Thread repair for mechanics: an incomplete guide to essential ...

One of the most talented mechanics I know observed me struggling with damaged threads and dispensed an opinion: “Stripped thread repair is one of the most basic skills everyone should know how to deal with.” That’s not revelatory. However, when I pronounced the part I was working on junk, he made a complex thread repair that at the time, to me, bordered on art.

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I still have the part. It hangs on the shop wall in a position of honor as an admonition that I have two ears and one mouth and I should use them in that ratio, but it also serves as a constant reminder of the importance of threads in the daily work of a mechanic. Think of how many miles of threaded items you’ve turned over the years, how critical their applications, how meager their maintenance. Threads (and the fasteners on which they reside) are literally holding the vehicles together.

Thread repair can be one of the most difficult tasks in all of auto repair. I’ve gotten much better at it with practice, but I’d like to present a few products and practices that have helped me. But first, let’s cover why there’s such a wide spread of skill levels on this type of work.

Mechanic or machinist?

Thread repair is not typically within the realm of common repairs for an auto mechanic. I mean, obviously we all need to know how to handle it as my mentor stated, but it’s difficult knowledge to acquire. Factory manuals don’t cover it. Most technical schools don’t, either. Nor is thread creation and repair really the focus of most adult education courses like welding or machining.

Threading operations are often the domain of the tool and die maker, a dying trade in this country. It’s related to machining, but it’s focused on creating tooling in a toolroom rather than production. Unfortunately, thread repair, especially on an item that is in situ, can be especially difficult, and most machining texts don’t account for poor tool access.

Many mechanics are not well-versed in threading operations for the same reason they’re not proficient welders—this is more the domain of the fabricator than the repair mechanic. Don’t feel bad if this isn’t your strong suit. I’m not a trained machinist, though I practice making chips fly when I am able. Remember, if you can get the subassembly free from the vehicle, you’ll often be money-up to bring the workpiece to a professional. There’s no shame at all in knowing the limit of your capability.

Work your plan

Whenever a repair goes awry and the service manual no longer is in play, I generally try to use an escalation of force, if that makes any sense. So in a “damaged threads” scenario, I’ll attempt to first salvage existing threads, and only if that doesn’t work do I look into overboring and cutting operations. Sometimes you must move to a drastic final step right at the beginning. Remember, though, that if you make several attempts in the correct order, you have given yourself multiple chances to engineer a repair that works. Personally, I like them odds better.

Thread files

Thread files are an excellent initial repair tool in some scenarios. They are, effectively, a file the shape of which mimics the correct thread pitch and angle. While I am sure there is some magician out there who can use these babies in female threads, I have found them helpful for male threads, specifically on large fasteners with damaged sections that aren’t within the leading threads. (On large fasteners, a die may not be easy or affordable to acquire.) I’ve had good luck “sneaking up” on the damage by getting the file to run in the good threads as it restores some of the damaged ones; that way I know the metal is being removed from the right spots.

If these are something you’d like to explore, you should know split dies exist, too, that work similarly. The file is a little more universal, though—usually a metric and SAE will take care of most fasteners you’ll see in common service.

Tap sockets

Tap sockets are particularly helpful for avoiding that angular rotation issue mentioned earlier. They’re effectively a tap-holding implement that allows you to use standard square-drive tooling you already own to run ‘em in.

In addition to the benefit of the reduced swing, the tap socket allows you to use extensions, creating axial distance, too. Ironically, that ability can often mitigate the need for something that can operate with reduced tool swing.

You can buy individual sizes or sets. (You can also try your twelve-point sockets in a pinch. It works, but the 60-degree corners are obviously not the same as the 90s on your taps.) I picked up a set of modestly priced adjustables (just two cover most taps I own) a few years back and by now, they definitely don’t owe me any money.

Thread restorers

Taps and dies are designed to cut threads, of course. To do so, they have sharp edges. Most common taps sold for general use are plug taps, which have a bit of a taper to them, as do corresponding dies. Thread restorers (sometimes casually referred to as thread chasers) are a bit different. The taps are almost like a blunt bottoming tap, in that there is little to no taper, and the threaded section isn’t sharp. The idea here isn’t to remove metal as standard taps and dies do, but to move the metal. These tools push the metal back where it came from; they re-form (restore!) the threads. A set is very handy in a service bay!

What is thread reinforcement?

Thread repair inserts are used to repair defective threads while maintaining the nominal diameter or to strengthen threads while maintaining the nominal diameter (thread armoring).

The following tools are required: Core hole spiral tap, special tap, insertion tool and edge breaker. Stainless steel thread inserts ensure very good corrosion resistance and heat resistance. The cross section is diamond-shaped.The wire is coiled into an elastic spring. At the bottom end there is a driving pin. When the thread insert is screwed in, the spring is slightly tensioned and holds the insert in the external thread. The thread insert is therefore larger when screwed in than in its original state. There are three sizes in total, each aligned with the nominal dimension ( 1xd, 2xd and 3 x d).

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Product advantages of the thread separating inserts?

The inserts are extremely durable and corrosion resistant. In addition, the tensile force is evenly distributed in the threaded insert. The inserts serve as additional armor.

Click here for the article: How to repair a broken thread in 11 steps

How to repair a broken thread in 11 steps

Here you can find the suitable thread repair kits in our shop

GSR Thread Repair System

If you are looking for a permanent and durable solution to repair a defective thread, then you are on the safe side if you choose the GSR Coil Repair System. This allows you to restore the nominal dimension of the defective thread. Often, the thread treated in this way is even more resilient than the original thread. This is then also referred to as “thread armoring”.  This is especially true for materials with low shear strength such as aluminum or other non-ferrous metals.

 Procedure for thread repair

First of all, once again the important advice to put on your personal protective equipment (PPE) before you start working. Please also follow the safety instructions for drilling and thread cutting in any case. The first step in thread repair is to determine the thread. Then you can provide the appropriate wire inserts (GSR inserts) and the necessary system tools. THE GSR CORE HOLE SPIRAL DRILL The first step after the determination is the selection of the right drill. In this case, we drill a core hole for a so-called insert thread (EG). In this case we speak of a so-called holding thread. We recommend a twist drill made of HSS material with a helix ground from the solid material. The common abbreviation is HSS-G. A splitpoint gate is not absolutely necessary for this application. GSR core hole twist drills comply with this recommendation. In the Google Play Store you can find the GSR thread calculator app. You can use this app to find all core hole sizes for all thread types as well as for insert threads and for taps.

Tap for cutting the holding thread (STI thread).

The profile of the holding thread is regulated in the DIN standard ( DIN 1-3).
For the production of a holding thread a special tap is necessary. In the German standard one speaks of EG thread, the international designation is STI thread. STI stands for Screw Tap Inserts. There are different types of taps to produce a holding thread: Hand tap set with pre-cutter, center tap and finish tap or single tap with long gate. For thread repair, we recommend the GSR single-cut tap for the insert thread. It has a 3-4 thread gate and is sufficient to cut a full holding thread. In addition, you will need a holding tool for the square of the tap. Adjustable tap wrenches or tool holders with ratchets do a good job here.

Insertion tool and tang breaker

These two tools are necessary to assemble the wire insert. They are made of tool steel. The insertion tool has a powder-coated handle and an adjustable cap nut. The threaded wire insert is mounted on the insertion tool and then screwed in. The tang breaker is burnished, has a knurled shank and a flattened head. It is used to break off the threaded tang.
The most popular sizes for thread repair are M6, M8 and M10, as well as M10 x1, M12 x 1.5 and M14 x 1.5.

It was invented by the US Americans in the s. It became known in Germany under the trademark Helicoil. The market share of Helicoil in Germany was once so high that Helicoil became the generic term for this type of thread repair. In the meantime, these systems are offered under different brand names (Bair Coil, V-Coil).  Since they are all based on a DIN standard, they are generally compatible with each other.

You can buy thread repair sets here in the store. Please ask for not so common sizes. We have a wide and deep assortment of other sizes in stock.

There are sets for single sizes ( e.g. M 6 ) and large thread repair sets for M 5 – M12.

Thread repair sets belong in every workshop in trade and industry.

Thread inserts and thread armorings are gaining more and more importance for additive manufacturing! You can find more information here:

Thread inserts/ thread armouring for 3D printing models

How do I repair a thread?

You can find step-by-step instructions in this article: 11 steps to thread repair

Contact us to discuss your requirements of Thread Repairing. Our experienced sales team can help you identify the options that best suit your needs.