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Repairing threads is not difficult and the tools to do it right are not overly expensive. But the problems and expense they can save makes them worth the expense and a bunch more.
Click image to enlarge

Repairing Inside and Outside Threads

Or learning to tame a disaster

Text, photos and video by Tom Hintz

Posted – 6-25-2011

Whether you fess up to damaging threads yourself or can conjure up a plausible denial you still have to fix it. The reality is that most of us have experienced the sinking feeling of a wrench slipping from tight to soft feeling when the metal gives way as portions of the threads tear loose. If the threads on a bolt are damaged it is generally a smaller problem because we can fix or replace that fastener relatively easily and cheaply. If the threads inside of a hole fail the repair options can seem costly and complicated. Chin up ye of little faith! In many cases repairing a stripped bolt or bolt hole can be easier, faster and even cheaper than you are capable of fearing.

Exterior Threads

The threads on a bolt can be damaged in many ways but often that damage is confined to a relatively small area. If there are a few intact threads at the end of the bolt to get a thread cutting die started we can run it down over the damaged threads to clean them up. The die will cut metal away where the threads are distorted which can reduce their mass and overall strength. That means that this is not a complete repair but can make the bolt useable in low stress situations or until a new one can be obtained if the expected loads are at all substantial.

These little dings (top left) will stop a nut cold. Force it past these and they could gall the rest of the threads. The smart money runs a die (top right) over them to "cut them straight". You may lose a little bit of holding strength but this procedure can give you time to get a new bolt if the damage is too great. This repaired bolt (lower left) is fine, plenty strong and screws in with finger power only.
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Re cutting threads is done exactly like cutting brand new threads. Work the die over the damaged area turning it down ½ to ¾ turn forward by a half turn or so backward to break up the chips. Continue that cycle until all of the damaged threads are re cut. Though not mandatory in some cases, I use a bit of cutting oil whenever cutting threads to help minimize tearing or galling.

Inside Threads

Tearing the threads out of a hole can sometimes create a tougher repair chore. Adding metal to the hole through welding to re-drill and re-thread it to the original dimensions is somewhere between very difficult and impossible for most of us. Drilling the hole large enough to use the next size threading tap can be an option if the hole in known to be far enough from internal passages or edges. Often a larger diameter bolt simply is not compatible with one or more pieces being secured. The good news is that there is an easy to do and cost effective way to restore the original thread size in the same location in many cases. Heli-Coil™ Inserts is that answer.

Re-Tapping the Hole

If the threads in a hole are not badly damaged or only a few threads near the top of the hole are stripped out you might be able to run a tap matching the existing threads into the hole to clean up the threads and re-cut those with worse damage. It is important to know that this kind of repair is only safe in low stress situations. The process of “cleaning up or re-cutting” the threads will remove some amount of metal which can lessen the overall resistance to stripping in the future.

Increasing Diameter

If compatible with all of the involved parts you might be able to increase the size of the bolt enough to cut new, full-depth threads in the hole. That usually means increasing the diameter by one or two bolt sizes to get full threads in the metal. Trying to simply cut new threads over the old ones just insures dramatically reduced load capacity.

This stripped hole (left) is perfect for Heli-Coils. It is not near internal passages or an edge and shows no cracks or other defects. I've started drilling this hole out for a Heli-Coil and you can see (right) how little material is being removed. I still will wind up with full-depth threads in the original size, 3/6 X 16>
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A standard tap and die chart will list the appropriate drill size for each tap. If you are not sure of how big to make the new hole, start with one size over the original, drill the hole and see if all of the old threads are gone. If not, go up one more step and repeat the process until you get a clean-sided hole. It is best to use a drill press to keep the hole straight and avoid removing material irregularly from the sides as often happens with a hand-held drill.
After drilling the hole out, cut the new threads remembering to advance the tap ½ to ¾ turn and then back it up a half turn to break the chips. Continue these steps until all of the threads are cut. If you are new to using these tools see my story “Using Taps and Dies.”

Heli-Coil™ Inserts

About 40 years ago I was introduced to Heli-Coil™ Inserts when I stripped out a Chevy starter bolt hole in the engine block which was still in the car where I really wanted to leave it. It was not possible to drill the hole through to use a nut on the other side and increasing the bolt diameter to cut new threads would mean weakening the mounting flange on the starter substantially. My savior was the counter person at my favorite auto parts store who gave me a Heli-Coil™ Insert kit to try. What had been looking more and more like a major disaster was fixed and the customer on his way about a half hour later.

Heli-Coils come in a huge range of thread sizes in both metric and inch scales. The small kit (left) comes with the coils, the right tap and insertion tool. You can fix several hole with this one kit.
The insertion tool has an edge that catches the tang on the inserts. (right) That insert makes installing them easy. You can knock off that insert (at the notch) with a simple rap of a hammer on a rod that fist down into the insert when installed.
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Installing Heli-Coil™ Inserts is surprisingly easy if you follow the simple steps. (See my full review of Heli-Coil™ Inserts) A major point in favor of the Heli-Coil™ Inserts is that you can return the hole to the original thread size without sacrificing holding power. Actually, installing a Heli-Coil™ Insert often makes the threads stronger than those that stripped out of the original material.

After determining the bolt size and thread was 3/8 X 16 we chose the matching Heli-Coil™ Insert kit for the example in this story. The Heli-Coil™ Insert packages list the size drill needed for that insert. (The drill size is also printed on the included tap) The 3/8” X 16 thread Heli-Coil™ Insert kit calls for a 25/64” drill, or 0.391”-diameter. The drill size for the original 3/8 X 16 thread was 5/16” or 0.313”. Because the Heli-Coil™ Insert drill is only 0.088” (less than 3/32”) larger overall than the original 5/16” bit we are enlarging the hole a tiny amount which dramatically reduces concerns about breaking into internal passages or obstructions. This can be a huge consideration when working on virtually any kind of engine or machine.

After carefully drilling the hole out and removing all debris we can cut the new threads for the Heli-Coil™ Insert using the special tap that is included with the kit. While the threads it cuts are special, using this tap is absolutely normal. Turn it in ½ to ¾ of a turn and then back it out a half turn or so to break up the chips and repeat that process until the hole is fully tapped. The hole has to be threaded deep enough so the insert is fully below the surface when installed. In the case of deep holes we can stack Heli-Coil™ Inserts as needed to fully thread the hole.

Next, turn the Heli-Coil™ Insert into the hole using the included tool. That tool catches on a tang at the bottom of the Heli-Coil™ Insert and as we turn it, that tension reduces the diameter of the coil slightly which makes turning the insert in very easy. When we back the insertion tool out the coil tries to expand to its original diameter and that locks it in place.

If the bolt passes through the Heli-Coil™ Insert you can snap off the tang at the bottom. Use a rod or something similar that is close to the inside diameter of the insert, set it on the tang and then give it a hard tap with a hammer to snap the tang off. The tang is notched at the factory to produce a weak spot where needed so the inserts will stack on each other correctly in deep holes.

Economical Hero

Video Tutor

Being able to repair internal or external threads can make you feel like a hero. Being able to accomplish this bit of mechanical magic for so little money justifies puffing your chest out even more. Sure taps, dies and Heli-Coil™ Insert kits cost money (about $30 for the simple Heli-Coil™ kit used in this story) but those expenses pale when compared to financial landslide of having to replace a major component or paying to have it machined – after you take it all apart and then try to get it back together again.

These procedures are relatively simple but the results are well worth any learning curve you have to navigate along the way. Keep in mind that you can use these threading skills to build custom projects, jigs and fixtures that could make your other work easier, more accurate or safer. That can make the initial expense even easier to take.

Related Content

Using Taps & Dies
Heli-Coil™ Insert Review

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