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Thread: Proper use of Loctite & general fastener information

  1. #1
    Murder Machine, Harmless Fuzzball TCinVA's Avatar
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    Proper use of Loctite & general fastener information

    Thread locking products seem to be a source of significant confusion. Hopefully this little writeup will help clear up confusion on the use of the Loctite product.

    As slide mounted dot optics become more common, more people are going to run into problems mounting optics. This usually manifests in fasteners that hold plates or optics coming loose or breaking off. Sometimes materials and manufacturing defects produce this outcome, but user error the most common and the most easily controlled reason why optics mounting fails.

    Fasteners -

    The first thing to understand is that fasteners work essentially like a spring. When you tighten a fastener down, the fastener itself elongates applying force on the bearing surface around the fastener's head to clamp down on whatever it is being screwed into.

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    The fasteners we are using on slides usually tighten down into a blind hole drilled and tapped into the slide itself. If a fastener snaps or breaks, extracting it from the blind hole usually requires specialized tools and knowledge to accomplish.

    If we stop a moment to think about the implications of fasteners stretching when tightened, and then we think about the consequences of breaking a fastener off in a blind hole, it would make sense to adopt a best practice of replacing fasteners whenever practical to do so.

    The materials and manufacture of the fastener will have a lot to say about how many times that fastener can be stretched or how much stretch it can endure without a catastrophic failure...but your typical end user has no way of measuring any of those things. Fasteners that have never been stretched provide the widest margin for error should they be over-torqued, and the bearing surfaces on the unused fastener are going to have the best engagement with the bearing surfaces in the tapped hole.

    From a risk mitigation perspective, simply replacing the fasteners whenever practicable to do so would be a solid way of avoiding a broken fastener and lessening the chances of the fastener loosening under use. This would become especially important if you are using an optic like the Trijicon RMR or some of the smaller optics that require removing the optic from the slide to replace the battery.

    Fasteners included with optics or firearms are usually being sourced from a big wholesaler like McMaster/Carr, meaning you can also purchase the very same fasteners yourself. You simply need to know the length, the thread pitch, and the shape of the fastener’s head for your use. Often the fasteners used by manufacturers of more budget style optics are of inferior quality, so purchasing your own might actually allow you to get a better-quality fastener that is stronger and more durable.

    Loctite -

    You might think that the threads cut into fasteners make complete contact with the corresponding threads cut into the nut or the mounting hole they are being screwed into, but that’s not quite the case. One side of the thread is actually forced into that close contact, but the other is not. This allows a gap to form.

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    If the fastener is installed in a static, dry environment this is not a problem. In our case, however, the fastener will be exposed to some fairly violent vibrations, impact, temperatures, and a humid environment. Remember that our fastener operates essentially like a spring. With a gap between the bearing surfaces, our “spring” can move, allowing the fastener to become loose. The more rapid and forceful the vibrations, the more quickly the fastener will come loose.

    Thread locking products made by or similar to the LocTite family of products function by trying to fill these gaps with an anaerobic adhesive. These products are a liquid or a gel in the presence of oxygen. When the fastener is tightened, air is driven out. Without the oxygen standing between the compounds in the adhesive, it borrows ions from the metal in the fastener and the work piece, forming stronger polymer chains. These chains network together eventually filling the gaps with a solid polymer substance that gives the threads of the fastener nowhere to move to. The solidification process is typically referred to as "curing".

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    Preparation -

    Anything that gets between the thread locker product itself and the surface of the metal hinders the ion-borrowing process critical to a proper cure. Dirt, debris, grease, oil, water, corrosion etc will reduce or completely eliminate the ability of the thread locker to form the bonds necessary to solidify. The hole and the fastener both need to be cleaned and degreased with some sort of volatile cleaner that won't leave any residue behind. High concentration isopropyl alcohol can be used for this as it performs the function and evaporates quickly without leaving behind an interfering residue. You want to go with the highest concentration you can get for this purpose because what isn't isopropyl alcohol is water.

    Loctite actually makes a cleaner/degreaser product that is compatible with their thread locker products in an aerosol form, which can be useful for blowing crud out of the blind holes themselves.

    Activator -

    The aforementioned ion-borrowing process requires surfaces that readily give up ions to the material. Some materials are more "passive" or "inactive" than others when it comes to handing out ions. Temperature also plays a role in this process. The lower the temperature the longer it takes to achieve a proper cure.

    The curing process can be significantly enhanced by use of Loctite's activator. The activator helps the cure set up properly on more passive substrates, promotes curing in lower temperatures, but most importantly to many of us it will actually significantly speed up the curing process by several hours. Typical advice given on the use of LocTite is to wait a full 24 hours before using the pistol to allow for a proper cure. That's not bad advice as it gives enough time for the product to cure on even passive substrates at lower temperatures.

    Use of the activator, however, can reduce the time for a proper cure down to three hours or less. That might come in handy at a class, a match, or in the field.

    Activator isn't strictly necessary, but if you want a little extra insurance it's a pretty good option.


    Application -

    The vast majority of the fasteners we deal with in mounting optics on slides go into blind holes either to hold the optic itself or an adapter plate on the slide. LocTite's recommendation for application in blind holes is to use a little of the liquid product applied on the fastener and then more applied directly to the blind hole. Air is displaced as the fastener is tightened. As it is displaced, the liquid LocTite is essentially sucked up through the spaces left between the threads of the fastener and the threads in the tapped hole to distribute evenly and fill the aforementioned gaps:

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    In this video you can see the process in action:



    Keep in mind that the practices shown above are directly from LocTite themselves.

    Filling the hole completely will just result in a lot of excess product spilling out of the top that then has to be cleaned up. I will generally fill the hole between 1/3 as shown in the video and 1/2 depending on how precise I can be with the dropper. If you can be more precise than that, by all means. I find it difficult to just hit the bottom third of the female threads in the holes we're working with and the typical applicators. We're often working with very small holes that don't have the capacity for much volume. If you get more in the hole than you want, you can wick some away with the tip of a paper towel.

    If you've used enough product there should be at least some excess to clean up. If you aren't seeing at least some of the product come out around the head of the fastener you were probably a little too sparing in your use. Again, remember that what we are trying to accomplish is filling the gaps between the male and female threads with adhesive so our "spring" doesn't have the ability to shift or move when it experiences vibration or impact.
    Last edited by TCinVA; 12-01-2021 at 02:33 PM.
    3/15/2016

  2. #2
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    I'm gonna say this should be stickied somewhere.
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  3. #3
    A machinist friend told me that there are several classes of thread fit.

    I forget how they are categorized, but it has to do with how much slop vs contact there is between the male and female threads.

    This seems to influence strength of fastening etc....

  4. #4
    Murder Machine, Harmless Fuzzball TCinVA's Avatar
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    Quote Originally Posted by 1slow View Post
    A machinist friend told me that there are several classes of thread fit.

    I forget how they are categorized, but it has to do with how much slop vs contact there is between the male and female threads.

    This seems to influence strength of fastening etc....
    Without question. But typically there's a lot less contact than laymen like us would believe. As little as 15% in some applications. I'd imagine it's higher than that in fasteners going into slides instead of nuts on bolts, but that has to be weighed against how many threads are actually being engaged when the fastener is stretched.
    3/15/2016

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    Quote Originally Posted by Tensaw View Post
    I'm gonna say this should be stickied somewhere.
    @LittleLebowski...

    @TCinVA You also just explained what I thought were cost saving measures on my LaRue mounting screws....

    pat

  6. #6
    Murder Machine, Harmless Fuzzball TCinVA's Avatar
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    Another note on fasteners:

    If possible, upgrading your fasteners to torx style heads instead of allen fasteners is a good idea, at least on the size fasteners we are usually dealing with IMO. The smaller Torx heads seem, at least in my use, less prone to stripping the head out.

    You also want to use good quality bits to tighten/loosen fasteners to prevent stripping the heads. I've found Chapman tool kits to provide excellent fit on fasteners that I encounter in gun-related applications. I have been very pleased with their #1000 Mity Master tool set for working on optics and revolvers.
    3/15/2016

  7. #7

  8. #8
    Great post, TC.

    There are two steps to thoroughly cleaning any surface in preparation for the application of adhesives.

    First, cleaning the surface of organic material, such as dirt, dust, food particles etc. This is done with a soap of some type. I like to use a mild solution of Dawn dish soap and water as hot as I can stand and thoroughly rinsing with hot water to heat the part to help to evaporate the water. I use Dawn because it not only lifts organics but cuts grease & oil as well.

    Next, is cleaning the part of oily residue with a solvent. I have found (as TC pointed out above) isopropyl alcohol works really well. Not only does it clean away any oily residues, but it breaks the surface tension of water and helps to remove any water left from the first cleaning.

    Then, let the part set for a half hour to let the solvent flash off.

    NOTE: Compressed air can be a good way to blow water or solvent out of your part. Be sure to use an oil free source. Not much air pressure is needed and wear safety goggles and be careful of splash back.
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  9. #9
    Site Supporter OlongJohnson's Avatar
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    I get the impression that lots of OEs are using the cheapest screws their purchasing managers can possibly source, rather than the best. It's been hashed out on this forum before, but going with a quality supplier (McMaster is like Home Depot -they don't make anything, they just buy and sell stuff) like McMaster-Carr, Grainger, Fastenal, Fabory, etc. will get you better quality parts for less money per part than going to your local hardware store.

    In some cases, the poor quality of OE fasteners comes from the engineers being permitted to design around custom-made fasteners (cough...Beretta...cough...Howa), rather than picking from some industrial standard. The result is a fastener made using methods that are economical in small volumes, but don't result in a quality part with expected mechanical characteristics.
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  10. #10
    Deadeye Dick Clusterfrack's Avatar
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    It's worth noting that many adhesives, including Loctite, cause crazing and stress fractures in certain polymers, including ABS.
    https://www.ellsworth.com/globalasse...ic-bonding.pdf
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