I have some questions regarding thread lockers. I was taught years ago every screw gets blue loctite. I have since evolved and began reading the instructions... Some manufacturers say just use torque, some say to use blue loctite some say use the vibra tite products. Let's talk scope cap screws and Pistol red dot mounting. Let's keep in mind duty related firearms that will sit in cars for miles and miles of good vibrations prior to being deployed.
Is it the thread of the screw that dictates the need of thread locking compound? Fine VS Coarse? Is it the amount of torque the head of the screw can take when removing. If you use thread locker and torque it in at 12 in. LB., it's going to take more than that to remove it and defeat the thread locker.
Thanks, I really appreciate learning from this group. There are so many extremely talented people here,
@Terry Cross @LongRifles Inc. @MikeRTacOps and many more.
I spent a great deal of time researching this very subject 15 or so years ago. That rabbit hole eventually leads to an introduction to a guy named Jerome Sailing up in the Pac North West. His job at the time was to do nothing more than design fasteners used in the airplane industry.
Screws and bolts work because they are a spring. The tension (tensile) is created because the helical feature on the screw mates with the opposing side and the fastener begins to elongate (grow in length) as the rotation increases. This is because the head of the fastener serves as the point where all the tensile load originates from.
Where things start to fall apart:
Were working with materials as an end-user that we don't always understand or even know what they are. Steels vary and so does aluminum. 7075 AL behaves much, much differently than 6061. A steel screw can easily deform the pocket that the head registers off of and this will mess with a given torque value. This is amplified by screws that have a small surface area under the head, such as the typical 8-40 screw used to attach an M700 base.
Next is the amount of torque applied to the fastener. The real goal here is not to torque the thing to a value of "X" but to turn it far enough for the fastener to elongate and apply the tensile load without going too far and causing it to yield or fail. It doesn't take much either.
Case in point: ARP (Automotive Racing Products) makes fasteners for race car stuff. Engines, etc... Rod bolts are a critical component to any engine. For a 2000 series bolt to apply the correct load it needs to grow .006" to .0065" in overall length when tightened. Less than 2 sheets of notebook paper. That's a fastener that sees an absurd about of abuse for millions of duty cycles. Now compare that to a 6-48 base screw.
Last, the pitch. Thread pitches are important because it has to do with the materials being used. Aluminum and plastics are weaker than steel. So, the thread pitch in those types of alloys are typically coarser. This is because there is a greater difference between the root diameter and the major diameter. The cross-section of the thread increases which spreads the loading forces over a greater surface area. The quality of the surface finish on the fastener hear matters big time as well. A shitty thread will chew on an opposing thread made of AL or plastic. It doesnt take long for the screw to erode the flanks of the internal threads and weaken it.
The bad side of this is that the root diameter of the fastener is also affected. A 6-32 screw for example is probably one of the worst as the root diameter is quite small. This means you have to be more careful as it's an easy screw to ruin or break. It's one reason why 6-32 thread taps are notorious for failing at a rate that exceeds a lot of others. It's just a shit design because the tool lacks the cross-section to support it properly as it does the work. The screws suffer the same condition. On that note know that there's a variety of ways to create a thread. Forming and rolling is always prefered when possible as it has higher material density and that makes for a stronger part. Cutting a thread is probably the weakest of all of them.
Thread lockers:
Thread lockers are basically sugar. Under pressure, the fluid transforms into a crystalline-like structure that will grow, fill a void, and exert some pressure. That pressure is what makes them stay put and why they require more effort to remove. It's not glue or adhesive. It's important to understand that. The bad side of LT is that it can break down if exposed to heat, excessive strain, or certain chemicals. It's why it is never used on things like head bolts, studs, or connecting rod bolts on an engine. Those depend on preloading the fastener during assembly.
All this is great, but what does it mean?
If you have 6-48 or 8-40 base screws, a dab of blue is not a bad thing. IF your base lacks any kind of lug feature that transfers recoil directly to the base it is a good idea to periodically check your screws AND I would recommend replacing them once a year if its on a light gun chambered in something big and nasty. (EDIT: Assuming this is a gun you shoot a lot, a safe queen is likely going to be fine although it never hurts to PM your stuff) I have personally witnessed scopes and bases flying off of rifles at events because of recoil-induced failure. None of those guns had a keyed base that interfaced with the receiver. They merely flopped the base to the top of the receiver and relied on the screws to hold everything together. Remember what I said: Screws work by acting as a spring in a tensile type load. During recoil, the load changes, and its also being applied at a right angle to the screw. Shearing forces are a completely different animal and it's really not how a puny 6-48 should be used when heavy optics are being attached to it. Adding the mass of rings and the riflescope works against you because it is now the job of those little guys to accelerate and decelerate the optic and all the related parts n pieces every time the gun goes bang. Rings are usually different because they work in tension. There are typically no shear loads applied or they are quite small. A typical STENAG base/ring setup might be the exception but those screws are typically pretty big and have plenty of cross-section to tolerate the loads being applied. Those rings are also often lugged so that they'll key into the base itself.
Base screws: The math that Jerome and I did suggests that the material we used at Nesika for guard screws could not exceed 40 inch-pounds. That was 303 stainless. Carbon steel (8620 or 1018 is very common) will tolerate a higher load. 50-60 inch-pounds seems to be the industry standard here. I would not recommend Loctite just because it's going to eventually lead to problems. These screws only work in tension as they should. Recoil is handled by the lug on the receiver and the well in the stock.
We can run down the whole list of possible parts, but this covers the big stuff.
Hope it helped.
