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Thread: On loose carrier key screws

  1. #21
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    Nice. We will be having a conversation about these new screws.

    Quote Originally Posted by Ned Christiansen View Post
    In this ’08 post I talked about how carrier key screws could be optimized:
    https://www.m4carbine.net/showthread...ier+key+screws

    I have sold some in small quantities over the years but now there is a new version that I have been providing to the industry at per-thousand pricing since 2016. They will start getting noticed pretty soon I think, and I think it’s time to explain them and let it be known that I’m ready to make them available to the individual and armorers, at less than what is mentioned in the old post.

    In testing, the new OCKS (Optimized Carrier Key Screw) offers a huge increase in breaking torque….. huge. Where the breaking torque spec is 55-100 inch pounds, I have had readings as high as 132 inch pounds breaking torque with screws installed at 50 inch-pounds, the bottom of the 50-58 inch-pounds torque spec. Nothing else I’ve tested has come close to this.

    A good many manufacturers continue to not get this simple requirement right. Some do a poor job of staking, some still don’t do it at all, some I believe still deny the need for it. Some don’t do it because they think it detracts from the “looks” of their BCG. Well those in particular are not, I’m pretty sure, getting much business from M4C-isti, but as we all know, that is still part of the market.

    Over the years I have worked with some manufacturers on several facets of manufacturing, and provided tooling to some—bench-top production stakers and hyrdraulic stakers. Some are doing great on staking without any help from me, I’ve helped some improve, and others have been unreachable. Out of frustration at continued loose-screw issues with many brands I decided to again explore the OCKS. Early and recent testing have shown that even with marginal staking, it offers a big improvement. If they won’t do the staking right, maybe they’ll at least stop using the offshore, roundy-headed, fake-knurl screws, was my hope.

    I’ve always been baffled by the ubiquity of the offshore screws. Given the price of cap screws and the many long-established American screw manufacturers of good repute, I have always wondered, “how much are they saving, maybe five cents a gun….? Ten cents? How the F can it be worth it when they surely are getting complaints about them coming loose, and warranty returns?” It’s one of those frustrating unanswered questions about the industry. My hope was to convince manufacturers that it would be a good idea, for less than a quarter per gun, to virtually eliminate this issue and eliminate warranty expenses, and show the educated segment of the AR-buying market that they are interested in making a good product.

    I have had some success at this (currently filling an order for 25,000), mostly with outfits that were already doing it right, and want to do it better, and want to be recognized for it. And the crusade continues.

    1. The Optimized Carrier Key Screw. Sharp, deep, effective knurling all the way to the top of the head, plus 12 splines. Measured over the splines, the diameter takes up any clearance space between screw head diameter and counterbore diameter. The splines essentially are pockets for carrier key metal to flow into when staking takes place, giving a gear-tooth-like engagement.


    2 Three OCKS; and, on the left is what in my experience is by far the most commonly used carrier key screw outside the high-end manufacturers. Maybe, maybe not a bad screw in other applications but note the indistinct, ersatz knurling. The top edges of the head are devoid of knurling and have a very rounded, polished-looking surface, making it nearly impossible for even a good staking job to get a grab on the screw head. Of the many, many (I want to say hundreds but no actual count) AR’s I’ve seen with carrier key screws that have come loose, the vast majority had this Asia-sourced brand of screw.


    3. What they look like staked. Notice how the staked metal has flowed around and into the splines giving positive, even visually-confirmed, metal-to-metal engagement. It’s actually even better than it looks since this carrier key has a pretty generous chamfering of the counterbores. But—carrier key metal has formed into the splines from the sides and even downward from the top. Secondary advantage, it’s easy to see at a glance: this one’s got the good screws and I can depend on them not coming loose.



    4. Carrier key sectioned without having first removed the screws, so that staked material is not wiped or deformed. Flow and engagement with the splines can clearly be seen. Metal has moved into the splines from the side and over the top. Also, engagement with the knurling is positive.



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  2. #22
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    Gunz, there should be a sample on your desk somewhere.

    Might be a while before these are on my site but they are available.

    These have a 9/64" socket. They will be available with the mil-spec 1/8" socket if there's a call for it. I kinda hope there's not as there is nothing to gain by it, but one mfgr testing it may be required to use it that way. It's my belief that the splines would not or might not technically violate "mispec". I guess that'll be up to the people who deal with that kind of thing on a weekly basis.

    I also believe that the milspec screw calls out a 1/8" socket only because at the time of the system's design, the 1936 standards were still in use and the fastener industry had not yet updated cap screw standards to what would become known as "1960 Series", still the industry standard. One notable change in this revamping and improvement to fasters was a change in the 8-32 SHCS from a 1/8" socket to a 9/64 socket.

  3. #23
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    The things you run into when you section parts! Ever heard about a gun that could not be made to work 100% and somebody determined it was "just a bad gun"? Tried their best to figure out what was wrong, could not find the problem and decided there was "just something about it"? Well it's always "something" but sometimes it's something you just assume it can't be. Here's an interesting one I ran into on a key while sectioning it for the staking study.

    The carrier key where it telescopes over the gas tube-- that hole is held to pretty tight tolerances, as it should. It needs to freely slip over the gas tube but at the same time be somewhat of a gas seal. Here we see that the reamer did not follow the hole. Having reamed I dunno, a bazillion holes in my toolmaking career, I can tell you that usually, under most conditions, a reamer works as intended and does pretty reliably follow the hole drilled for it. This time, for some reason, it didn't; it wandered off pretty radically leaving part of the drilled hole unreamed, meaning this part of the gas seal is not a seal. Gas can leak around the swell on the end of the gas tube and back to the relieved part of the GT's diameter, and out the front. The gas tube is show "about" where it would be when the system is in battery.

    Enough to short stroke the gun? Probably not, most of the time..... but maybe, if every other aspect of the system is not 100% up to snuff, in other words if, say, the gas rings are worn and everything is filthy (which yeah, it's gonna get filthier faster with this leak), this could be enough to put it outside the window.

    Note also the reamed part probably does not meet the finish requirement. The reamer was dragging chips around causing grooves although there doesn't appear to be anything really hanging "into" the hole that would cause immediate gas tube wear. I've seen them much rougher inside-- take a rough one, chrome it, and you have just created the perfect tool for reducing gas tube diameter. Throw in some misalignment like this and it's harder to get a perfect gas tube / carrier key relationship.

    This one was before I was tracking carrier key sources in testing so I'm not sure who made it; anyway it was perfect in every other aspect. This would be a hard imperfection to catch in manufacturing. I suspect either the key shifted in its fixture or the reamer upon entering the drilled hole, encountered a chip that got it started off center, maybe coupled with a reamer at the end of its life.

  4. #24
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    Brilliant, simple and effective.
    Kudos for the remarkable and innovative design.

  5. #25
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    Bravo Sir, bravo!

  6. #26
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    Nice to see such thought put into this portion of the rifle. Great idea!

  7. #27
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    Very interesting idea! Ned, is there any way to accurately test the removal torque vs a standard milspec screw?

  8. #28
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    Sweet...I must have some.

    Ned you mention it "Might be a while before these are on my site but they are available"

    What give you call/PM? Do you have a price? Bulk price perhaps?
    Gettin' down innagrass.
    Let's Go Brandon!

  9. #29
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    Very cool! Thanks for taking the time to post this also

  10. #30
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    Quote Originally Posted by Ned Christiansen View Post
    Gunz, there should be a sample on your desk somewhere.

    Might be a while before these are on my site but they are available.

    These have a 9/64" socket. They will be available with the mil-spec 1/8" socket if there's a call for it. I kinda hope there's not as there is nothing to gain by it, but one mfgr testing it may be required to use it that way. It's my belief that the splines would not or might not technically violate "mispec". I guess that'll be up to the people who deal with that kind of thing on a weekly basis.

    I also believe that the milspec screw calls out a 1/8" socket only because at the time of the system's design, the 1936 standards were still in use and the fastener industry had not yet updated cap screw standards to what would become known as "1960 Series", still the industry standard. One notable change in this revamping and improvement to fasters was a change in the 8-32 SHCS from a 1/8" socket to a 9/64 socket.
    There is no such thing as a "Military Spec" 8-32-UNC-3A, alloy steel cap screw, undrilled head, with phosphate coating.

    The closest thing to a Military specification for a "8-32-UNC-3A, alloy steel cap screw, undrilled head, with phosphate coating" was MS35459, but back in 1962 the #6, #8, #10 and 1/4 inch sizes were deleted from the specifications, then in 1964 the both specifications were declared "inactive". The military currently uses standard cap screws that are made to Aerospace Industry Standard NAS1351 or NAS1352, a commercial standard. According to ASME B18.3, and NAS1351/NAS1352, 9/64 is the standard key size, as are the few remaining active military standard cap screws (cadmium plated or drilled CRES screws). The obsolete MS35456 and MS35455 did call out a 1/8 inch socket.

    That is why the military carrier key screws do not have an "MS" number, but an M16/M4 specific part number: 8448508.

    So, while everyone assumes they are standard ASTM A574, 2520 pound tensile proof load, 8-32-UNC-3A, cap screws, or NAS1352-08-4 screws they aren't. Until you can see the requirements set out on DWG #8448508, you just don't know the actual proof load is supposed to be.

    Your splines might not cause the screw heads to exceed the dimensional tolerances of ASME B18.3, or NAS1351 or NAS1352, you might check, however, those castellations probably will.

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