Bore diameter of stripped upper

[StainlessSlide]

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?

[lysander]

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.

[Todd.K]

The stresses in forging can also require a specific order of operation, as well as heat treatment at specific points in the manufacturing.

[StainlessSlide]

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.

[StainlessSlide]

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.

[curious1]

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.

[StainlessSlide]

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.

[lysander]

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.

[lysander]

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.

[StainlessSlide]

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-events/the-impact-of-anodize-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.

[Molon]

It is not uncommon to encounter upper receivers in which the bore does not have a uniform diameter throughout the entire travel of the bolt carrier (note that I didn’t say “entire length”). This is easily demonstrated with pin gauges.

The first demonstration uses a Larue Stealth upper receiver, which is an expensive billet upper receiver. This particular upper receiver has had 2,317 flawlessly cycled rounds fired through it using a Young Manufacturing National Match bolt carrier. The receiver was thoroughly cleaned prior to this demonstration.


https://www.ar15.com/media/mediaFiles/28568/larue_upper_receiver_001_resized-2358537.jpg


A 0.998” pin gauge easily slides into the front end of the upper receiver.

https://www.ar15.com/media/mediaFiles/28568/larue_upper_receiver_example_001_resized-2358532.jpg


The same pin gauge easily slides into the aft end of the upper receiver.


https://www.ar15.com/media/mediaFiles/28568/larue_upper_receiver_example_002-2358545.jpg


However, when I tip the receiver to 90 degrees vertical, instead of the pin gauge sliding all the way through the bolt carrier travel section, it wedges in the mid-section of the upper receiver.


https://www.ar15.com/media/mediaFiles/28568/larue_upper_receiver_example_003-2358533.jpg


The next example uses a brand new SOLG forged upper receiver. A 0.999” pin gauge slides easily into the aft section of the upper receiver.


https://www.ar15.com/media/mediaFiles/28568/solg_upper_receiver_example_001-2358576.jpg


As before, when I tip the receiver to 90 degrees vertical, the pin gauge wedges in the mid-section of the upper receiver.


https://www.ar15.com/media/mediaFiles/28568/solg_upper_receiver_example_002-2358580.jpg


…..

[JediGuy]

Is there a good tool to bring uniformity if it is lacking?

[lysander]

Is there a good tool to bring uniformity if it is lacking?

There really is no need to do anything, so long as a bolt carrier passes freely.

[StainlessSlide]

There really is no need to do anything, so long as a bolt carrier passes freely.

Passes freely under what conditions? Clean, dirty, external debris? In a windy desert with fine sand occasionally blowing, and you've been there for a few days? I do want uppers that pass the .9981 gauge at least, all the way through (or at least to the forward most BCG contact).

Of course the dry lube and some anodizing will wear at the BCG rail contact arcs, but may remain on the rest of the bore. The gauge will measure the bore diameter at the unworn points. So the effective BCG clearance may be greater than the gauge indicates, for a broken-in upper.

I like your explanation that the area under the ejection port is flexing outward when the bore is machined, and so is not cut as deeply. Finger pressure often gets the gauge through, which may mean this area is flexing outward to accommodate.

Thank you Molon, it's good to know that this is widespread. You've probably noticed that this doesn't happen at random, it's usually in the ejection port area. I've also been detecting it on some brands. As I mentioned, some won't pass the .997 GO gauge, it hanging up in this area.

[lysander]

The gauge is cylindrical, the bolt carrier rides on four rails, so if a bolt carrier passes through the diameter at the important points (where the rails ride), then the minimum clearance criteria has been met, with that bolt carrier.

For normal reliability, the largest bolt carrier and smallest bore only have to have a 0.0001" clearance.

[StainlessSlide]

The gauge is cylindrical, the bolt carrier rides on four rails, so if a bolt carrier passes through the diameter at the important points (where the rails ride), then the minimum clearance criteria has been met, with that bolt carrier.

For normal reliability, the largest bolt carrier and smallest bore only have to have a 0.0001" clearance.

Sure, that's the geometry, but do you really think that a tenth clearance is enough? I don't know of any other sliding part intended for field use where a tenth would be enough, e.g. 1911 slide rails. True, the force on the BCG is coaxial with the rails, unlike the 1911 case, but still...

Or maybe you're allowing for the dry lube wear in the contact arcs, which would fairly quickly add .0006 to the minimum clearance. But I don't think this is enough, either.

And I don't see the BCG phosphate thickness in your calculations. I'm not finding a spec for this.

[StainlessSlide]

After a merry chase through the specs I found an expected bolt carrier phosphate thickness of 5 to 10 microns, or about .0002" to .0004". The dimensions we have been using are before finish, so when we factor this in, the BCG could have an interference fit even after the upper passes the .9981 gauge.

The drawing gives MIL-STD-171 Finish 5.3.1.2, which refers to MIL-DTL-16232G for the phosphate layer.