Using Tanged Screw Thread Inserts:
You’re at the end of your job, one more bolt to go and, wouldn’t you know, the threads blow out. You remove the bolt and the threads are coated with aluminum. This means the threads are stripped, not the bolt. If you only lost the top two threads, you could probably get by, but if it goes any deeper than that, you need to re-drill & re-tap that hole for the fastener. That’s when knowing installation for screw thread inserts comes in handy.
Just tapping will not be enough. The threads are now unreliable and while you might be able to get a little bite with a tap, it won’t last. Your best chance of salvaging the part is to repair the threads with a screw thread insert.
A screw thread insert (STI) is a coiled wire-type insert that installs into the re-drilled & re-tapped hole, giving you the correct diameter and pitch threads to accommodate the original fastener. The most common thread repair insert is the tanged insert. These have a diamond-shaped cross section with 60° threads on the inside and a special feature on the outside that grips the walls of the hole.
How to Install a Tanged Screw Thread Insert:
Installing a tanged screw thread insert requires drilling out the hole to match the STI tap drill diameter, tapping the hole for the new coil with an STI tap, installing the screw thread insert, & removing the tang. You can purchase inserts individually or in kits with drill, tap, installation tool, & a quantity of inserts.
The proper installation depth of a screw thread insert is 1/4 to 1/2 turn below the top of the original hole.
Remove the fastener from the hole. Inspect the hole and make sure that the threads are indeed damaged. With safety goggles on, use compressed air to blow out the hole, or use a little brake cleaner.
Using the correct size bit, drill out the hole. Make sure you drill to the end of the hole, but not deeper than you need to.
Thread the new hole with the correct STI tap for the size of insert you need. These taps are not standard 60° threading taps, they are specific STI (Screw Thread Insert) tools. Unlike a standard tap, the threads are the same, but the diameter is slightly larger, yielding more grip for the insert.
Here is where the process differs a bit depending on your thread pitch, coarse or fine.
Coarse thread – Using the installation tool, mount the screw thread insert to the tool, rotate the insert until the tang aligns with the drive notch on the tool. Thread the insert into the hole to the proper depth and remove the installation tool. When using a coarse thread screw insert, install the insert onto the installation tool until the tang hits the stop. By the very design of the insert, it will hold itself in place without the use of threadlockers.
Fine thread – Install the screw thread insert into the pre-coil installation tool. This tool compresses the insert to the correct size. Thread the installation tool into the insert inside the pre-coil body. Spin the installation tool until it is fully threaded.
Hold the pre-coiled assembly to the hole being repaired. Thread the installation tool so that the insert threads out of the pre-coiler and threads directly into the repaired hole. Remove the installation tool when the screw thread insert reaches the proper depth.
The insert is now installed but the tang must be removed. Using a punch that fits snugly into the insert, place the punch into the insert until it reaches the tang. Hit the punch with a hammer. This should knock off the tang. Use tweezers or a magnet to retrieve the tang from the hole.
Screw thread inserts have been used for high-stress, high-use performance applications for a long time and have performed well. The automotive industry has used screw thread inserts in original designs, not just for repairs. They are inherently stronger than some fastener joints and are super reliable.