If you work on cars, trucks, motorcycles, or any other piece of machinery, you’ll no doubt experience that dreadful feeling when a wrench suddenly goes “soft” tightening a bolt. Immediately you know the threads stripped. In the rare instance, the offending threads can be tuned up by carefully running a tap through them, but that’s seldom the case. Most often you’re faced with repairing the threads.

There are a couple of ways to fix the damage, depending upon the circumstances. Visit SummitRacing.com and you’ll find hundreds of different variations of thread repair kits, ranging from Loctite thread forming products to dedicated spark plug repair kits. In between is a huge range of Heli-Coil or Fix-A-Thread systems—each with kits dedicated to the specific type of thread you’re trying to repair.

With a formed thread repair by way of a liquid repair system, you’re essentially making a mold of the original thread, using the fastener in question. The process goes something like this:

When you can access the backside of the damaged component, cover the bottom of the bore and fill the damaged thread bore about half way or so with Loctite Form-A-Thread. Apply the supplied release agent to the threads of the screw or bolt. Then, with a “jiggling/twisting” motion insert the fastener. Loctite states this improves the thread conformation. Then you allow 30 minutes to cure. Sometimes you have to clamp the fastener in place while the product cures.

Using a Thread Insert to Repair Damaged Threads

Of course, this isn’t the only means to fix threads. Another method is to use an insert. How does mechanical thread repair (by way of an insert) work? According to OEM Tools:

“The thread inserts are made with high strength stainless steel wire. Uninstalled the thread insert is greater in diameter than the tapped hole into which the insert will be installed. During installation, the compressed coil spring design reduces in diameter, then springs back to permanently anchor the insert into place. Since the thread insert is larger than the original female thread, it provides greater contact area that is generally stronger than the original female thread.

“The coil spring design adjusts axially and radially to fit expansions and contractions within the parent material, eliminating stress concentration problems that typically arise with solid style inserts. Locking, keying or swaging the insert is not needed with using coil style inserts.”

When using mechanical thread repair kits, a special tap is included. The reason is the Screw Thread Taps are specifically designed to be slightly larger than a normal tap. This provides the correct hole diameter for thread repair insert installation. And because of this, you have to drill out the damaged thread. Repair kits (Heli-Coil or Fix-A-Thread) spell out the drill size necessary.

Essentially, you can use these thread repair kits for alloys, steel, plastic, or magnesium. The actual wire in the Fix-A-Thread kits is manufactured from stainless steel, with a diamond shape cross section wound to the form of a spring thread. You’ll note that there’s a tail or tang on the inserts. This engineered so that the supplied tool can wind the insert into place. Once installed, the tang is broken off.

As you can see, there are a number of different ways to safely and securely repair threads. The fix isn’t really painful either. In fact, we’ll walk you through a typical thread repair in the photos below.

And you may enjoy this article as well: How to Use a Tap & Die Set to Cut Internal & External Threads

thread tap & repair tool on a workbench
This is an OEM Tools Fix-A-Thread kit, and there are various versions of this tool to correspond with different thread sizes.. This particular kit was sourced to repair damaged threads on a Holley carburetor bowl drain plug.(Image/Wayne Scraba)
drilling out a stripped hole on a holley fuel manifold
If necessary, drill to clear out the damaged thread. Each kit identifies the recommended drill size required. It’s a good idea to check your drill bit selection beforehand! It’s also a good idea to double check the threads on the fastener to ensure you order the right kit. (Image/Wayne Scraba)
thread tap and cutting fluid on a workbench
Each kit includes a special tap designed to properly rethread the hole/thread size. Tapping fluid is recommended for all tapping applications. (Image/Wayne Scraba)
thread tap repairing thread in a fuel port
Place the provided tap into a tap wrench (handle). Tap the bore to the required depth. (Image/Wayne Scraba)
oem tools thread repair tool
The insert simply slides on the tool as shown, with the tang centered inside the slot on the tool. You’ll note the collar on the tool is adjustable. It can slide down on the tool shaft if necessary to square the insert. (Image/Wayne Scraba)
inserting thread repair tool into drilled hole
Using the tool handle, carefully rotate the insert into the tapped hole with constant light pressure until the top of the insert is a quarter to half turn below the surface. (Image/Wayne Scraba)
threaded insert repair in a drilled hole
Disengage the tool. (Image/Wayne Scraba)
hammering thread repair insert in a drilled hole
Reinsert the tool so that the edge makes contact with the tang inside the bore. Thrust the tool down against the tang sharply to remove it. (Image/Wayne Scraba)
tiny piece of removed thread repair insert
The tang will break. This is what remains. (Image/Wayne Scraba)
holley fuel manifold on a big block v8 engine
Here’s the finished job and, best of all, it works as normal and it doesn’t leak! (Image/Wayne Scraba)
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Author: Wayne Scraba

Wayne Scraba is a diehard car guy and regular contributor to OnAllCylinders. He’s owned his own speed shop, built race cars, street rods, and custom motorcycles, and restored muscle cars. He’s authored five how-to books and written over 4,500 tech articles that have appeared in sixty different high performance automotive, motorcycle and aviation magazines worldwide.