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Takedown pin retention


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I'm putting together a lower using a Magpul AR15 build kit and have run into a problem with the rear takedown pin.  After inserting the detent and spring, the takedown pin will not push back into the receiver.  I made the mistake of tapping the pin with a brass hammer and sheered off both the detent and the inside of the receiver.  I tried grinding down another detent flat rather than a point but the pin still will not slide past it.  I'm not sure if the spring is too stiff or the hole the detent goes into is too deep.

Has anyone run into this issue before?  Any suggestions on how to resolve it?


I tried to insert 2 pics but apparently Google Drive shares don't work.

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Is it possible that you're using the wrong spring from the kit?

It could be that the spring is too stiff, in which case cutting off a couple coils might make all the difference. But it could also be that the detent hole is too deep. I've never heard of that being a thing, but that doesn't mean that it isn't possible. It's also possible that what we have here is something we call "tolerance stacking", where a combination of things add up to the problem, like maybe the spring is a little too heavy and the detent hole a little too deep or the detent pin being a bit out of spec.

Pics could be useful. Many of us here use IMGUR for pic sharing since PhotoBucket went stupid.

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Thanks Longhair for the reply.  Indeed, tolerance stacking could be the cause.  One thing after another, after another...
I went out to IMGUR and uploaded the pics.  Hope this link works for both of them.


I have a Colt Sporter from the 80s.  The pivot pin on that one has no retention so I could leave the detent and spring out of this one.

I'll try cutting down the spring first to see if that helps.

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On IMGUR, if you click on the "Get Share Links" button, then copy the BB Code (the one with IMG tags), you can paste it into your post and display the images in the post instead of linking to them.



Takedown PinTakedown Pin

Takedown Pin


Looking at your pics, I would probably make a new longer detent pin, and be careful not to make it too pointed, and cut down the spring commensurately.


Also, if your Colt doesn't have a detent, it's probably because someone lost it when they took off the stock or buffer tube. That's exactly why I do what I do with the tap and setscrew, so parts don't launch themselves into never-never land.

Edited by Longhair
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Thanks for the tip about IMGUR, I'll try that and will post a pic when I fill the detent hole in the takedown pin with JB Weld.  I tried a standard detent, pushed it in with the spring but put no pressure on it at all.  The pin still wouldn't move.


There is no slot milled in the Sporter pivot pin.  I'll include it in my next photo.  Gonna try and get to the JB Weld today.  Should have more pics in a few.

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FWIW, I wouldn't do the J-B Weld thing.

Personally, I would likely get a #45 drill bit and cut about a 3/8" piece off the shank (longer than standard so it won't be affected by the broken-out bit on the inside of the receiver), and if it didn't have a little bevel on the end i would grind or file one. That should not lock into the hole at the end of the slot, but would rest in it enough to act as a detent should (with the spring shortened appropriately).

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Huh.  Well that's a good suggestion.  Something to try before I muck it up with JB.

I think I have a numbered drill bit set - I'll drop one in the detent hole and use a little finger pressure on it as a test.

I'll get back to you...

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I took the takedown pin, spring and detent out of another AR.  Had the same problem.  I'm baffled. 



Apparently you need a membership on IMGUR to see a "Get Share Links" button.  I couldn't find it anyway.  I didn't take the time to create one.


I can't really see any difference between the two takedown pins.  The left one is the problem pin, the center is from another AR and the third is the pivot pin from my Sporter.

Sooo, I decided to try the JB Weld.  This is a SBA3 MOE EPT PISTOL LOWER BUILD KIT from Palmetto State Armory.  I'm not sure who to complain to; PSA or Magpul.

Anyway, I'll let the JB Weld dry and report back on my progress tomorrow.

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7 minutes ago, Eighty said:

I did try the blunt end of a #45 drill bit.  Even stepped it up to a #41 but it just wouldn't push back in.  It's weird; all the combinations worked fine with the detent hole when the pin is pushed in!

Very strange indeed.

I wish that I had it here in front of me to check out. I'm the type that can't have something that doesn't work the way it should without finding out why.

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The JB Weld solved the problem.  I still haven't determined what was wrong but without a depth gauge or other measuring caliper there's no way to check for a manufacturing defect - and certainly not now with the JB in there.  I could still kick myself for busting out the inside of the aluminum receiver but at least it's now functioning.  I'll have to run a mag or two through it but the feel of it seems as though the pin is not going to work itself out. I don't know if this string of code starting with "blockquote class=" will insert the two pics...


<blockquote class="imgur-embed-pub" lang="en" data-id="a/AyVu6mc"  ><a href="//imgur.com/a/AyVu6mc">Lower Takedown Pin Detent Broken</a></blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>


Here's the link to the last set of pics.  https://imgur.com/a/AyVu6mc


Thanks again Longhair for the suggestions and being a sounding board for me!

Eighty (NinetyMinusTen)

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