Stripped Bolt and Screw Repair

How to fix stripped threads
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Ever try to tighten up a bolt and it just won't? Or maybe you are putting the final torque on one and suddenly it gives way to being loose again. Whatever the situation, it is almost always accompanied by a sinking feeling in your stomach. Now what do you do?

Many times it is an easy repair, and almost any situation can be fixed with some sort of threaded insert. This term refers to exactly what it sounds like. It has threads on the inside that match whatever size of bolt hole you are trying to repair. The outside is also threaded, but of course a larger size. Usually, the offending bolt hole is drilled out on a drill press, and re-tapped to match the outside threads of the threaded insert. Then the insert is screwed into the hole, and you are ready for action again.

Steel Thread Insert

The most common threaded insert is called a "Heli-Coil", but this is a brand name. The industry name for this item is a Steel Thread Insert, or STI. It looks very much like a compressed spring. The coils of the spring form the inside and outside threads for the insert. If you were to cut the wire of this little spring, however, you would see that instead of round wire, a 60-degree diamond shape is used. This shape matches the common shape of most threads.

These are not the strongest inserts available. If you are repairing a high-torque bolt hole in a weaker metal such as aluminum or magnesium, then some other sort of solid, rather than spring-like insert, should be used.

These STIs are very small, so if space is an issue then they're a good choice. They are relatively easy to install, and the installation kits won't break the bank. STIs are a very good idea in difficult to reach spots, or in softer metals that may require repeated installation and removal of the actual bolt.

Most installations of a Steel Thread Insert are pretty straightforward. Drill, tap and install. But it is not unusual that the part to be repaired is not easily placed on the table of a drill press or milling machine. Sometimes the offending part may already be installed in the vehicle and in a hard-to-reach spot.

One of the biggest mistakes made in drilling out the stripped bolt hole is improper alignment or crooked drilling. It might seem like the bolt hole that is already there will guide the drill bit. It does help, but you can very easily start to lean in one direction or another as you are drilling. This is especially the case in soft materials, such as aluminum and magnesium.

So, obviously it is important to have a way to hold the drill very straight. It may be helpful brace your arms against something solid, and have a friend help line things up. Sometimes it is possible to put some kind of square block against the surface that you are drilling into to help visually align things. A straight and true hole is the beginning of a good job.

Another problem that you may encounter when working with soft metals is the tendency for the drill bit to grab in a pre-existing hole. The reason that this happens is that the flutes, or spirals of the drill start to act like the threads on a wood screw. Since the material is soft, it tends to form a very large chip very quickly. This makes the drill bit start to behave more like a tap than a cutter, and it will start to thread itself into the material that you are drilling, which can be very bad.

The drill bit may break and become stuck in the part being drilled. An even worse scenario is when the surrounding material of the part splits or cracks from all the stress of a drill bit being screwed into it. In either case, the part will have to be removed and replaced or repaired.

The reason that the drill pulls itself into the material is because the material is very soft compared to the sharpness of the drill bit. When the sharp edge of the drill catches in the material, it just continues to pull down and thread in. This potential difficulty can be avoided by taking a small sharpening stone or diamond file and slightly dulling the ends (not the sides) of the two cutting flutes on the drill. The stone should leave a small flat spot along these two edges, making them much less sharp, and thus less likely to grab.

Next let's talk about what you should use to power the drill bit. Obviously, it would be best to put the whole mess on a drill press or a milling machine. When that is not possible, then you might be inclined to use an electric hand drill. This is often a good choice. However, if you are going to be working with those soft metals, and drilling small holes, the electric drill may just have too much power or spin fast, making it difficult to react fast enough if the bit becomes stuck. Also, in cramped locations there may not be room for a power drill.

Hand Drilling

In the photograph here, you will notice that the drill being used is just a drill chuck held by hand. The bolt holes that were being repaired were all in magnesium and were all a small diameter. This is a real recipe for breaking the drill bit and also cracking the metal.

By turning the drill by hand, it was possible to keep the drill from pulling into the part and drilling too deep into the material. You might be surprised at how little force is required to drill out a soft metal in this manner. However, it is probably not a good idea to try it on bolts over about .300 inches in diameter.

In the photo below, note that there is no room for a conventional drill to fit. By using this small drill chuck by hand in this location, it was possible to reach the recessed bolt hole.

Hand Drilling Magnesium

A small angle drill would have fit this spot, but the part being drilled was a very expensive transmission already installed in the vehicle. Drilling crooked, breaking the bit off in the transmission, or drilling through to the internals were just not options to be lived with. It is a little slower to drill these holes by hand, but the chances of success are greatly improved.

It is also good practice to turn the tap backwards about every turn in to break the chips up as you go in. Otherwise the chips will have a tendency to become long strings of metal and clog up the tapping process. Of course, if a traditional tapping handle will not fit in the location in which you are working, try holding the tap in the same drill chuck that was used to drill the hole, and turn it by hand.

Hand Tapping with Drill Chuck

A point to remember is that a tap for a STI is special. It has no other use. Most STI taps will have the letters "STI" printed directly on them to help identify it as a special tap. Trying to use one of these taps for anything else, or trying to use a regular tap before installing a STI will result in some serious problems. Most machine shops keep these taps far away from regular taps so that they won't be used by mistake.

After completing the drilling and tapping, it is time to install the STI. There are a few different kinds of installation tools but they all function in a similar manner. The insert has a tab that the tool hooks onto, and essentially pulls the insert through the threads. It is possible to cross-thread a STI. My advice is simple: Don't do that.

Special Tap/STI Mark

Once again, take care that everything is straight and true before threading it in. Since the STI is designed to be installed, not removed, then it stands to reason that they can be hard to get out if you make a mistake. The key to avoiding the hassles is to pay very close attention and avoid rushing through these processes. So when you are hit with that terrible feeling of a bolt not tightening up, don't despair. When the proper care is taken, even very challenging parts can be repaired and used again, and many times the thread repair is superior to the original threads.

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