It may turn out that you do not have a tap of the appropriate size at hand, and the part with damaged threads is bulky and firmly attached to a machine or mechanism, for example, like a clutch housing to an engine and gearbox.
It seems the situation is hopeless. But it turns out that there is a fairly simple but effective way to restore the threads in place using an ordinary steel bolt of the appropriate size, even possibly unscrewed from the same damaged hole.
To implement this method, we will need a mini grinder with a small and thin disk. If it is missing, then you can get by with an ordinary hacksaw for metal, since in principle there is nothing to saw.Here are some specific examples of using this method of restoring internal threads.
Restoring car exhaust manifold threads
Imagine an exhaust manifold that is secured to the engine block with bolts or studs in several places. Damage to even one hole in such a part poses a significant problem. Even disconnecting this unit from the engine is often difficult: over many years of operation, due to high temperatures, it becomes tightly stuck to its seat. And then you still need to find a craftsman with the right tap, pay for the work...
There is only one thing left: fix the problem yourself, since a crumpled profile in the hole will not allow you to screw a bolt into it, and may even ruin the thread on it.
You can use the “native” one, just unscrewed, or another one, but exactly the same in size and profile.
We clamp its head in a bench vice so that the rod points upward. Then, using a hacksaw for metal, we cut a groove of about 3-5 turns exactly in the center of the rod.
Next, generously lubricate the cut bolt and the damaged area with machine oil, and screw this kind of tap into it, first by hand as far as you can, then with a wrench, screwing it in and out several times in a row.
As a result, the threaded protrusions on the bolt, cut by a groove, fit into the recesses and gradually cut off the creases.
After unscrewing our unique tap, we are convinced that it itself remains intact, and what is even more important, it has completely restored the profile in the hole.
Now you can screw this or another similar bolt into it completely with one hand and secure the part in the place intended for it.
Hood hinge attachment
Sometimes this defect occurs in the hole to which the hood hinge is attached. We do exactly the same as with the exhaust manifold.
We fix the fastening bolt in a vice and make a groove on its rod, adding lubricant, after which we tighten it by hand as long as possible.
Next, we continue the process using a wrench, screwing and unscrewing the homemade tap several times.
At the same time, the cut profiles of its thread tidy up the profile in the hole, performing the functions of a tap. By removing the bolt, you can make sure that the threads on it and inside are in perfect order. The bolt can now be easily screwed into it by hand. If you then tighten it with a wrench, it will provide the necessary tightening force.
Restoring threads on an engine block
The problem in question can also occur on the engine cylinder block, especially if it is made of aluminum alloy. A clogged or crumpled thread in the hole prevents the bolt from being installed properly. He either doesn’t screw in or tends to go wrong.
We repeat the procedure we used with the exhaust manifold and the hood hinge assembly. The rod of a bolt clamped in a vice is cut from the end with a hacksaw for metal to a depth of several turns.
Then, having removed metal particles from it and generously lubricated it, we screw it by hand into the hole with damage. As soon as the bolt is securely in there a few turns, we take the wrench in our hands and carefully and gradually screw it in and out until we go all the way to the end.
We make sure that the thread in the hole has been restored and that it has not been damaged on the bolt. You can prove this by screwing it into place to the full depth by hand.
Conclusion
Considering that millions of bolts are produced every day in the world, this surprisingly simple method of restoring threads in holes can be considered an outstanding technical solution.
