Speaking of AR-15's

The #1 community for Gun Owners of the Northeast

Member Benefits:

  • No ad networks!
  • Discuss all aspects of firearm ownership
  • Discuss anti-gun legislation
  • Buy, sell, and trade in the classified section
  • Chat with Local gun shops, ranges, trainers & other businesses
  • Discover free outdoor shooting areas
  • View up to date on firearm-related events
  • Share photos & video with other members
  • ...and so much more!
  • boisepaw

    boisepaw
    Jan 5, 2015
    380
    Eastern shore, MD
    Are either cerakoted? If so, the cerakote can mess with tolerances. Might need a light tap with a plastic hammer.

    I have an upper thats off and the front pin hole had a lip on it. I took a light file to the lip and the pin goes in fine.

    Small variances like that shouldn't cause issues with shooting. The main issue you'll have with a 7.62x39 are the magazines. Sometimes they don't feed right and the cartridge will hang up. Mine was reaI bad and I had to grind back the feedramps on my barrel extension so the bullet tip wouldn't catch.

    Sorry I didn't respond earlier.

    Neither are cerakoted. I had tried the light tap...becoming a less light tap after a couple of taps. No luck.

    I can check the pin hole but putting it back together with the original upper doesn't seem to be a problem...
     

    boisepaw

    boisepaw
    Jan 5, 2015
    380
    Eastern shore, MD
    Is this the Radical branded upper that was posted on here not too long ago by chance?

    It isn't. I have a Palmetto upper that I bought a couple months ago. i inquired ALSO about the Radical upper but decided to pass on it.

    Your thoughts about the Radical uppers? I was getting mixed signals online about the quality of their uppers.
     

    boisepaw

    boisepaw
    Jan 5, 2015
    380
    Eastern shore, MD
    First off, have you had that upper on other lowers?

    Have you had any other uppers on that lower?

    Does the rear of the upper fit flushly with the lower receiver extension when you use only the front pin? If yes, shine a light into the rear pin hole on the 'port' side. You shouldn't see any of the rear takedown lug through the hole. Meaning, you don't want to see anything that would interfear the rear pin going all the way through the two holes(the upper and the lower holes). My suspicion is the problem might lie back there.

    Sorry I didn't respond earlier and thanks for the additional questions...

    1) I have NOT had that upper on other lowers. I just bought the upper a couple months ago. And I just have not EVER interchanged uppers and lowers on my AR's...

    2) It does seem to to fit flushly but I can sit down with it and go over the things you suggested. When I had one pin in I eyeballed the second hole and it looked clear but I didn't really look all that carefully.
     

    ToolAA

    Ultimate Member
    MDS Supporter
    Jun 17, 2016
    10,500
    God's Country
    Sorry I didn't respond sooner.

    The upper is a PSA...Palmetto State Armory. I am not seeing any misalignment visually. The upper seems to snug into place just fine and I can get one pin into place but not both.


    That sucks but it’s a telltale sign that one or both parts are very slightly out of spec. It really doesn’t take much to cause interference.

    Take a look at the lower technical spec drawing. Both pin holes are supposed to be drilled 0.250±.001” and they are supposed to be round within .001” and perpendicular to the axis within .002” then their position away from each other measured from center to center can be 6.375±.002” max. Normally there is a bit of statistical average that happens during machining. One hole may be +.001 the next one may be -.001 and they usually cancel each other out in a sort of way. However occasionally you may have an unusual combination of tolerances all at the extreme. Say both holes are drilled .249 and they are at the max limit .002 out of perpendicular to the axis and the overall distance between the holes is right at the minimum 6.373”. If the upper is also at the limit of those tolerances but in the opposite direction you may have .003” misalignment between the upper and lower pin holes. That’s less than the thickness of a sheet of copy paper but enough to cause some interference.


    c0e4a1436b50a02c0aa0e0197345ec34.png


    What can you do. Without gauges or QA fixtures it’s not going to be easy to find out which part may be out of spec. If you had a bunch of different uppers and lowers you could swap them around to see if there was any pattern that emerged. It may indicate which specific part was wrong by the mere fact that it didn’t fit onto several parts.

    An easy fix would be to lightly hone down the diameter of the rear takedown pin. Maybe just chucking it into a drill using some 220 grit sandpaper you can remove .002” from the diameter and get them to work, without trying to ream out the hole on the upper or lower.
     

    outrider58

    Eats Bacon Raw
    MDS Supporter
    Jul 29, 2014
    49,818
    Sorry I didn't respond earlier and thanks for the additional questions...

    1) I have NOT had that upper on other lowers. I just bought the upper a couple months ago. And I just have not EVER interchanged uppers and lowers on my AR's...

    2) It does seem to to fit flushly but I can sit down with it and go over the things you suggested. When I had one pin in I eyeballed the second hole and it looked clear but I didn't really look all that carefully.

    If you are having fitment problems with two halves of AR's, it is well past time you had. It would most likely tell you immediately where the problem lies. Do this with any part that 'won't fit' in a gun. It will usually tell you whether it is the part or the gun. Occam had a razor...
     

    Users who are viewing this thread

    Latest posts

    Forum statistics

    Threads
    274,935
    Messages
    7,259,606
    Members
    33,350
    Latest member
    Rotorboater

    Latest threads

    Top Bottom