The Ultimate Firearms Destination for the Gun Lifestyle

Threadlocker How-To // The Glue That Binds Us

There’s a saying in Formula One that a 15-cent screw can ruin a race just as fast as a blown motor. The little things matter, especially in a 13:1 compression engine spinning at 18,000 rpm. Guns don’t exactly run on gentle unicorn toots either; they’re an instrument of intense heat, pressure, and extreme acceleration in an area no bigger than a Pringles can. If your life depends on your firearm, or a major match is looming on the horizon, you’ll want to prep your gun just like any F1 pit crew would, and that means securing all those 15-cent screws with threadlocker.

I was recently at a pistol match with my Sig XCarry, my slide freshly hacked up to accommodate an old Insight MRDS I had bouncing around the bottom of the parts bin. The fitment of the sight was rock solid. However, in the middle of my second stage — BOOM — no dot. I did my best to salvage the remainder of the stage, but blindly shooting your way through a plate rack and a Texas star is a tricky business. I thought maybe my early GWOT-era dot had sh*t the bed, but after a secondary inspection, it seemed the two screws that kept the battery door shut had simply come loose — a small oversight that cost me a decent stage score. Had it been a defensive shoot, it could’ve cost me my life. I thought to myself, “What a boner move! A dab of blue Loctite would’ve prevented all that.” Any of that sound familiar? Well, then read on — I’ve got some primo nuggets of knowledge for your ass-pack of facts.

THREAD BASICS

Before we jump straight into the ins and outs of threadlocker, let’s do a quick review of threads and screws. There are various types of fasteners, but the most prevalent of these used in guns would be those with an isosceles “V” thread form, between 6-32 to ¼-28, and come with either a head or as a set screw (a screw with no head and a machined or broached drive integral to the body). If you’re not quite sure what all the numbers mean, let’s decode it.

thread diagram

For Imperial threads, the first number is the theoretical major diameter of the thread body. I say theoretical because the sharp points of V threads are truncated to varying degrees in order to allow clearance between the tip, or crest of the male threads, and the root, or bottom of female threads. The second number is the threads per inch, or TPI. That is, counting from crest to crest, how many rotations are formed about the body in one linear inch.

Metric threads diverge slightly, as the second number is the pitch of the threads, or the distance between one crest to the next. You might also see some thread callouts with a tolerance attached in certain examples. These classes of fit range from loose to tight as a 1, 2, or 3 and A or B (External or Internal). The thread helixes in either a clockwise or counterclockwise direction denoting a right- or left-hand thread. The existence or the absence of the head on the screw distinguishes which direction torque is being applied. Headed screws are designed to fasten a part externally to another, while set screws fasten one object inside another. Prime examples of this would be attaching a scope base to the top of a bolt-action receiver with headed screws or anchoring a gas block to an AR barrel.

thread measurments

Set screws exert compressive force while headed screws stretch, much like a spring, to exert linear tension. Torque is the rotational force we apply to a screw based on its design parameters to achieve the desired effect. The optimal torque setting directly correlates to the material and finish of the screw — the diameter and the pitch. While it’s common knowledge that too little torque can be detrimental for obvious reasons, did you know that too much torque can be even worse?

Stretching a screw past its nominal yield strength (commonly about 75 percent) can weaken the integrity of the screw body, reducing the clamping strength over time and exposing it to repetitive lateral shear forces, which, coincidentally, occur quite often perpendicular to screws in firearm applications. So the next time you think it best to white knuckle a screw with your hex wrench, think twice. “Well, how much torque should I be using then, John?” you might ask. Before we delve into that, first and foremost, get an adjustable torque wrench in inch-pounds and a set of bit drivers for every screw head type you plan on using.

torque wrench soldering iron threadlocker

A soldering iron can be used to break down threadlocker on individual screws without spreading the heat around too much.

Torque wrenches are relatively inexpensive, so if you don’t have one, there are plenty of brands and models available. On the topic of specific torque values, I’ll always consult the manufacturer that specified the use for that particular fastener. Their design intent might differ from the typical torque value you might expect from a certain screw size, such as securing a polymer body MRDS to a pistol. Now, if you’re some poor degenerate with no torque wrench, outside the realm of any form of internet or mail-order system and just need to figure it out, I find a prudent practice is to thread the screw in with two fingers until resistance is met, then go another quarter to half of a turn and evaluate for tightness. Then, pray to the machine gods for forgiveness, heathen.

 

ON THREADLOCKER

You’ve probably started to wonder to yourself, “If I’m applying the proper torque to a screw, what’s the purpose of threadlocker?” Short answer: insurance.

There are a few factors that contribute to a screw coming loose, and it just so happens most of these occur with firearms. Shock, vibration, and thermal expansion are all colluding to undo your meticulous efforts with every round fired.

I wouldn’t say the best way of putting threadlocker on screws is necessarily tribal knowledge, but from where I’m sitting, it sure seems elusive to most. I can’t tell you exactly how many thousands of guns I’ve handled or disassembled, but I can confidently comment on the American firearm owner’s love affair with the subtle art of way overdoing it. What we should strive for in a gun build is the careful balance of assembled rigidity and ease of disassembly. At some point, that screw you thread in might need to come out, so it’d be great if we could accomplish that without the associated blood, sweat, and tears.

noveske installation

Screws on rifles and threadlocker go hand in hand — just make sure you use it properly.

So how do we achieve this delicate equanimity as gun builders?  Enter Loctite Threadlocker. Manufactured by the Henkel Chemical Company, Loctite produces an array of phenomenal products that are surprisingly more extensive than the few options you likely see in the glue aisle at your local home improvement store. These single component, anaerobic adhesives are comprised of liquid resins that cure to a tough solid when exposed to metal ions in the absence of air. The anaerobic cure mechanism allows adhesive to flow along threads to fill the grooves of fasteners without curing prematurely. Any excess threadlocker that overflows the threads during application will remain liquid and can be easily wiped away.

As Loctite cures, it forms polymer chains that find their way into all the empty space between threads. Thread friction increases with the application of threadlockers as they bond to imperfections in metal’s surface. A fully cured application of threadlocker prevents lateral movement and protects the joint from corrosion that can result from moisture, gases, and fluids. Using threadlocker, much like bedding a rifle, provides 100-percent contact between metal surfaces, whereas a typical nut and bolt assembly may have as little as 15-percent metal-to-metal contact. Available in different strengths for different applications, threadlockers will hold critical clamp load pressures even in the most severe environments.

SELECTION

While Loctite isn’t the only threadlocking compound available, it’s the most widely used in all the shops I’ve seen. For the purpose of simplicity, I chose to focus on that product. If you prefer to use Permatex or another brand I’m unaware of, be sure to follow manufacturer guidelines. Loctite Threadlockers are available in low-strength formulations (Purple) for easy removal, medium-strength grades (Blue and Green) that can be removed using common hand tools, and high-strength formulations (Red) that offer the highest holding abilities. While none of these could be considered truly permanent, Loctite Red requires direct exposure to temperatures of up to 450 degrees F to remove, which might limit its use near anything that tends to melt or burst into flames.

loctite threadlocker

Loctite advises that the strength and viscosity of the adhesive needed for an application are directly related to the size of the fastener used. Purple is used on screws up to ¼ inch in diameter, for tiny screws or ones that might need future adjustment. Blue is typically meant for fasteners up to ¾ inch in diameter, but I’ve found through experience that it’s a good general-use threadlocker for just about any threads you’d typically have in a gun. Red Loctite is ideally used on larger thread sizes up to an inch, but since we typically deal in the smaller sizes, I find it best to reserve its use to only the most extreme examples. How extreme you might be wondering? All of the fasteners in the miniguns we build at my day job get Red. It doesn’t get more extreme than 3,000 rounds per minute, and I haven’t heard any first-hand account of any screw or bolt coming loose.

During my research for this article, I came across an interesting bit of information that perfectly explained some frustration from my early time as a gunsmith that you may have experienced as well. Apparently, the type of metal used on the fastener is critical to the performance of the adhesive. If the surfaces to be bonded are made of two inactive metals such as stainless steel, magnesium, black oxide, anodized aluminum, or passivated titanium, a primer may be required to facilitate cure. I’d wager a guess that, like me, if you’ve ever been underwhelmed by Loctite, this might be the root cause.

offbrand threadlocker

Automotive stores will typically have more options than a big-box hardware store.

APPLICATION

While some of the newer offerings of Loctite offer some oil and solvent resistance, I find that thoroughly degreasing all the threads to be bonded using a strong solvent is an absolute must. My favored method is removing all the screws I needed to coat at once and dipping them in my small steel bench ramekin full of MEK. I let them sit for a few minutes, then remove and blow dry with compressed air. I strongly advise the use of rubber gloves and that this be done in a well-ventilated area. This stuff is super gnarly and will for sure give you face cancer.

For the most effective bond, the adhesive must coat the complete length of the thread engagement area. Loctite is a thixotropic formula, which means it is thick and viscous under static conditions, and as it spreads along a surface under capillary action, it becomes thinner and will seem almost self-guiding. You’ll find a modest drop on a section of threads will find its way around the circumference and “creep” up and down the helix. When threaded in, this action compounds and the adhesive will work its way up and down the length of the threads quite easily. 

enough threadlocker

If a little is OK, then more is better, right? Wrong.

For large parts, applying to both faces provides the most adequate adhesive application as well as for blind-hole assemblies such as cap screws — threadlocker should be applied to both the bolt and the mating threads. If applied only to the screw going into a blind hole, air pressure tends to force the liquid threadlocker out of the hole as the bolt is torqued down. The anaerobic process hardens in minutes at room temperature and fully cures within 24 hours.

When determining the amount to use, it’s important to remember that less is more, and dipping the screw to the head will not make it “more better.” The space between threads allowed by tolerances is actually quite a bit smaller than you think, and the location that the adhesive is applied is much more important than the amount. If you’re trying to determine which level of adhesion fits your specific needs, start on the low end and work your way up gradually and try experimenting a bit if you have the time and resources.

If it comes time to remove a screw that has been secured using Loctite, first ensure you’re using the best-fitting bit for the job. Any play between your screwdriver bit and screw allows space for deformation and/or breakage of one or the other under the increased torque load you’re likely to encounter with the use of threadlocking adhesive. If it’s still being stubborn, I alternate between using a heat source, and a soak of either solvent or Kroil. If you’re going to resort to heat, don’t go straight for the MAPP gas torch. Try to keep heat localized around the female threads — start with a heat gun and gradually work up to higher heat. Some advocate using a soldering iron to heat the screw directly. You’d be surprised what the thermal expansion coefficient in some metals are.

ROCKSETT

Contrary to popular belief, some parts of a gun can get really hot. I know; shocker, right? If your range sessions strangely resemble the shootout scene from the movie Heat, it’s a good bet your gun will be a tad on the warm side. Suppressed rifles can easily achieve temperatures in excess of 800 degrees around the muzzle, which would cook any application of Loctite Red.

If you’ve ever bought a SureFire can or a muzzle device, you’re well aware of the little translucent tube that accompanies it, which states simply and in bold red “ROCKSETT.” Rocksett is a water-based cement containing silica manufactured by Flexbar Machine Corp. It originally started out as a clear version of an adhesive used for bonding machine components in high-temperature environments. Its proprietary formula is both nontoxic and impervious to most chemicals. At its inception there were a few gunsmiths who were quietly using it, but with the rise of the internet and search tools, Rocksett really caught on among gun users.

Sig top

Improper application or selection of threadlocker can lead to situations like this: three of four screws broken off in this slide!

Unlike Loctite, the structure of Rocksett can withstand temperatures from -350 to over 2,000 degrees F. This makes it ideal for use around the muzzle, but the Director of Sales at Flexbar, Lou Valenti, assured me it can be used effectively in any threadlocking application. This is made possible by the somewhat counterintuitive way it’s removed: a soak in hot water.

The correct application of Rocksett uses a thin or sparing amount, because it tends to dry from the outside in somewhat like a skin hardening. Once application is complete, the water in the formula needs to evaporate in order to leave only the silica behind. This is a physical process rather than a chemical one. The drying/curing process is benefited by forced warm/hot air movement across the surface. Rocksett recommends that for a light application, 24 hours at room temperature will suffice, followed by 20 minutes of heat at 175 degrees for a full cure.

If the application requires a fair amount, better results can be achieved by a gradual bake from room temperature to 300 degrees F in linear steps over the course of six hours. Excessive application is problematic for Rocksett. While excess Loctite can be wiped away, too thick an application of Rocksett won’t cure throughout, encapsulating uncured adhesive. When any additional heat is applied, the trapped water will boil, forcing its way out and destroying the bond. As is expected, ensure all parts to be bonded are free of oil and contaminants.

LOOSE ROUNDS

Having a strong familiarity with bolts and screws can serve all aspects of your tinkering lifestyle, and since fasteners are everywhere in our daily lives, we can better understand how they hold things together. You may even feel the urge to go put threadlocker on everything now, but don’t think you absolutely have to. When I’m putting a gun together, I often don’t use Loctite, unless it’s a part I suspect might be subject to excessive vibration or heat, or has shown a predisposition to come loose in the past. I trust my torque wrench implicitly when I’ve prepped my fasteners and surfaces correctly.

If you plan on using Loctite or Rocksett on your next build but are uncertain if it might be the right choice, don’t hesitate to contact either company. I found both to be enthusiastically forthcoming with product information, and if you can get an engineer on the phone at Henkel, they’ll actually help you determine the exact color and grade that’ll work best for whatever you’re doing. Once you have everything dialed in, enjoy the confidence that comes with a well-built gun.

 

[Editor's Note: This Article First Appeared in RECOIL #50.]


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