If metal expands when heated, how does heating a bolt loosen it?


Spoiler alert: The puzzle is answered in the update at the end of this post.

Most solids expand when heated and contract when cooled. Water/ice is anomalous in that it expands when cooled, at least near its freezing point. If you’ve been unfortunate to forget a bottle of water in the freezer, only to find that it has broken once its contents have frozen, then you’ve seen this effect first-hand. Similarly, if we don’t clear the water from our outdoor pipes in winter, we run the risk that they might burst should the water within freeze. You can also observe this effect by carefully filling an ice-tray up to a mark, then noticing that upon freezing the ice has expanded so that it is now higher than the mark.

Why does water have this unusual behaviour? Its molecular structure and hydrogen bonding tell the story.

Ordinary solids (that is, virtually everything except water/ice) expand upon heating and contract upon cooling. Molecules jiggle about, and when a...

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Thermal expansion is an important property of many materials, and few materials break the pattern of expanding as they increase in temperature. However, solid water and, at low temperatures, even liquid water, actually shrinks as it heats up. Water is an exception, however, with a relatively unique molecular structure and properties that cause its solid state to be less dense than its liquid state.

Thermal expansion is not only a common property but also an important one. Different metals expand at different rates with temperature change, which is a feature used in bimetallic strip thermometers. While thermal expansion is useful in this way, it is also often a problem in construction, particularly when several different materials are used or when heating is likely to be uneven. Shrinking and expanding metals produce stresses in structures, which can cause structural instability over time.

Learn more about...
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At this point, the bolt may still not budge. I bet your getting frustrated, Right? Don’t fear, there’s still plenty more things to try!

Our next step is to use a torch to heat the bolt up. The idea is to have one side of our bolt expand from the heat and break the rust. This can be easily done with a small propane torch. Be careful, this method WILL ruin the heat treatment on stronger bolts.

Heat it up as hot as you can get it without melting the bolt. Remove the heat and let cool or pour warm water on the bolt to speed up cooling. Once it is cool, try out the breaker bar again. With any luck, The heat has broken the rust seal on the bolt allowing it to turn.

This is not a good idea around flammables or rubber...

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We've all come across the stubborn bolt. You know the one. That bolt that needs come off, like, now, but for no visible reason, the nut won't turn. Turns out, there's a simple trick: add a bit heat and get it turning quick. Here's how I make it happen.

Why does this happen?

The problem with a rusted bolt is that the spaces between the threads get clogged with minerals and oxidized metal. Trying to turn the nut around these stuck joints can result in stripping, which transforms a simple task into a real pain in the ... bolt.

That's where a bit of heat comes in handy. Metal expands slightly when heated, so warming up the nut a bit makes it ever-so-slightly larger, loosening the hold enough that it will be able to break free. It's important to keep in mind what you're heating here, make sure it's only the nut, if possible – or the female threads, and not the bolt's male threads.

Here are the steps:

Look around. We all get...

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Using a pipe cheater bar and propane torch to loosen stuck nuts and bolts.

To loosen a nut or bolt that won’t come off, start by spraying the connection with penetrating oil. Allow the oil to soak in for a time, then try loosening it with a wrench.

To allow for more torque on the nut or bolt, slip a piece of metal pipe over the handle of the wrench to give you more leverage when turning it.

If the nut or bolt still refuses to budge, try heating it up with a propane torch. Heating the connection causes the nut and bolt to expand and contract, which can help break the bond between them.

Apply heat to the joint with the torch for several minutes, allow the nut or bolt to cool, then repeat. When heating a connection with a torch, make sure to keep all combustible materials away from the area to avoid a fire.

Watch this video to find out more.

Further Information

Joe Truini: What do you do when you have a nut or bolt...
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All analogue thermometers work because most (not all) solids, liquids and gases expand when heated and contract (up to a point) when cooled. The old mercury filled ones and the ones with red dyed alcohol expand a liquid up a narrow tube with a vacuum at the top and a scale showing the temperature. The dial type use a spiral of expanding metal which gets longer and moves the needle. Digital thermometers use a junction of two differing metals which generates an electrical current which changes with temperature (this is called the Seebeck effect).
By heating the nut (and avoiding the bolt as much as possible) the metal expands, this increases the size of the hole in the middle, releasing pressure against the bolt and allowing you to turn the nut while hot. Wear heat proof gloves, eye protection and have extinguishing equipment and cold water to hand. The level of heat required is often described as "cherry red" and can cause some types of metal to permanently change their...

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Applying heat with a propane torch can expand a nut (or threaded opening) and loosen it. If you've tried everything and still can't free a bolt or nut, try heat. But first make sure the fastener is a good distance away from hoses, belts, gas tanks or anything else that might burn up your car or kill you in an explosion.

If the area is soaked with penetrant, spray it with nonflammable brake cleaner to remove it before you apply heat. Keep a fire extinguisher nearby. Once you've established that it's safe, aim the flame at the bolt head or nut, not the surrounding metal. Heat for about 15 seconds, but don't get it cherry red. Then spray the bolt head with water to cool it quickly. Continue spraying until it no longer steams. The expansion/contraction cracks the rust, so add more rust penetrant, let it soak and then add vibration by tapping it with a hammer or other tool. Then try to loosen...

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Any time you replace an old faucet, you have to take off the locking nut that holds the faucet tight to the countertop. But often that nut will be frozen due to corrosion or mineral buildup from years of water seepage. Here are some nut-freeing tricks from Richard Trethewey. This Old House's plumbing and heating expert, listed in order from easiest to difficult.

1. Tighten the nut. Moving the nut in any direction is progress. Then try to loosen it using a correctly sized wrench.

2. Tap with a hammer. Jarring the nut can break its bond to the bolt. To make sure you hit the nut itself and not the surrounding threads, place a center punch on the nut and strike the punch with a hammer.

3. Apply heat. Metal expands slightly when hot, which may be enough to crack the nut free. You can do this with a hair dryer, heat gun, or propane torch. (If using an open flame within 12 inches of anything flammable, shield it with a flame-resistant fabric.) Try to turn the nut...

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Subsections herein are provided as links, as follows -


Purpose of This Treatise

As we will discuss below, there are two schools of thought regarding the aircooled VW heating system: 1) it isn't worth a damn, and 2) it is adequate and can be made to work well. The purpose of this treatise is to explore the latter premise. It is motivated by my own experience (and frustrations!) with the VW heating system.


Two Schools of Thought

As we indicated at the outset, there are two schools of thought regarding the VW heating system: the first and most common is that it isn't worth a damn; the second is that is adequate and can be made to work well. Following are some comments on these opposing views:

The VW Heating System Isn't Worth a Damn.

Most people's view of the VW heating system: You are cold in the winter; your girlfriend/wife doesn't want to ride in "that" car; you are no longer amused by the ice crystals that form on the inside...

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Presented below is a glossary of terminology on topics related to nuts and bolts, threaded fasteners and tightening techniques. If you have any questions about any of the content or you think we've missed something why not send us an Email on the topic.

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

We have a web site dedicated to training, have a look at


- for additional information on bolting technology.

ACORN NUT A nut (so-called because of its shape) that has a domed top so that it prevents contact with the external thread. AEROTIGHT NUT A torque prevailing nut of all metal...
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Microfiche and Parts Sources
http://www.knmotorcycles.com/ K&N Cycles will give VMOA members a nice discount.
www.pcwracing.net Has a gear puller that works, plus has a 'solution' to the starter clutch issue, which I used.

Fiche from BikeBandit
2004 prices

Crankcase cover fiche from BikeBandit

Stator fiche from BikeBandit

Starter Motor fiche from BikeBandit

Starter Clutch Assembly from BikeBandit

Same operation as outlined above, 'by the book.'
or extracted from various manuals/sources with personal ideas.

Vmax Starter Clutch Servicing Steps in Order
Source information from Clymer's, Haynes',...

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All N-Series Tractors - Ford-Ferguson 9N, 2N, and Ford 8N

Engine rebuilding is already covered in detail, in the shop manuals, and on many other sites. I've provided a link to more detailed info near the bottom of this page.

Rather than duplicate all that information, what I will do here is provide a few tips that may not be covered in the other manuals. Compression checks should be done prior to deciding an engine rebuild is necessary. Some engine problems may appear terminal, but are easy to fix. My procedure for getting and interpreting compression numbers is here: Engine Compression Check.

IMPATIENT PEOPLE BEWARE - Rebuilding an engine is not particularly difficult. However, it is very, meticulous work. Each step must be painstakingly completed, and re-done if necessary, as many times as it takes to complete. Every single step must be completed properly before moving to the next. Start cutting corners, and engine failure is virtually guaranteed. The point...

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Aircraft preheat FAQ's

"Long Engine Life Starts With Reiff"

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Frequently Asked Questions

These are the questions we get the most, with straight answers. If you have a question that is not listed here, please call and ask.

How long will it take your system to get my engine warm?

Which system should I get?

Can the preheater be left on continuously, i.e. what about condensation?

Can I use your cylinder heater system with my non-Reiff oil sump heater?

Why is your price so low? / Why is your price so high?

Why did you develop the HotStrip, is there something wrong with silicone pads?

Does your system require an STC or Form 337?

Does your system have a thermostat?

How long will a Reiff Preheat System last?

What about the other brand's claim that heating the head is best?

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