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Thread: Mil-Spec Gas Tube Roll Pin

  1. #21
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    I know we stickied the 9310 Bolt thread in Tech, but if this discussion develops further, I’d love to see this as “easy to find.” Great info if we can confirm “generics” that work.

  2. #22
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    Quote Originally Posted by titsonritz View Post
    carrier key screws
    Actually, no.

    The gas key screws are not standard MS cap head, socket screws.

    The screw, per drawing number 8448508 is a 1/8 socket, 8-32 UNC-2A.

    All military standard cap head socket screws in 8-32 UNC use a 9/64 hex wrench.

    Screw, 8448508 has a thicker head wall thickness, and has more specific strength requirements. There used to be a specification for such thick walled cap screws, but it was cancelled a long time ago.
    Last edited by lysander; 04-12-21 at 11:07.

  3. #23
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    You know, I find it a wee bit amusing that people get all excited about the addition of a tenth of a percent (0.10 %) of molybdenum to the steel used in the bolt, yet blithely glossy over the steel composition of other parts.

    Commercial cap screws made to ASTM A574 or NAS 1352 standards are made from one of several steels, AISI 4137, 4142, 4145, 4340, 8740, 5137, or 51B37M.

    The gas key cap screws (8448508) are specified to be made from AISI 4037.

    So, what it's all steel with the same yield strength, right?

    Well, the strength and hardness are the same but their are other properties that might have a bearing on the behavior of the screws and explain why Colt's was so specific in material choice. The elongation at break is 5%, and 36% reduction in area for 4037, the other alloys have values in the 17% elongation and 53% reduction. This means that 4037 is a very "stiff" alloy, in takes a lot of force to stretch the material a small amount

    Also, the required torque on these screws in 58 in-lbs, which means the stress in the threaded portion of the screw is 157,800 psi, that is very close to the yield strength of the material. So, additional loads, say from the gas pressure, might push the stress into the region of plastic yield, which means the screw will stretch and stay stretched. AISI 4037, with its low elongation, will stretch and set to a smaller percent that the other steels.

    Oh, but is it not also the case with Carp 158/AISI 9310? No. The mechanical properties of Carpenter 158 and 9310 are remarkably similar, elongation being 13% and 15% and reduction being 49% and 52% respectively. Impact properties are also very similar.

    So, what's on my bolt carrier? Screws made to ASTM A574, simply because no third party sells real 8448508 screws, that I know of, even some of the one advertised as "Colt's" had 9/64 socket wells. If the screw has a 9/64 socket well, it is not the correct part, it should be 1/8 inch.

  4. #24
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    I purchased spare gas keys from BCM. They came with the screws. When buying gas key screws separately I go with the OCKS screws.
    Last edited by prepare; 04-13-21 at 04:06.
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  5. #25
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    Quote Originally Posted by prepare View Post
    I purchased spare gas keys from BCM. They came with the screws. When buying gas key screws separately I go with the OCKS screws.
    Both of these use ASTM A574 screws.

  6. #26
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    Quote Originally Posted by lysander View Post
    Both of these use ASTM A574 screws.
    I appreciate the technical breakdown on the screws.

    Why are real 8448508 screws unavailable?
    Last edited by prepare; 04-13-21 at 18:40.
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  7. #27
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    Quote Originally Posted by prepare View Post
    I appreciate the technical breakdown on the screws.

    Why are real 8448508 screws unavailable?
    I don't know that they aren't.

    Note the difference in size of the hex well:


  8. #28
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    Quote Originally Posted by titsonritz View Post
    1. Gas Tube- 5/64 x 5/16 Part No. 93740A622
    2. Trigger Guard- 1/8 x 5/8 Part No. 98296A879
    3. Forward Assist- 3/32 x 5/8 Part No. 98296A849
    4. Ejector Pin- 1/16 x 3/8 Part No. 98296A023
    5. Bolt Catch- 3/32 x 7/16 Part No. 98296A837
    6. Socket Head Cap Screw (Gas Key) 8-32 x ¼ Part No. 91251A190
    7. Pointed Set Screw (Gas Block) 10-32x 3/16 Part. No. 90289A337

    Thank You!

  9. #29
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    Quote Originally Posted by lysander View Post
    I don't know that they aren't.

    Note the difference in size of the hex well:

    They must be difficult to obtain. Fortunately there are a couple equivalents and perhaps the OCKS is even a better choice.
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  10. #30
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    The TM doesn't call for any sealant to be used when installing the gas key.
    There doesn't seem to be an industry standard or SOP in regards to using a sealant?
    Last edited by prepare; 04-17-21 at 16:54.
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