Page 1 of 2 12 LastLast
Results 1 to 10 of 17

Thread: Bore diameter of stripped upper

  1. #1
    Join Date
    Jul 2008
    Posts
    131
    Feedback Score
    0

    Bore diameter of stripped upper

    I recently purchased a stripped upper that did not allow a Colt bcg free movement. The bcg dragged just a little on part of the cycle, even with no bolt.

    This sparked an interest in the upper bore diameter. An old print specified that the bore must pass a .998 diameter, 3” long plug gage (after the finish is applied). I had one made by Vermont Gage, and I found that some commercial uppers wouldn’t pass it.

    Some wouldn’t even pass a .997 gage. The gage stuck where the ejection port begins (when moving the gage forward), and where it ends (when moving the gage aft).

    It appears that the strut underneath the ejection port is sometimes displaced inwards, narrowing the bore at this point. Pulling outwards on this strut allows the .997 gage to move through.

    By the way, the spec on the bcg rails is .9945+-.001

    Does anyone have any thoughts?

  2. #2
    Join Date
    Mar 2015
    Posts
    1,725
    Feedback Score
    0
    The diameter of the bore in the upper receiver is specified to be 1.000" +.002"/-.000 for the first 1.110" (the threaded end) and 1.000" +.006/-.000 for the remainder, prior to anodizing. The anodizing is required to be .0008" to .0012" thick, reducing the bore to 0.9988", then a coat of solid film 0.0002" to 0.0003" thick is applied, bringing the final minimum diameter down to 0.9982". (The maximum diameter is 1.0008" for the first 1.11" and 1.0048" for the remainder.)

    Those dimensional requirements have not changed since at least 1972.

    By the way, the gauge dimension is 0.9981 +.0001" with a gauge length of 2.880". This is not only to ensure the diameter meets the minimum, but the bore straightness.

    If the web below the ejection port is bowed inward, it indicates the order of machining of the upper was incorrect. The 1.000" bore should be bored as the first operation, when there is the maximum material, if the bore is machined after the ejection port and bottom slot, then the web is unsupported and will deflect outward, rather than being cut.

  3. #3
    Join Date
    Jul 2006
    Location
    Oregon
    Posts
    4,629
    Feedback Score
    2 (100%)
    The stresses in forging can also require a specific order of operation, as well as heat treatment at specific points in the manufacturing.

  4. #4
    Join Date
    Jul 2008
    Posts
    131
    Feedback Score
    0
    Quote Originally Posted by lysander View Post
    The diameter of the bore in the upper receiver is specified to be 1.000" +.002"/-.000 for the first 1.110" (the threaded end) and 1.000" +.006/-.000 for the remainder, prior to anodizing. The anodizing is required to be .0008" to .0012" thick, reducing the bore to 0.9988", then a coat of solid film 0.0002" to 0.0003" thick is applied, bringing the final minimum diameter down to 0.9982". (The maximum diameter is 1.0008" for the first 1.11" and 1.0048" for the remainder.)

    Those dimensional requirements have not changed since at least 1972.

    By the way, the gauge dimension is 0.9981 +.0001" with a gauge length of 2.880". This is not only to ensure the diameter meets the minimum, but the bore straightness.

    If the web below the ejection port is bowed inward, it indicates the order of machining of the upper was incorrect. The 1.000" bore should be bored as the first operation, when there is the maximum material, if the bore is machined after the ejection port and bottom slot, then the web is unsupported and will deflect outward, rather than being cut.
    Lysander, you are subtracting only one thickness of anodizing from the diameter, when you should be subtracting two thicknesses.

    There is also the ambiguity that the thickness of an anodize coating is not the same as the growth of the surface. So do they mean the surface grows by .0008 to .0012, or that the aluminum oxide layer is that thick (which may be difficult to gauge)?

    Conventional wisdom says that the surface growth is about half the oxide thickness, which would be some justification for the gage diameter. But this must depend on porosity and other factors.

    Also the BCG rail spec has changed, it used to be .9945-.001, without the plus tolerance.

    Because of these ambiguities I am using a Class ZZ go gauge at .9985, so then a maximum diameter of .9987.
    Last edited by StainlessSlide; 03-05-23 at 13:23.

  5. #5
    Join Date
    Jul 2008
    Posts
    131
    Feedback Score
    0
    By the way, the uppers that don’t pass the .997 gauge claim to be “M4 Contract Uppers”. But some that are so described pass fine. Of course, a bore on the high end would pass even with some bowing.

    The machining order is the first explanation that comes to mind. I haven’t yet tested Colt uppers. Surely these would have the correct order. I’ll post results when I can.
    Last edited by StainlessSlide; 03-05-23 at 13:34.

  6. #6
    Join Date
    Jul 2006
    Location
    Martinez GA
    Posts
    73
    Feedback Score
    0
    The old drawings say all dimensions apply before finish. The gauge note is on the drawing where the unfinished dimensions are and it appears you are trying to drop it through a finished upper that is closed up a couple thousandths or more with the finishes. The .998" gauge dimension I believe is for checking the unfinished upper.

    I would think if it was to be gauged after finish it would be noted that in the notes or at the gauging note.
    Last edited by curious1; 03-05-23 at 15:30.

  7. #7
    Join Date
    Jul 2008
    Posts
    131
    Feedback Score
    0
    Quote Originally Posted by curious1 View Post
    The old drawings say all dimensions apply before finish. The gauge note is on the drawing where the unfinished dimensions are and it appears you are trying to drop it through a finished upper that is closed up a couple thousandths or more with the finishes. The .998" gauge dimension I believe is for checking the unfinished upper.

    I would think if it was to be gauged after finish it would be noted that in the notes or at the gauging note.
    This may well be true, I hadn't thought of that. Since the minimum diameter is 1.000. the minus .002 on the gauge could be checking for bend.

    But in that case the arithmetic says that an upper that barely passed the .998 gage before finishing would barely pass a .9962 after maximum thickness finishing, even using the half-and-half rule of thumb for anodizing buildup. This would lead to a minimum bcg clearance of just .0012, using the new bcg spec.

    You raise an excellent point, but there is ambiguity. As a (sandy ) desert rat, I'll stick with my .9985 go gage after finishing. My upper that requires gentle pushing to pass the .997 gauge is dragging on my Colt BCG.
    Last edited by StainlessSlide; 03-05-23 at 16:56.

  8. #8
    Join Date
    Mar 2015
    Posts
    1,725
    Feedback Score
    0
    Quote Originally Posted by StainlessSlide View Post
    Lysander, you are subtracting only one thickness of anodizing from the diameter, when you should be subtracting two thicknesses.

    There is also the ambiguity that the thickness of an anodize coating is not the same as the growth of the surface. So do they mean the surface grows by .0008 to .0012, or that the aluminum oxide layer is that thick (which may be difficult to gauge)?

    Conventional wisdom says that the surface growth is about half the oxide thickness, which would be some justification for the gage diameter. But this must depend on porosity and other factors.

    Also the BCG rail spec has changed, it used to be .9945-.001, without the plus tolerance.

    Because of these ambiguities I am using a Class ZZ go gauge at .9985, so then a maximum diameter of .9987.
    Anodizing both penetrates the substrate and adds to the surface. Half the anodizing thickness is on the surface and half the thickness is below the original surface. So, if the required anodizing thickness is specified at 0.0008" thick the surface will increase 0.0004". Holes will close up by one anodizing thickness, half on each side.

    The aluminum oxide layer is not conductive, it is not that hard to measure the thickness of the anodizing. It is the non-conductive layer that is measured, and defined as the thickness of the anodizing.

    I do not know what revision you are referring to but Rev U dated 21 April 2011 gives the dimension as 0.9945 +/- .001", it has changed from earlier revisions. (EDIT) In any case, a bolt carrier with diameter of 0.9955" would still pass through a bore 0.9982" in diameter.
    Last edited by lysander; 03-06-23 at 11:49.

  9. #9
    Join Date
    Mar 2015
    Posts
    1,725
    Feedback Score
    0
    Quote Originally Posted by curious1 View Post
    The old drawings say all dimensions apply before finish. The gauge note is on the drawing where the unfinished dimensions are and it appears you are trying to drop it through a finished upper that is closed up a couple thousandths or more with the finishes. The .998" gauge dimension I believe is for checking the unfinished upper.

    I would think if it was to be gauged after finish it would be noted that in the notes or at the gauging note.
    It is important to have all the drawings. There is the Quality Assurance Provisions Sheet (QAP) for every drawing that tells you what gauges are to be used to gauge what dimensions and when they are to be gauged. QAP 9349063 (for the A2 upper) and QAP 12972670 (for the M4 upper) state that the completed upper shall have major characteristic items 108 and 109 (the 1.000" bores before and after the 1.11" point) gauged with gage drawing # 8443767. Since this inspection is to be performed on an upper ready for acceptance by the purchaser, the gauge must pass a bore with both the anodizing and the solid film lubricant applied.

    As the tolerance stack has shown that the 1.000" bore in the maximum material condition (MMC) is 0.9982", a 0.9981" gage will pass through, provided the bore is straight.
    Last edited by lysander; 03-06-23 at 12:13.

  10. #10
    Join Date
    Jul 2008
    Posts
    131
    Feedback Score
    0
    Thank you, Lysander, for clearing this up. It does indeed appear that they are assuming the 50/50 rule of thumb for type III anodizing buildup, and they are gaging after finishing.

    They seem to be going for a minimum BCG/upper clearance of .0027, using the later BCG rail diameter of .9945+-.001. The max clearance is wayyy larger at .0113

    I saw the older .9945-.001 rail diam. spec. on the drawing rev K dated 2/17/2000. Naturally this would increase the minimum clearance by .001. I’m splitting the difference with my .9985 gauge.

    Do you have a source for the gage drawings and the QAPs? I wonder why the upper drawing soecified a .985 gauge, while the gauge drawing is .9981. Maybe the upper drawing refers to a Class ZZ gauge, max. dia. .9982, the same as the gauge spec.that you mentioned.

    By the way, it appears that 45% buildup 55% penetration is a more realistic assumption for type III - https://www.anoplate.com/news-and-ev...on-dimensions/ - but this only affects the bore diameter calculation by a little more than .0001 on the decreased material side. Still, this is the level of accuracy we were considering.

    If these anodizing assumptions depend on the quality of the type III process, they should be given explicitly.

    Of course anodizing thickness could be easily measured with an eddy current technique. I’ve done this a few times, surprising that it slipped my mind.
    Last edited by StainlessSlide; 03-07-23 at 07:18.

Page 1 of 2 12 LastLast

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •