tools can perform at high speeds.
• There are also battery-powered versions available for handheld installations, as in automobile or aircraft repair. Some of the more advanced electrical and pneumatic tools also have built-in prewinders, which compress the coil thread insert before it is inserted into the tapped hole.
• Manual tools are available for occasional uses, such as repairs.

Coil Thread Inserts Insert Lengths and Drill Sizes – In order to gain the full load value of the bolt, you have to choose the helically wound insert that is the right length for the job. A coil thread insert size calculator has been developed using the ASME B18.29.1–1993 standard and the HASM 33537 standard to determine the right insert size for maximum efficiency.

Coil Thread Insert Sizes – In addition to coil length, you need to be sure you have the right coil thread insert width for the job. Determine whether you need tanged or tang-free inserts, the size of your tap holes, and the insert material that’s best for the job.

Coil Thread Insert Kits – For smaller applications and repair jobs, you may need a self-contained insert kit, which includes manual insert tools and a variety of inserts.

 

Once the hole is drilled, you need to tap the hole using an STI to create a screw thread. You can then use an STI Go or No Go gauge to ensure the pitch is correct through the entire tap hole.

The standard type of STI tap creates threads that match the coils of the insert. These taps are usually used on a CNC machine or high-precision equipment to ensure that the fit is exact. When preparing a hold for a screw thread insert, a drill bit is used first to open the hole to the correct minor diameter. The STI can then be inserted to create the proper threads to receive the insert.

There are different types of taps with various designs, including two-thread chamfers and four-thread chamfers, which are useful for drilling blind holes, and fluted taps, which remove chips of material as they turn.

Now you can install the insert itself. The coil thread insert is fitted on the end of the tool and screwed into the hole. If you are using tanged coil inserts, the tang will fit in a notch at the end of the bit to provide a secure grip to rotate the coil. With a tang-free coil, the bit should grip the notch in the coil. Once the insert is secured, another tool is used to grip and break the tang; to prevent having loose metal, it is important the tang be removed and not just broken off.

Now the coil is ready to accept the bolt or fastener. As the bolt is screwed into the tap hole, the coils or locking coils will flex outward, generating pressure on the bolt and the parent material for a secure seal. The spring-like design of the coil transfers the shear load into radial load across the entire length of the insert, which virtually eliminates failures caused by high stress.