Adhesives have their places in AR-15 construction projects, and application ranges from what I think is crucially important to what I think is cautiously wise.
Because of the best-known brand name association, it’s usually called “Loctite,” which, like “Kleenex,” is a name associated with a product, but not the product itself.
These products are known as “threadlockers.” There’s also Permatex brand and others.
Again because of the common association with Loctite, there are different formulations available and each might have a unique name from each manufacturer, but they are usually referenced by their color.
Color is sort of a universal means to denote threadlocker “strength.” Each bottle of threadlocker will be color-coded to indicate its strength.
What Threadlocker Colors Mean
Let’s take a look at each of these colors and what they indicate:
- Blue: a medium-duty glue that’s probably the most common. Blue means the fastener can be removed with the same hand tool that installed it. It might take a little more effort, but it won’t be stuck.\
- Purple: a slightly lower-strength threadlocker than blue that’s formulated for smaller-diameter fasteners, like set screws. It gives up its hold a little easier than blue, which is a bonus if it’s holding small screws (which have small driver fittings).
There’s not a big difference in the effective holding power of purple and blue, just, again, that purple works best on small (smaller than 0.25-in. diameter) threaded pieces and blue works best on larger threaded areas.
Either of these threadlockers dissuades unwanted movement otherwise primarily induced by vibration. That is their job.
Again, something like a trigger adjustment screw (common application) can still be turned, it just makes the screw “sticky” in a way of looking at it. These have pretty much the same effect as the nylon insert inside a nut, as is familiar to mechanics.
- Red: Heavy-duty. Folks, this is stout stuff. Don’t use it nilly-willy, think it through. It takes a whopping lot of heat, for longer than you might imagine, to break loose (especially a larger surface area parts pairing). And when I say a whopping lot, that means it’s a good 500-degrees worth. On the other hand, whatever it’s used on is forever done, the parts are patently solidified. There are different “levels” of red from different manufacturers, but all are considered “permanent” in their applications; there are small differences in formulations based on fastener area size.
- Green: Considered “medium-to-high strength,” while blue is medium strength, green threadlockers are ideal for preassembled fasteners (e.g. set screws).
Applied over a large area (like threads on a collar), blue or purple may well require some heat to turn loose, but a heat gun is usually adequate.
Three Things to Know About Threadlockers
Now, in general, there are a few things to know about threadlockers, and this applies to virtually all of them regardless of strength.
- It is absolutely necessary to get the threads and the threaded both down to their bare metal. Use a residue-free degreaser, like brake cleaner, electrical contact cleaner, denatured alcohol.
- Do not shortchange or disbelieve the manufacturer instructions: it takes a FULL 24-hours to cure! That’s not “all afternoon,” or even “overnight.”
- Threadlockers won’t set or cure in the presence of air. You can drip a few drops on the bench and it will stay liquid until some great time passes and some evaporation occurs. Unlike epoxy (exactly unlike epoxy), threadlockers work best where there is only a tiny gap between parts. Epoxy doesn’t really work well at all unless parts have been “roughed up” — it has to have a gap to seal to get maximum hold.
This also makes excess threadlocker pretty easy to clean up and off unwanted areas.
Another thing to know, and keep in mind, is that these threadlocking compounds also inhibit corrosion. A mating of steel and aluminum can, over time, effectively weld the parts together.
There are also instances on an AR-15 where a steel screw is threaded into aluminum, and other areas where these metals make flush contact.
I’m not saying to use threadlocker for this reason, but it is a validation to use threadlocker! It won’t hurt a thing and might save a day.
Applications for Threadlockers
So, where to use which, and when? Put us all together and I imagine we could brainstorm a few pages worth. However, there are a few specific points and processes in a build where I highly recommend the use of threadlocker.
The first that comes to mind is installing the lower receiver extension tube (“buffer tube”) for a rifle-stock configuration. These loosen easy, so use blue.
It’s not necessary installing a CAR-style buffer tube because these have a castle nut that’s also a lock nut, in effect, but it cannot hurt.
A drop of purple is a great detail touch for many small screws, like handguard rail screws, gas block set screws, trigger adjustment screws, and the like. It’s only a little extra effort.
Anyone’s eagerness to use threadlocker has a lot to do with experience, and if you’ve experienced loosened fasteners at inopportune moments, well, that’s what threadlocker helps prevent.
If you are among an increasing number who don’t want to stake bolt carrier key screws, and I am one of you, then red is the ticket for installing this part.
Some competition rifle builders use it on the barrel extension (actually gluing the extension into the upper receiver). I used to do that, but no more.
Reason? It doesn’t make a whit of difference when it comes to accuracy. Believe me, I’ve tested…
Another also competition-oriented use of red is to install a muzzle device using a minimum of torque. The idea is to eliminate the otherwise small amount of muzzle constriction that results from tightening the device in place.
Do you use threadlockers? If so, what color do you use most often? Let us know in the comments below.