Stand in for 6-32 socket cap screws

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  • Patapsco

    Banned
    BANNED!!!
    Jan 16, 2019
    16
    SHCS are usually a tighter class 3A fit rather than 2A which is normal for most other screws. Make sure they're grade 8.
     

    clandestine

    AR-15 Savant
    Oct 13, 2008
    37,031
    Elkton, MD
    Hell it's all good. I believe it was you that told me in another thread for an AR 308 set screw gas block that carbon is like clotting blood. Plus I've got a dimple jig now.

    I was researching for suppressing a 300blk and adjustable block was the first thing I saw along with an article from pew pew tactical discussing the different types of blocks and the merits of set screw and clamp. I ultimately went with clamp style because of equal pressure around the barrel and a supposed better gas seal. NOW I'm thinking that even though that may be true of clamp gb's, the hardware goes through more stress when tightening. Not only do the screws have to clamp, they also have to DRAG somewhat don't they? If thats the case then it's no wonder that they're snapping the way they are.

    My apologies. I misread. I though we were discussing set screw gas blocks.

    That is correct, clamp on cause less barrel stress and they seal better. They should not be trusted for a hard use gun unless they are shielded with a rail.

    For clamp on, the value depends on the grade of bolt and the manufacturer. SLR for instance greatly lowered the torque value on their clamp one down to something like 14IN# due to the bolts breaking.
     

    Patapsco

    Banned
    BANNED!!!
    Jan 16, 2019
    16
    Grade 8's are hard, but brittle, and Chinese grade 8's are trash. Do not buy based on grade 8. Choose by advertised and tested strength (ksi) value.

    I believe there is a minimum elongation in the SAE spec for gr 8. Gr 8 usually means min 130 ksi yield / 150 ksi tensile. Tensile strength and hardness correlate extremely well. Brittleness and elongation to failure negatively correlate perfectly since brittleness is not a real value.
     

    omegared24

    Ultimate Member
    Dec 23, 2011
    4,747
    Ijamsville, MD
    I just stopped by Ace Hardware (Clarks...Ellicott City area) and they have an extensive selection. I picked up some gas block set screws to try out.

    The guy working that section told me that Ace is sourcing from a Chinese manufacturer now so he is unsure of the quality.

    They do carry grade 8 but again, who knows if it's any good.
     

    Boxcab

    MSI EM
    MDS Supporter
    Feb 22, 2007
    7,867
    AA County
    I believe there is a minimum elongation in the SAE spec for gr 8. Gr 8 usually means min 130 ksi yield / 150 ksi tensile. Tensile strength and hardness correlate extremely well. Brittleness and elongation to failure negatively correlate perfectly since brittleness is not a real value.

    With no policing or penalties. SAE is not a .gov agency, nor do they have legal/arresting powers.

    It should be noted as well, that quality fasteners will also have better geometry to reduce stress concentrations. Rolled instead of cut threads, threads that stop before the head, shanked, (Threads should never be put in shear), generous radii under the head, made from forged vs. cut blanks, etc. A fastener needs to be more then just strong. It needs to resist cyclical loading, bending, corrosion, galling, heat cycling, extreme hot and cold (think Titanic) , etc.

    The more expensive bolts will have the grain structure oriented in the proper direction, running continuous along its length and through its geometry changes.

    Strength can be achieved in different ways (Heat treating, metallurgy and geometry). You can have strength without brittleness.

    Just a side note, the FAA does not allow grade 8 fasteners anywhere near aircraft (maybe one or two non-critical locations). Never minding the lack of true quality control, you do not want to use a fastener that as the first sign of overloading, is catastrophic failure. No warning, no indicators, just bang, your dead. Well designed hardware will bend before it breaks. Brittle hardware will not.

    As any engine builder can tell you, they never use grade 8 on anything important... head bolts, bearing caps, and such. They work fine for holding on water pumps, but most racers would not even use them there... because they are brittle, fail without warning and will fail at the most inopportune time.

    Grade 8 hardware has been around for years and is widely used. It is generally used where they are never subjected to really high loads on any one bolt. The loads can be shared over a pattern of fasteners. Use them at a max loading of 30% of break strength and you will be fine. But when weight, and size (the gun fasteners of OP) are limitations, and you will be taxing the limits of the fastener, go with a well engineered one. Just realize that the USGov has put out a number of warnings on the quality of imports, which covers 80% of the Grade 8's.


    Carroll Smith has a great book on the topic. My copy is over 20 years old.

    https://www.amazon.com/Fasteners-Plumbing-Handbook-Motorbooks-Workshop/dp/0879384069

    Stanley Dzik, pricey, but considered a bible on the subject,

    https://www.amazon.com/Aircraft-Hardware-Standards-Manual-Engineering/dp/0879940123

    The FAA has a number of notices warning about fastener problems and the supply chain if you care to dig through them.




    .
     

    Patapsco

    Banned
    BANNED!!!
    Jan 16, 2019
    16
    With no policing or penalties. SAE is not a .gov agency, nor do they have legal/arresting powers.

    It should be noted as well, that quality fasteners will also have better geometry to reduce stress concentrations. Rolled instead of cut threads, threads that stop before the head, shanked, (Threads should never be put in shear), generous radii under the head, made from forged vs. cut blanks, etc. A fastener needs to be more then just strong. It needs to resist cyclical loading, bending, corrosion, galling, heat cycling, extreme hot and cold (think Titanic) , etc.

    The more expensive bolts will have the grain structure oriented in the proper direction, running continuous along its length and through its geometry changes.

    Strength can be achieved in different ways (Heat treating, metallurgy and geometry). You can have strength without brittleness.

    Just a side note, the FAA does not allow grade 8 fasteners anywhere near aircraft (maybe one or two non-critical locations). Never minding the lack of true quality control, you do not want to use a fastener that as the first sign of overloading, is catastrophic failure. No warning, no indicators, just bang, your dead. Well designed hardware will bend before it breaks. Brittle hardware will not.

    As any engine builder can tell you, they never use grade 8 on anything important... head bolts, bearing caps, and such. They work fine for holding on water pumps, but most racers would not even use them there... because they are brittle, fail without warning and will fail at the most inopportune time.

    Grade 8 hardware has been around for years and is widely used. It is generally used where they are never subjected to really high loads on any one bolt. The loads can be shared over a pattern of fasteners. Use them at a max loading of 30% of break strength and you will be fine. But when weight, and size (the gun fasteners of OP) are limitations, and you will be taxing the limits of the fastener, go with a well engineered one. Just realize that the USGov has put out a number of warnings on the quality of imports, which covers 80% of the Grade 8's.


    Carroll Smith has a great book on the topic. My copy is over 20 years old.

    https://www.amazon.com/Fasteners-Plumbing-Handbook-Motorbooks-Workshop/dp/0879384069

    Stanley Dzik, pricey, but considered a bible on the subject,

    https://www.amazon.com/Aircraft-Hardware-Standards-Manual-Engineering/dp/0879940123

    The FAA has a number of notices warning about fastener problems and the supply chain if you care to dig through them.

    .

    Some of what you have said is correct but some is not. When someone refers to a grade 8 fastener, they are referring to a fastener that meets the SAE spec, not a counterfeit. Your comment is the equivalent of me telling someone "Use only unleaded gasoline" and then you saying "No, actually some gasoline marked unleaded is really leaded".

    Wrt thread geometry, this is also controlled by SAE specs which point back to Fed standards, so again, if the thread geometry is correct then the fastener is out of spec. 30% is just a number pulled from nowhere as a substitute for proper analysis.

    If you want to discuss material properties and testing I am more than happy to do so. You are of course correct that not all relevant mat props are captured in strength (or elongation as I mentioned) but the SAE spec does control elongation iirc.
     

    Patapsco

    Banned
    BANNED!!!
    Jan 16, 2019
    16
    I was not able to find the actual drawing for the gas block screw but I think it will be in Gas Tube Assy (9390016) if anyone can find that document. It should have the material spec called out on it.

    Closest I could find was the handguard screw which is an AN501 screw. For reference the material on these only specs a minimum tensile strength (no elongation) and permits any other steel meeting this strength. Not a whole ton of material control there.
     

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