[Instruction] How to unscrew a broken bolt: 100% working way

Most modern connections of various mechanisms are assembled using threaded fasteners - bolts, nuts, screws, etc. The threaded connection is quite simple and reliable, it can be said that it has passed the test of time.

With the right approach to the choice of material and its manufacturing technology (for example, use in the production of hardening), threaded connections on bolts and nuts can provide a reliable connection for decades.

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Why do bolts break?

Despite a lot of advantages, threaded connections have one significant drawback: bolts break quite often

In an industry such as the automotive industry, threaded connections are used very widely. Almost all parts of the car are assembled using threads. Despite the greater simplicity and low cost welding, it cannot completely displace the thread, since the latter has the ability to quickly disassemble the structure. 

When a bolt breaks, situations arise when part of it remains in the hole and it is quite problematic to remove it from there.

The article addresses the issue how to get the rest of a torn bolt out of a threaded hole using the example of working with a car cylinder block.

The top of a stripped head bolt can often be unscrewed from the threaded hole, since in 90% of cases the bolts break in the direction of twisting

There are few reasons why bolts fail. The main one is excessive force when twisting. Using wrenches with a long handle, huge forces can be applied to the bolts, which, in the end, will lead to an excess of force over the yield strength, and the bolt will simply collapse at its weakest point. Most often, this is the first third of the thread length, starting from the bolt head.

When assembling some devices, it is important not only to select the correct bolts for thread and strength class, but also to tighten them with the correct force. Moreover, it should be no more and no less than that indicated in the assembly instructions. Usually, the bolt is tightened until it stops, and then tightened.

The tightening force is measured in newtons per meter or in kilogram-force per meter (obsolete method). As often happens, the masters may not have the right key, and what is at hand is used.

It may well be that a hypothetical bolt, for example, with an M8 thread, which should be tightened with a key with a nominal force of 3.8 kgf m, will be tightened with a key with a force of 4 kgf m. Doing so may break the bolt. Even a 5% difference in torque often results in bolt breakage.

If you attach a bolt to the engine, you can see that its failure occurred exactly above the cylinder block, that is, there are thread remnants in the block and in the head

The lower part of the bolt remains in the structure. At the same time, since the thread is already in its regular place, and the bolt is simply screwed in, it can be located in both twisted parts at once. 

That is, even dismantling the entire structure could not remove the remains of the bolt. The only possible solution in this situation is to extract the broken thread in some other way.

At the same time, it must be understood that when removing the bolt, in no case can the internal thread of the structure be damaged, Otherwise, nothing can be screwed into this hole anymore.

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Ways to remove bolts

Broken Bolt Extractors

There are several ways to unscrew a broken bolt. Let's consider them in more detail.

Using the top of a head bolt

When deformed, peculiar “antennae” are formed on it, with which you can catch on the rest of the bolt in the hole

The easiest way to unscrew a broken, but well-lubricated bolt when tightening is is to take advantage of its already broken off upper part. 

The probability of success when using this method is small, but worth a try, especially if the bolt breakage occurred at the initial stage of tightening.

Using a small screwdriver or chisel

Removing damaged bolts

You can try to unscrew the remnants of the bolt if you use a screwdriver or a small chisel. In this case, there are two options: either simply unscrew the broken part, using the bumps at the place of the break, or make some kind of small slot on the upper side of the broken part, clinging to the edges of which and twisting.

The latter method is implemented only if the material from which the chisel or screwdriver is made has a higher strength than the material from which the bolt was made.

Using the Extractor

The extractor is a drill with a long conical tip

This is the most effective way, because with its help it is almost always possible to achieve results. Features of the extractor device are that they are screwed into a hole located in the central part of the unscrewed bolt. In this case, the thread present on the extractor has the opposite direction to the thread of the bolt (the so-called. "left thread"). 

Usually, in the sets sold there are several drills of different diameters.

They all have the same shape, while on the other side of the tapered thread there is a rectangular mount for a drill or tap.

In the process of screwing into the bolt, the extractor wedges it and at some point stops. Further rotational movement is transmitted through the extractor to the bolt and unscrews it.

Consider the sequence of actions that need to be done to unscrew a torn bolt using an extractor.

No. 1 Reaming the rest of the bolt

It is recommended to make a sleeve of small length, from the outer part of which to cut a thread with a pitch equal to the thread of the hole into which the broken bolt was screwed

In this case, two drills with a different tip shape are used - with an obtuse and a sharp angle.

Unfortunately, this method is not always applicable, because for its implementation it is necessary to have a lathe at hand. Therefore, a different method is used. 

The first is used to level the top of the bolt remnants, and the second is used directly to drill the hole for the extractor. In this case, the diameter of the drill with a tip having an obtuse angle should be slightly less than the diameter of the threaded hole. The diameter of the second drill should be equal to the diameter of the extractor.

No. 2 Twisting the extractor

It is undesirable to use pliers, as they lead to "licking off" the rectangular end of the extractor

In extreme cases, you can weld a nut of the appropriate size to the end of the extractor in order to unscrew the bolt using a regular wrench

Further rotation of the extractor leads to the unscrewing of the remnants of the bolt

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[Instruction] How to unscrew a broken bolt: 100% working way