If you're having trouble removing a stuck bolt, common advice is to heat the bolt up. But if metal expands when heated, wouldn't heating up the bolt just make it harder to remove? How does heating the bolt get it unstuck?

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    Depends. Is it seized (from rust) or over-tightened? Over-tightened is your answer; seized is Ecnerwal's. – Mazura May 11 '16 at 22:11
  • @Mazura my original thought was over tightened, but I didn't originally specify that so it's a bit late to add it now. – Tester101 May 12 '16 at 0:06
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    If you're talking about a nut on a bolt, heating the nut more than the bolt will cause the nut to enlarge relative to the bolt. This action is simple and straight-forward -- you should simply take care to concentrate the heat on the nut, not the bolt. For other scenarios you need to think a bit about the situation and concentrate heat on the part that you want to expand. Sometimes using cooling of some sort on one part helps. – Hot Licks May 13 '16 at 0:26
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    Well if you heat it enough that it melts... – nnnnnn May 15 '16 at 1:49
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    The common (sense) advice is to heat the nut, not the bolt. If you really think what you propose here and in your answer actually works, post some video of it with measured torques. – Fizz Sep 28 '17 at 8:06

10 Answers 10

up vote 70 down vote accepted

The answer is surprisingly simple: the bolt expands, but the nut expands more.

What is happening here is good old thermal expansion:

  • The bolt is heated and expands outwards, its radius increasing
  • The nut is heated and... expands outwards, its radius increasing

Now, since the nut's radius is slightly greater than the bolt's, and since the increase is proportional to the rest length, the nut expands a little more.

Iron has a thermal coefficient in the 10-5 / K ballpark. It means that for each 1 K increase in temperature you have an increase in size of 10-5: a 1 m rod becomes 1.00001 m long.

If your bolt has r = 1.5 mm, and the nut has R = 1.501 mm, what happens is temperature is increased of 500 K? Well:

  • r = 1.5*(1+500*10-5) mm = 1.5075 mm
  • R = 1.501*(1+500*10-5) mm = 1.508505 mm

As you can see, before heating R - r = 1 μm, while after R - r ≈ 1.001 μm. It increased!

Please note my numbers are quite wild and used just to make an example. I am sure that I got the starting values wrong, but I hope they help to get the message across anyway.

  • If there was a gap of 0.001mm between the bolt and the nut, the nut would be loose anyway, so you wouldn't need any heat to loosen it. – alephzero May 11 '16 at 20:33
  • Does this hold true when there's no nut involved? For example, if the bolt is threaded into a solid piece of stock, as depicted in my answer? – Tester101 May 11 '16 at 20:34
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    @alephzero well there's a disclaimer about me and numbers :P Tester101, this holds for a solid piece as well. As the metal expands, the holes in it expand too. – Vladimir Cravero May 11 '16 at 20:46
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    @alephzero - Not if it's rusted shut. See Ecnerwal's answer. – Mazura May 11 '16 at 20:49
  • This makes sense to me with a nut and a bolt, as they'd both heat to about the same temperature. However, if the bolt was in a large hunk of material, I would think it would be difficult to get the hole to the same temperature as the bolt. – Tester101 May 12 '16 at 9:38

The secret is constrained expansion.

Here's some cruddy diagrams to help explain how it works.

Bolt stuck in a hole
Bolt stuck in a hole

When the bolt is heated, it expands. Since the shaft of the bolt is constrained, it can't expand inside the hole.

Bolt expanding
The bolt expands in the direction of the green arrow, but cannot expand in the direction of the red arrows.

As the bolt cools, it contracts. The contraction, however, is not constrained. This means that the bolt can shrink in all directions, making the bolt slightly smaller.

Bolt contracting
The bolt is able to contract in all directions.

Once the bolt has cooled, it should be smaller and easier to extract.

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    @JPhi1618 If there's a nut involved, expanding it with heat could help. Though I don't know many people that like to play with hot nuts. – Tester101 May 11 '16 at 17:47
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    Also, the bolt is likely to cool more quickly than the larger mass around it, enhancing the phenomenon. – isherwood May 11 '16 at 18:13
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    -1 this answer is absurd, you're claiming the screw shrinks back to smaller than it was at the same temperature!? If that were true, it would be possible to make metal of any density by heating/cooling it in smaller and smaller holes. The real answer is probably "expansion of the hole (due to heat conduction), plus uneven expansion breaking any bonds caused by rust" – BlueRaja - Danny Pflughoeft May 11 '16 at 21:07
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    @BlueRaja-DannyPflughoeft not exactly. The diameter of the threaded portion of the bolt (the constrained portion) will be very slightly smaller, while the length of the bolt will be very slightly longer. We're not talking about a huge change here, we're talking about thousandths of an inch. – Tester101 May 11 '16 at 21:59
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    @BlueRaja-DannyPflughoeft The answer makes sense to me - essentially, the bolt gets "cold-formed" [though not really so cold per se] to the slightly relatively (since it hasn't expanded as much) smaller hole. – Random832 May 11 '16 at 22:26

The actual reason this usually works is that rust is significantly larger than the steel it's rusted from, which is why the bolt is stuck in the first place. In some other instances the reason heat works is that the bolt was applied with a threadlocker that requires heating to remove (if it comes out with no sign of rust, that's a pretty good bet)

Many forms of rust contain "chemically bound water" and will lose that water (and shrink) when heated sufficiently.

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    A further factor along these lines is that metals can cold weld (steel to aluminium for example). Differential thermal expansion can crack this join. – Chris H May 11 '16 at 20:40
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    Heat is nature's greatest release agent. – Mazura May 11 '16 at 20:44
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    not arguing, but I'm interested in learning more. Is there a source that says this? – DrewJordan May 11 '16 at 20:45
  • @DrewJordan - Not that I can find; mechanics aren't really the 'literature' type. They just know it works. IMO it's 95% "release agent" and 5% "constrained expansion." – Mazura May 11 '16 at 21:15
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    @DrewJordan Explained to me by a blacksmith/armorer. Plenty of info about bound water, heating to release bound water (ceramics info has a lot), and a bunch of info about making some sort of nanoparticles (ohh, trendy - also irrelevant) - not much that gets into the volume change resulting from removing the bound water (lots that mentions the volume change of iron/steel to rust.) – Ecnerwal May 12 '16 at 1:59

Metal arranged in a ring expands outward when heated. Imagine a ring of thin wire being heated--it expands primarily along its length, making both the inner and outer diameters larger. The same occurs with the material around a bolt hole.

Generally, I try to heat the surrounding piece and not the bolt itself. However, even if the bolt is heated directly, conduction will usually result in heating of the surrounding material, and therefore expansion of the bore.

More sciencey info on that

Consider a washer, or some other metal ring or disk with a hole in it. When the ring is heated, we expect the ring to expand, and experiments confirm that it does expand. But does the hole in the ring expand, contract, or stay the same size?

...[T]hink about what you do when you are trying to open a Mason jar, and the screw-top metal lid is stuck. You either tap on the lid with a spoon (to try to jar loose any part of the lid that is stuck), or you place the lid under hot water. You do the latter because you know the metal lid will expand more than the glass jar, and so it will be easier to get the lid off.

And by saying the metal lid will expand more than the glass jar, what we really mean is that the hole in the lid will expand.

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    This is exactly what my mechanic resource online say; heat the surrounding part, not the bolt. Twist while hot. However, I think your conjecture about just heat the bolt and rely on conduction is misleading because it is the exact opposite of what your main point is. – Zach Mierzejewski May 11 '16 at 20:09
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    No, it's an alternative that's sometimes the only option and still may work. Let's not get too pedantic, shall we? After all, the goal is to get the bolt out. :) – isherwood May 11 '16 at 20:15
  • "or you place the lid under hot water." Don't I do that because the contents were hot when sealed, and now at room temperature they are stored at a lower than atmospheric pressure which provides a force that acts to retain the lid? And heating the contents (or at least the air above them) reverses that process? – Doug McClean May 12 '16 at 16:37
  • We're talking about friction between the threads of the lid and the threads of the jar. Slight pressure inside or outside has relatively little effect on lid rotation. – isherwood May 12 '16 at 18:19

In my experience, you have to heat a frozen bolt until it is blistering, red hot and getting soft, and remove it while it is hot and soft. Heating the bolt and allowing it to cool has never helped me. As the metal contracts, the bolt seizes; it usually doesn't loosen... it probably makes the situation worse.

The same is true for drinking glasses that have gotten stuck together... cold, contraction is the cause of the seizing.

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    I don't let it cool, but red hot is way too far; you risk shearing the bolt. Then what.... drills and taps. – Mazura May 11 '16 at 20:58
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    @mazura when the bolt is hot, it's going to draw and slide like hot cheese. shearing is a problem for cold cheese. Cheese crumbles and breaks when cold, not so much when it's hot. But yes, it could still happen... if it shears, you can take comfort in knowing that it would have happened either way, and less likely when it's soft and stretchy though. – Ben Welborn May 13 '16 at 13:17
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    This is totally the correct answer, which sucks because its at the bottom with 0 votes, and all the other answers about a difference in expansion are just garbage. Whatever your bolt/nut material, and whatever you heat up more, expansion of the material the bolt is screwed into will expand in to the cavity, not expand the size of the cavity. The heating/cooling argument might make sense, but I really doubt the effects are significant. Plus its not what people do. No, the reason is because hot metal has far less spring tension on the thread, which is what is actually holding the bolt in. – J.J May 15 '16 at 12:10
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    @J.J yeah, they think they are scientists. I'm an actual scientist, and I've worked in the forge, and under more vehicles than I can count. But whatever, the truth is not always popular. – Ben Welborn May 16 '16 at 13:00

@Vladimir Cravero (sorry not enough rep to enter a comment)...

I think clarification of the answer is needed. The nut isn't expanding "more", it ends up larger but the % increase is the same.

r = 1.5*(1+500*10-5) mm = 1.5075 mm         
R = 1.501*(1+500*10-5) mm = 1.508505 mm         

        start   after heat      increase amt    % inc
bolt    1.5     1.5075          0.0075          0.5000%
nut     1.501   1.508505        0.007505        0.5000%

My perception of the effect of heating is that not only do the bolt and the nut or block expand, but also the space between them expands as well, dont' forget about that.

        start   after heat      increase amt    % inc
space   0.001   0.001005        0.0000050       0.5000%

slightly larger space between as well, easier to remove. :)

  • It is expanding "by more" though. – frnhr May 14 '16 at 0:51
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    This is in my opinion, the only correct answer here. The bolt increases, the nut increases, and the space between them increases. And in these exact words, is how I was taught about the principle by my Physics teacher. – Douglas Held May 16 '16 at 4:05

I think there are multiple factors that contribute to this effect but I think one has not been mentioned. One other way to get a stuck bolt to release is to shock it by hitting it sharply. Generally this is something you do something large like a valve but I think the underlying problem is the same. For rust, I would expect that this can shatter the brittle structure of the oxide. Another factor is that there are two types of friction. There's static friction and kinetic friction. Consider a heavy (filled) cardboard box on a floor. If you try to slide it, it will initially be 'stuck'. Once the box starts moving, it slides much more easily. This is the same reason it's bad to lock up the brakes on a car. Once the rubber starts sliding, the friction is significantly reduced.

Temperature is the measure of the average kinetic energy of the molecules of a substance. That is, the molecules are moving in any substance warmer than absolute zero and the faster they are moving, the higher the temperature. When you heat something, you are adding kinetic energy to the system. This is literally causing the molecules of the bolt to move more and more quickly. In a solid, the molecules are not moving freely in space and essentially vibrate. The following image is a depiction of how metal molecules move when heated.

enter image description here

I think it's possible that this energetic movement could, by itself create the same effect as the shockwave caused by a sharp knock. That and an uneven change in size of the bolt and nut could break the static friction and/or shatter the brittle rust. I know that if you have a rusted cast iron skillet, one solution is to put it in a hot fire and the rust will simply fall off.

Because heat does not travel instantly, the nut will expand more than the bolt... if you time things right... which is not trivial. For a bearing rather than a nut/bolt, this [induction] heating is an industry method of removal, as shown in this video for example, and even more so for affixing. The removal action is instantaneous in this case, once the bearing ring is heated enough. The issue with a nut/bolt is that a lot of heat may have transferred to the bolt, perhaps before you are done removing the nut. Quoting from a practitioner of this art "you want to heat the nut and not the bolt".

The issue is further compounded by the fact that there isn't one single method of doing this. You can see in this other video that the nut gets much whiter than the bolt, which means it gets much hotter when heated. The catch is that by the time the nut is removed, neither is glowing anymore [in that last video], so we can't visually tell their temperature [difference]. Air however is a much better insulator, so I suspect that the bolt cools faster than the nut because it makes contact with more metal, which acts as a radiator. A video with a thermal camera would be definite proof, but I couldn't find one. The description of that last video also says that corrosive bonds are loosened by heating which may well be true as well, but I haven't checked the science on this bit; this claim also assumes that those bonds are not immediately restored by cooling.

And for the scenario depicted in the questioner's own answer: it doesn't work like that in practice. If you watch the 2nd half of this half-hour video, the dude is carefully heating the frame around the bolt itself, and it takes a lot of time, patience and care to succeed when the "nut" is a large piece.

I've got a simple answer that no one has said the head of the bolt expands off the surface loosening the tension off the threads thus making it loose enough to turn off. Sometimes bolts are too tight even when they are not rusted.

Put a penny in a door jamb and close it. The door will be almost impossible to open, because friction will hold it in place. The flexing of the rest of the door will prevent it from moving. A rusted bolt is essentially the same principle - many small bonds formed on the threads of the bolts by oxidized metal prevent it from turning.

The heat and expansion of the metal simply served to break those bonds. It has nothing to do with thermodynamics or any other science-y nonsense. It is the simple mechanical action of the expanding metal breaking the rust.

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    So how does the heat break those bonds without any "science-y nonsense"? Magic? – JimmyJames Jun 20 '17 at 20:34
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    How is friction less "scienc-y" than thermal expansion? Are you aware that friction and pressure are directly related? Your own example demonstrates this. Who said anything about rust anyway? – isherwood Jun 20 '17 at 21:21

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