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Is there a threadlocker compound that immobilizes the bolt while cool and requires high heat to release, that can withstand very high heat and thousands of heat-cool cycles without needing to be reapplied and without deteriorating? The application is automotive-exhaust-related.

P.S. This is important factoid that I left out (since I'm getting advice on doing it a different way): this is a "non-tamper" security application. The thief would have to heat the bolts to loosen them.

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    $\begingroup$ That just sounds like welding or brazing to me. Or more feasibly, jam nuts. $\endgroup$ – DKNguyen Jun 9 at 14:25
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    $\begingroup$ @PeteW: specifically trying to make a cat-anti-theft device that uses security-head bolts harder to defeat, by immobilizing the bolts. $\endgroup$ – Tim Jun 9 at 18:48
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    $\begingroup$ hmm... maybe use a Torx or "Volkswagen" bolt with round-OD head, then fill the drive socket with solder? Alloys with a variety of melting temps are available... $\endgroup$ – Pete W Jun 9 at 18:51
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    $\begingroup$ @PeteW: That suggestion might bear fruit. Whatever the material is, it can't be something that the heat of the cat itself will melt away. $\endgroup$ – Tim Jun 9 at 19:06
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    $\begingroup$ @AnoE Took me a while to realize that multiple people weren't being autocorrected from car to cat, but that cat was short for catalytic. $\endgroup$ – DKNguyen Jun 11 at 1:05
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I don't mean to discourage you, but the catalytic converter thieves in my area use battery operated reciprocating saws to cut the exhaust pipe, then disassemble the cat from the pipe when they get back to their den of thieves. If you've ever disassembled an exhaust system with rusty nuts, you'll agree that a quick cut with a Sawzall is way easier than struggling with rusty threaded fasteners.

It's noisy, yes, but it's quick.

So what's next, case hardened tool steel exhaust pipes to defeat the Sawzalls? The thieves will just start using plasma cutters. It's a never ending tit-for-tat.

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  • $\begingroup$ The anti-theft devices are designed either to resist the sawzall blade (e.g. CatClamp with its steel rope) or to prevent the sawzall from accessing the vulnerable pipes. $\endgroup$ – Tim Jun 9 at 23:46
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    $\begingroup$ If you don't care about damaging the car you can have a cat off in seconds with modern battery-operated tools &/or brute-force, security nuts will cause you and your mechanic more inconvenience than any theives. $\endgroup$ – John U Jun 10 at 14:39
  • $\begingroup$ @JohnU: These devices are designed to deter a sawzall, either because the material is very hard (a plate) or because it is not rigid and is very difficult to cut with a sawzall (steel rope). That leaves the bolts as the weakest element, hence the security bolts. But if a thief brings the right bits along even they are easy to remove. Hence the threadlocker idea. $\endgroup$ – Tim Jun 11 at 11:37
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If it is that critical, then go for locking wire.

Used on all the exhaust fixing nuts on a friend's airplane as if they come loose they go through the prop and not nicely...

Will handle heat/cool cycles no problem.

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  • $\begingroup$ Yes, this is of course the clear answer given the question as originally stated. But I'm specifically looking for a theft-deterrent solution using thread-locker which would require the catalytic converter thief to bring a torch. These bolts are securing a cage around the cat. $\endgroup$ – Tim Jun 9 at 18:36
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    $\begingroup$ @Tim I would not need a torch - have busted (aka sheared) 3/4" studs which would not undo so I just tightened till they broke. So I could still have your cat. $\endgroup$ – Solar Mike Jun 9 at 18:40
  • $\begingroup$ but wouldn't that break the head of the bolt off, leaving the bolt still threaded? $\endgroup$ – Tim Jun 9 at 18:50
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    $\begingroup$ @Tim Properly design dictates that only one hole between two objects being fastened to each other has a thread. The hole on the side with the head is unthreaded, or both holes are unthreaded and a nut is used. Think about it, if both holes were threaded they would jam since the thread is not continuous. Even if they were lined up and tapped at the same time, it would still jam since there is always some thread tear out at the ends of a hole. $\endgroup$ – DKNguyen Jun 10 at 1:20
  • $\begingroup$ @DKNguyen: The device has a hinged clamp that opens up like the letter "C". On the side opposite the hinge it has a smooth unthreaded section of tube and a threaded section of tube that are aligned axially. You squeeze the clamp shut, and insert the bolt through the smooth section and tighten the bolt into the threaded section. The bolt has a dome security-head so there's not much "purchase" for the tool. It would be hard to shear the head off if the bolt was immobilized with thread locker. $\endgroup$ – Tim Jun 10 at 13:25
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Apart from the locking wire in Solar Mike's answer there is a host of other thread locking mechanisms to pick and choose (because in all likelihood mechanical/form locking is a more secure method if you are worried about thermal expansion).

I am adding the following table for a quick reference

Type Photo source
Positive self locking Locking plate enter image description here Wikipedia
tab washer enter image description here wikipedia
pruning shears locking blade mechanism enter image description here Casacenina
Castle nuts and cotter pins enter image description here proven productivity
Safety Wire (or locking wire from Solar mikes) enter image description here Aviation hunt
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  • $\begingroup$ Safety wire made of something that could not be easily cut, and which could be chemically welded to prevent untwisting, would work, if such a material exists. $\endgroup$ – Tim Jun 9 at 18:38
  • $\begingroup$ after reading your update (ie. intention for anti-theft device), yes I agree that safety wire is probably the least applicable. although you should be able to make it more difficult with some of the above solutions . However, in IMHO, the solutions from Harper is probably the closest to the original solution you had in mind. (although MTA's view is also very valid - i.e. its going to be very difficult to find a solution that acts as a deterrent for a determined thief). $\endgroup$ – NMech Jun 10 at 10:09
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    $\begingroup$ Your answer sent me looking for fastener types and I found this: brycefastener.com $\endgroup$ – Tim Jun 24 at 10:56
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Nowhere is it written that all fasteners must be hexes.

The answer is, you use some sort of bizarro fastener such as High Security Torx. Or better, a dog's breakfast of different obscure fasteners on the off chance they have one of them.

To answer your next question, McMaster-Carr or Fastenal.

For that matter, you could just choose stud sizes which are same as wheel lugs, and use lug locks.

Saws and grinders can be defeated by different materials.

Saws are instantly dulled by any abrasive, i.e. out of the concrete, ceramic or refractory families.

Concrete cutters are instantly fouled by acrylic.

A very potent cut stopper is a concrete-like substance, reinforced with a short lengths of acrylic fiber, which provides both reinforcement and assures fouling.

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  • $\begingroup$ My thinking was like this: a committed thief could have a wide range of security bits and operate inconspicuously but a blow torch would draw attention, especially at night. $\endgroup$ – Tim Jun 9 at 23:44
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    $\begingroup$ Nah, cat theft isn't like that. They must be ready to flee on a moment's notice, so if they hit a stumbling block on any given car, they just go to the next car. It's very much like the old adage: "I don't have to outrun the bear, I just have to outrun you". As soon as they see the expanded metal grating guarding your cat, they won't even look to see what kind of bolts they are lol, they'll just got "nope" and next car. $\endgroup$ – Harper - Reinstate Monica Jun 10 at 0:09
  • $\begingroup$ Then all I'd need to do is make a convincing looking replica of a security device and a window sticker that says "CAT protected with BUMAX ULTRA bolts" :) $\endgroup$ – Tim Jun 10 at 13:36
  • $\begingroup$ The reality is they cut the cats off in seconds with a battery-operated saw or grinder and very little is going to stop them doing that. $\endgroup$ – John U Jun 10 at 14:37
  • $\begingroup$ @JohnU ...and, there's an XKCD for that. "defeating the armor on THIS cat" is a distraction that misses the objective: coming back to the shop with 20 cats. Failure is getting arrested. A guarded cat means a car owner who's serious, means silent alarm means cameras means God knows what. Get the heck outta there and find other targets. $\endgroup$ – Harper - Reinstate Monica Jun 11 at 0:35
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If it's your design, use materials with a different heat expansion rate.

For example, a steel bolt and a brass or aluminium nut.

You'd need to cut them so that they were tight when cold, i.e. make the nut thread in the hot state. This might only be practical for decent sizes, perhaps M8 or more.

That said, such things are hardly a significant deterrent for determined thieves who are prepared to cut the part off. And for petty thieves, the device should be visible, like security bolts, so that they wouldn't bother. For a thread-locked nut, they will attempt it and will probably damage it if locking is actually good.

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