A HeliCoil is a coiled-wire type of thread repair insert used to create internal screw threads to accommodate standard-sized fasteners.
Some of the advantages of using a Helicoil include...
According to published data, inserts are available as small as a #2 screw size, and as large as 1 1/2" diameter. Corresponding metric sizes are also available.
Each insert is offered in several different length options.
As its name suggests, a HeliCoil is a helically coiled stainless steel wire that has a diamond-shaped cross-section.
The wire cross-section allows the external portion of the wire to screw into the receiving threads in the housing during installation.
On the inner portion of the wire, the spacing between the coils of the insert form new conventional 60° threads.
The coils of the insert are compressed during installation as the insert is threaded into the housing.
Like a coiled spring, the compressed windings of the installed insert attempt to expand against the walls of the threaded housing. The resulting tension between the insert and the walls of the housing helps to retain the insert and keep it from unthreading.
A small notch is formed into the insert that slightly reduces the coil cross sectional area at that location. When the time comes to remove the tang, the notch will cause it to break-off at that point.
As the insert is installed, be sure to allow the pre-coil body to float as needed to allow the windings of the HeliCoil to align with the threads in the housing.
One of the reasons that HeliCoils fail in service is that they are improperly installed initially.
A good rule-of-thumb is to install the HeliCoil so that the top of the insert is 1/4 to 1/2 turn below the top surface of the workpiece. One easy way to determine the installed depth is detailed below...
Before beginning the HeliCoil installation process, look closely at the STI threads cut in the hole. Notice the location where the STI thread first starts to enter the hole.
Make a small temporary reference mark on the surface of the housing that is rotated 3/4 turn from the location where the STI thread first enters the hole. This mark is used determine the installed depth of the insert.
Install the insert using the process previously described, and screw the HeliCoil into the housing until the (non-tang) end of the insert is rotationally aligned with the temporary reference mark.
Following this process, the top of the insert is installed 1/4 turn below the top surface of the housing.
If a deeper seating of the insert is desired, rotate the installation tool up to 1/4 additional turn.
The end of the punch or bar should have a square end (without a chamfer).
Strike the end of the tool with a sharp tap with a hammer. The tang should break-off at the notch in the insert that is designed for that purpose.
On larger diameter inserts, it is sometimes easier to use a pair of needle-nose pliers to twist-off the tang.
Once installed, the internal threads of the HeliCoil insert are designed for use with standard sizes of fasteners. To work correctly, the spacing of the coils of the HeliCoil wire is installed with the same pitch (spacing between threads) as the fastener for which it is intended.
Because the external thread diameter and pitch is determined by the dimensions of the installed fastener, the resulting external thread does not correspond with a standard tap size.
A "Screw Thread Insert Tap" (also known as an STI Tap) is designed specifically to be used in the installation of some types of thread inserts, including HeliCoils. An STI tap is different than a standard fastener thread tap; they can not be used interchangeably.
To illustrate the difference between a standard tap and an STI tap...
A standard tap used to create threads for a 1/2" NF fastener has a major diameter of .500" with a pitch of 20 threads per inch.
The corresponding STI tap for that particular insert has the same thread pitch of 20 threads per inch, but has a larger major diameter. (In this example, the STI tap major diameter measures .568".)
The following methods are recommended to help secure an insert into the housing...