You are just about finished with your most recent maintenance project, one more bolt to tighten and…. dang it, wouldn’t you know the threads just blew out in the cylinder head. You remove the bolt and it is coated with aluminum, which means that the threads in the head are stripped, not the bolt. Now you are in for work. Stripped threads are rarely any fun, but they are a part of working on vehicles. If you only lost the top one or two threads, you can get by, but if it goes any deeper than that, you need to restore it. That’s when knowing how to install a Helicoil insert comes in handy.
Fixing Stripped Threads
In this instance, a tap is not going to cut it, the threads are wasted and while you might be able to get a little bite with a tap, it won’t last. The best bet is to repair the threads with a HeliCoil insert. A HeliCoil is a coiled threaded insert that installs into the part, providing the correct threads for the original fastener. There are several types of thread repair inserts, but the most common is the tanged insert. These inserts have a diamond-shaped cross section with 60-degree threads on the inside and specially designed threads on the outer ring, which grips the walls of the hole.
HeliCoil Insert Types
Free running inserts are fully rounded inserts that are slightly larger than the hole they install in. When installed, they grip the walls of the hole through the spring-loaded action. This allows the fastener to easily install and be removed as well. These do not require any special staking or locking. Screw locking inserts have a series of flat spots on the spiral threads, which put pressure on the fastener, holding it in tight. They are reusable, and still maintain their grip on the fastener.
How To Install A HeliCoil Insert
Installing a HeliCoil insert requires drilling out the original hole to match the new coil, tapping the hole for the coil, installation, and tang removal. You can purchase the inserts individually or in kits. You need a master kit for the size of threads you are working with if you do not have a tap and die set.
Pro Tip – The proper installation depth of a HeliCoil insert is ¼-1/2 turn below the top of the original hole.
Remove the fastener from the hole. Inspect the hole and make sure the threads are damaged. Use a little brake cleaner or compressed air to blow out the hole.
Using the correct size drill bit for the insert, drill out the hole. Make sure you drill to the end of the hole, but not deeper than necessary.
Thread the new hole with the correct tap for the size of insert you have. These taps are not standard 60 degree thread 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 alters depending on the thread pitch of your fastener, coarse or fine.
Coarse thread – Using the installation tool , mount the HeliCoil insert to the tool, rotate the insert until the tang mates with the drive notch on the tool. Thread the insert into the hole to the proper depth. Then remove the install tool.
Fine thread – Install the HeliCoil insert into the pre-coil tool. This tool compresses the insert to the correct size. Thread the install tool into the insert inside the pre-coil body. Spin the installer tool until it is fully threaded.
Hold the pre-coiled assembly to the hole being repaired. Thread the installer tool so that the insert threads out of the pre-coiler and threads directly into the repaired hole until it reaches the proper depth and remove the installer tool.
At this point, the insert is installed, but the job is not over. For all tanged inserts, 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 pop the tang off. Use a magnet or tweezers to remove the tang.
HeliCoils do not require thread sealants or thread locker in order retain the insert, but if you feel the need to use sealant or thread locker, a light coating may be applied to the outer threads, making sure the inner threads are clean. Once installed, HeliCoil inserts are very reliable and trustworthy. They have been used for high-performance applications under high stress, they perform very well in most situations.
Check out all the tools & equipment available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on HeliCoil thread inserts, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.
A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.