View Full Version : Witness marks in upper from BCG cam pin
Is this typical?
I just noticed so I checked my RRA NMA2 that I've fired 10x as many rounds thru and it is there as well though much less obvious.
About 200 rounds
http://www.danpassaro.com/img/s8/v83/p1632814525-4.jpg
http://www.danpassaro.com/img/s10/v112/p1750820073-4.jpg
http://www.danpassaro.com/img/s9/v87/p1656748000-4.jpg
http://www.danpassaro.com/img/s10/v113/p1712497300-4.jpg
Yes it's completely normal.
Hmmm …… well ok.
I just wonder why have this cutout in the upper for the cam pin if it is going to strike anyway ……
Cutout centered too far forward?
It's settling in. Getting comfy.
What is your experience level with the AR15 FOW?
Just this and my A2. Self researched.
This one I built.
(As far as lube, I had wiped the area to get a better pic)
I had gone thru your single post on Inspection
I am asking because this is in fact fairly normal.
Just this and my A2. Self researched.
This one I built.
(As far as lube, I had wiped the area to get a better pic)
I had gone thru your single post on Inspection
sinlessorrow
08-16-13, 02:12
Normal, it will get to a point and stop. The big cutout is because the cam pin needs a place to go when the BCG is in battery as it cams to the side when the bolt locks in place.
Here is a good discussion related to cam pins
Cam-pin wear as a contributor to bolt failure?
https://www.m4carbine.net/showthread.php?t=134117
sinlessorrow
08-16-13, 09:12
Here is a good discussion related to cam pins
Cam-pin wear as a contributor to bolt failure?
https://www.m4carbine.net/showthread.php?t=134117
Good bolts break lugs generally before failing at the cam pin slot.
Normal, it will get to a point and stop. The big cutout is because the cam pin needs a place to go when the BCG is in battery as it cams to the side when the bolt locks in place.
I know. But either the cutout radius is too small or the center of the cutout is too far forward.
So I really see no reason for it to be 'normal' in the design.
I may as well take the edge of a file and put in a relief to avoid having the parts 'help' each other out as a form of break-in.
steyrman13
08-16-13, 11:02
Is this typical?
I just noticed so I checked my RRA NMA2 that I've fired 10x as many rounds thru and it is there as well though much less obvious.
About 200 rounds
http://www.danpassaro.com/img/s8/v83/p1632814525-4.jpg
http://www.danpassaro.com/img/s10/v112/p1750820073-4.jpg
http://www.danpassaro.com/img/s9/v87/p1656748000-4.jpg
http://www.danpassaro.com/img/s10/v113/p1712497300-4.jpg
Is that a crack in the upper? Right passed the marks you are concerned with?
Is that a crack in the upper? Right passed the marks you are concerned with?
That's the bolt carrier and I was just wondering the same thing based upon the third picture down.
steyrman13
08-16-13, 11:20
That's the bolt carrier and I was just wondering the same thing based upon the third picture down.
Ahh small phone, small pics. Look in first pic too though
Just a thought here and perhaps someone with more knowledge like IG will respond:
Your 20" A2 RRA has a rifle length gas system, which will yield a smoother and slower-operating bolt. The BCM carbine bold is operating more quickly and violently. I'd wager that has something to do with the wear difference you're seeing.
Is that a crack in the upper? Right passed the marks you are concerned with?
grease line
grease line
grease + carbon = lapping compound
grease line
What are your thoughts on the third pic? I was thinking grease on that too but the line seems too distinct and irregular. Bad machining finish perhaps?
Secondary from your now settled issue but still possibly noteworthy.
sinlessorrow
08-16-13, 12:03
I know. But either the cutout radius is too small or the center of the cutout is too far forward.
So I really see no reason for it to be 'normal' in the design.
I may as well take the edge of a file and put in a relief to avoid having the parts 'help' each other out as a form of break-in.
Parts rub and wear until they mate perfectly. Its normal.
What are your thoughts on the third pic? I was thinking grease on that too but the line seems too distinct and irregular. Bad machining finish perhaps?
Secondary from your now settled issue but still possibly noteworthy.
Just the camera angle. That 'line' is where the rail of the BCG rides and pushed it's grease down.
I may as well take the edge of a file and put in a relief to avoid having the parts 'help' each other out as a form of break-in.
Don't do this.
Don't do this.
It's already happening.
But if I take off 0.5-1mm I'll prevent the cam pin from slamming the upper and putting undue stress on the BCG slot, the cam pin of course, potentially the bolt, etc
When I first assembled it all the bolt was pretty snug in the carrier and so I figured that was the cause of the harder-than-the-A2 effort to cycle the bolt but now it seems it was the cam pin striking the upper.
It is better now but only because it has nearly clearanced itself a nice little slot.
So you are filing into the anodizing and anodized hardened aluminum of your upper, exposing the softer aluminum below, because you don't want the gun to function as intended?
So you are filing into the anodizing and anodized hardened aluminum of your upper, exposing the softer aluminum below, because you don't want the gun to function as intended?
No. The cam pin has already done that.
Ridgerunner665
08-16-13, 12:32
If it ain't broke, don't fix it...
That is really stupid, especially after IG comes in and tells you it is normal. Everyone that knows more about these rifles than you tells you not to monkey **** with it and you are going to go ahead and do it anyways.
It's your rifle I guess.
That is really stupid, especially after IG comes in and tells you it is normal. Everyone that knows more about these rifles than you tells you not to monkey **** with it and you are going to go ahead and do it anyways.
It's your rifle I guess.
I haven't done shit. You on the other hand refuse to accept that the cam pin has ALREADY cut into the upper.
It may be widespread, ie: 'normal', but as a mechanical engineer I would never consider an interference issue such as this to be acceptable practice.
This isn't two rough surfaced parts wearing on each other, such as the bcg rail contact with the upper, this was/is an actual interference.
Your commentary is like saying "don't worry about having M4 cutouts in the barrel extension, eventually enough bullets will smooth out that transition"
(which is unlikely due to material differences, just illustrating a comparison)
thopkins22
08-16-13, 13:45
I haven't done shit. You on the other hand refuse to accept that the cam pin has ALREADY cut into the upper.
It may be widespread, ie: 'normal', but as a mechanical engineer I would never consider an interference issue such as this to be acceptable practice.
This isn't two rough surfaced parts wearing on each other, such as the bcg rail contact with the upper, this was/is an actual interference.
Your commentary is like saying "don't worry about having M4 cutouts in the barrel extension, eventually enough bullets will smooth out that transition"
(which is unlikely due to material differences, just illustrating a comparison)
In my job we buy pieces of equipment costing hundreds of thousands of dollars that do the same thing...that is to say they have initial unpredictable wear. These tools are critical to our safety and the success of projects worth millions of dollars. We determined it doesn't matter, that the tools work as advertised, and we rock on. You should do the same with your rifle.
It's not bad engineering unless it creates a problem. It doesn't create a problem, is normal, and doesn't effect the function of the weapon before it happens, while it's happening, nor after it's finished. Bad engineering would be to take something that works and works well, and start screwing with it to improve it simply because you don't like the way it looks or it makes you uncomfortable without ever actually quantifying how it's bad.
Just to give perspective, you buy a shoe that fits your feet but its usually after the shoe is worn for a while that the shoe feels the most comfortable even though the shoe is in my size I do not question why one area seems to get wear marks, or the old glove analogy. I understand your concern but it would seem to make logical sense that parts need to settle in, not all contact points are universal throughout an AR, different builds or completes, parts, different tolerances= different contact points. As long as it runs well and is not a major issue I would just keep on moving. All of my firearms have signs of wear or being broken in after actual usage. My DD shows some of the same marks as yours does.
In my job we buy pieces of equipment costing hundreds of thousands of dollars that do the same thing...that is to say they have initial unpredictable wear. These tools are critical to our safety and the success of projects worth millions of dollars. We determined it doesn't matter, that the tools work as advertised, and we rock on. You should do the same with your rifle.
It's not bad engineering unless it creates a problem. It doesn't create a problem, is normal, and doesn't effect the function of the weapon before it happens, while it's happening, nor after it's finished. Bad engineering would be to take something that works and works well, and start screwing with it to improve it simply because you don't like the way it looks or it makes you uncomfortable without ever actually quantifying how it's bad.
I wouldn't be changing anything.
Consider:
It is only about 200 rounds thru the sytem and it is still contacting the upper based on adding some grease and seeing it wiped away, and a 'step' in effort when pulling on the charging handle, therefore its obvious that it is still clearancing itself (and will do so to a certain point).
I was simply considering taking that eventual additional clearancing, since it is going to happen anyway, to that certain point manually in order to avoid having it done with the cam pin directly. Everything that is connected to the cam pin is participating in the process, I don't like that.
thopkins22
08-16-13, 14:36
I wouldn't be changing anything.
Consider:
It is only about 200 rounds thru the sytem and it is still contacting the upper based on adding some grease and seeing it wiped away, and a 'step' in effort when pulling on the charging handle, therefore its obvious that it is still clearancing itself (and will do so to a certain point).
I was simply considering taking that eventual additional clearancing, since it is going to happen anyway, to that certain point manually in order to avoid having it done with the cam pin directly. Everything that is connected to the cam pin is participating in the process, I don't like that.
I hear you...but I'm saying that it's going to be difficult to know when to stop, and that everything that's participating in the process is doing it against relatively soft metal...I truly think you'll be going down a dangerous road when there aren't any demonstrable downsides to leaving it be.
I hear you...but I'm saying that it's going to be difficult to know when to stop, and that everything that's participating in the process is doing it against relatively soft metal...I truly think you'll be going down a dangerous road when there aren't any demonstrable downsides to leaving it be.
Well I certainly wasn't considering going full retard with a dremel or anything lol.
Just a swipe or two, literally, with a small file, test and see, perhaps another swipe-and-done type of thing.
Powered by vBulletin® Version 4.2.5 Copyright © 2024 vBulletin Solutions, Inc. All rights reserved.