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Thread: Crack in my new lower (pics)

  1. #1
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    Crack in my new lower (pics)

    Hello all,

    I'm not new to shooting but am definitely new to building AR's... this is the result of my first attempt, and I think I have a problem. I need some advice.

    I just assembled a PSA LPK into a brand-new Aero Precision lower, and after I finished I noticed what appears to be a small crack along the edge of the forward "ear" that holds the roll pin for the bolt catch. I know this was not here before assembly as I inspected the lower very thoroughly at the store (Surplus Ammo & Arms in Lakewood WA). When I put the lower together, I secured it in a magazine well vise block and used a standard pin punch along with a small hammer to install the roll pin. I initially tried using the "vise grip" method but wasn't having luck getting the pin started, so I changed to using the hammer and punch. I have a factory-assembled BCM that I used for comparison throughout assembly and also consulted a pictorial instruction set I found on Arfcom.

    Both ends of the roll pin looked the same to me, with neither more tapered than the other. I drove the pin in starting from the rear of the receiver. I'm not sure if that makes any difference but I've seen photos of it being done both ways. The pin was hard to get started, went easily through the bolt catch, and then became hard to drive again as it went into the opposite "ear" of the lower receiver. I thought I did fine as I put no big scratches in the receiver and the bolt catch functions properly. And then I saw the crack.

    I know I must have done something wrong. Do you think this is a superficial "finish" crack, or is it going to propagate until the bolt catch "ear" eventually breaks off? Is there anything I can/should do about this? What did I do wrong, and how can I avoid this on future builds? Thank you all for your time and help.

  2. #2
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    If you're going to work on or build your own, I would recommend purchasing yourself a set of roll pin starter punches.

    http://www.brownells.com/.aspx/pid=2...TARTER-PUNCHES

  3. #3
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    Is it a crack in the metal or just the outer anodizing? Hard to tell from the pics. Can you flake it off with your fingernail?
    Next time you hear the word "FREE", try replacing it with "TAXPAYER-FUNDED".

    "War is a terrible thing, but there are things much worse than war. A man who has nothing for which he is willing to fight, nothing he cares about more than his personal safety, is a miserable creature who has no chance of being free unless made and kept by the operation and sacrifice of better men than himself." -Col. Bruce E. Wallace

  4. #4
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    Quote Originally Posted by DJ_Skinny View Post
    Is it a crack in the metal or just the outer anodizing? Hard to tell from the pics. Can you flake it off with your fingernail?
    Looks like damaged aluminum to me. Also damage around the hole on the rear side in the 3rd pic.

  5. #5
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    A tiny amount of lube doesn't hurt either.

  6. #6
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    And just a note, my buddy recently used a PSA LPK with a RRA lower (I don't personally recommend either but that's neither here nor there). He could not get his bolt catch pin started using two different LPKs. He managed to get his FFL to swap lowers, and he got it to work. Take that anecdote with a grain of salt, but it makes you wonder if there has been an issue with PSA LPKs of late. Likely not, but it's a bit curious to see this happen to two people within a week apart of each other.

  7. #7
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    Quote Originally Posted by Mule81 View Post
    Hello all,

    I'm not new to shooting but am definitely new to building AR's... this is the result of my first attempt, and I think I have a problem. I need some advice.

    I just assembled a PSA LPK into a brand-new Aero Precision lower, and after I finished I noticed what appears to be a small crack along the edge of the forward "ear" that holds the roll pin for the bolt catch. I know this was not here before assembly as I inspected the lower very thoroughly at the store (Surplus Ammo & Arms in Lakewood WA). When I put the lower together, I secured it in a magazine well vise block and used a standard pin punch along with a small hammer to install the roll pin. I initially tried using the "vise grip" method but wasn't having luck getting the pin started, so I changed to using the hammer and punch. I have a factory-assembled BCM that I used for comparison throughout assembly and also consulted a pictorial instruction set I found on Arfcom.

    Both ends of the roll pin looked the same to me, with neither more tapered than the other. I drove the pin in starting from the rear of the receiver. I'm not sure if that makes any difference but I've seen photos of it being done both ways. The pin was hard to get started, went easily through the bolt catch, and then became hard to drive again as it went into the opposite "ear" of the lower receiver. I thought I did fine as I put no big scratches in the receiver and the bolt catch functions properly. And then I saw the crack.

    I know I must have done something wrong. Do you think this is a superficial "finish" crack, or is it going to propagate until the bolt catch "ear" eventually breaks off? Is there anything I can/should do about this? What did I do wrong, and how can I avoid this on future builds? Thank you all for your time and help.
    You should be fine. The crack didn't completely penetrate the tab, or else it would have come off and moved in the direction the pin was being driven in. In addition, the force that was applied while driving in the pin is much greater than any forces that will be applied during normal operation of the rifle. Even when the bolt is locked open once a magazine is emptied during live fire, the majority of the catch's surface area is pushed forward by the bolt and rests flush in the recess. This will prevent any excessive force from being exerted on just the tab alone. Even hitting the paddle with the palm of your hand to release the bolt should not be an issue. At this point, it is strictly cosmetic.

  8. #8
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    A few observations.

    This happens quite bit when using the wrong technique and tools.

    1. Lube your pins before installing.

    2. Using the correct punches (starters followed by roll pin punches).

    3. I highly recommend getting a Magpul trigger guard and putting it in between the ears when starting your roll pin. It will help when starting the pin. Or you can get a piece of solid plastic or Delrin and make something to insert in between.

    4. Use a nylon/brass hammer. Tap the pin until it is flush and then place the trigger guard of your choice in the right position and finish driving through. I use the starter punch until it almost hits the receiver. Then use your roll pin punch.



    Owner/Instructor at Semper Paratus Arms

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  9. #9
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    Are you sure the lower wasn't out of spec? I have seen it more than once.

    Quote Originally Posted by TangoSauce View Post
    And just a note, my buddy recently used a PSA LPK with a RRA lower (I don't personally recommend either but that's neither here nor there). He could not get his bolt catch pin started using two different LPKs. He managed to get his FFL to swap lowers, and he got it to work. Take that anecdote with a grain of salt, but it makes you wonder if there has been an issue with PSA LPKs of late. Likely not, but it's a bit curious to see this happen to two people within a week apart of each other.



    Owner/Instructor at Semper Paratus Arms

    Facebook: https://www.facebook.com/SemperParatusArms/

    Semper Paratus Arms AR15 Armorer Course http://www.semperparatusarms.com/cou...-registration/

    M4C Misc. Training and Course Announcements- http://www.m4carbine.net/forumdisplay.php?f=141

    Master Armorer/R&D at SIONICS Weapon Systems- http://sionicsweaponsystems.com

  10. #10
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    Quote Originally Posted by Texas_Mike View Post
    You should be fine. The crack didn't completely penetrate the tab, or else it would have come off and moved in the direction the pin was being driven in. In addition, the force that was applied while driving in the pin is much greater than any forces that will be applied during normal operation of the rifle. Even when the bolt is locked open once a magazine is emptied during live fire, the majority of the catch's surface area is pushed forward by the bolt and rests flush in the recess. This will prevent any excessive force from being exerted on just the tab alone. Even hitting the paddle with the palm of your hand to release the bolt should not be an issue. At this point, it is strictly cosmetic.
    I agree. It looks like a minor forging flaw. It certainly isn't critical, as that post isn't subjected to significant loads. If I had already put in the effort to fully assemble the lower, I probably wouldn't bother with getting a replacement, provided there are no other flaws on that lower.

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