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Repairing mistakes on an 80%
Weaponeer Forums : AR15 Series

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  Brim

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Posted: May 12 2017 at 7:56am | IP Logged Quote Brim

I already asked this question on the homegunsmith forum, but only got a couple responses. More help and options is better than less, so here goes:


Question for all of you experts on here.

A while back, I found myself stuck out of town for a few months due to work. Being bored in the evenings, I decided to make an 80% lower into something more. Being away from home and therefore not having access to my normal big tools, I decided to see if it could be done with just a Dremel and an electric hand drill.

I know, I know... Bear with me here.

It turns out that you CAN do it with just a Dremel and a hand drill, but...

...not using a drill press, when I drilled the hammer and trigger holes, the bit wandered slightly. Long story short, rifle actually works fine - but i had to install the self-contained match trigger set I was saving for my next build. This also required making the hammer and trigger pin holes a bit larger so they were in the right place with the self-contained unit. Like I said - it actually works perfectly, with no problems - but it is ugly on the sides with those oblong holes, and that bugs me. Plus I want my match trigger set out of there so I can use it on my better build.

So, the question:

How can I fix the sides, so that I can pop a regular mil-spec lower parts kit in there?

I was thinking of TIG welding the hammer and trigger pin holes closed, since I have heard you can weld aluminum with a TIG. Then I can re-drill the holes (with my drill press and jigs this time), and be in business.

Will that work? Do those of you who know about TIG welding have any reasons that won't work? Will the heat created by TIG welding embrittle the aluminum of the reciever - or soften it too much? Will warping likely be a problem? Any other problems I am not thinking of?

Is there a better way to fix it than welding the holes closed and re-drilling?

I have a perfectly-working but ugly gun right now. I'd prefer a perfectly-working gun that is NOT ugly, but I REALLY don't want a non-working gun. Am I better just taking a lesson from Elsa and "Letting it go" instead of trying to mess with it?
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  bikergunnut

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Posted: May 12 2017 at 7:51pm | IP Logged Quote bikergunnut

Tig sounds like the cure. Drill some holes in a piece of scrap the same thickness to practice, it won't be a waste of time.
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  backbencher

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Posted: May 12 2017 at 8:51pm | IP Logged Quote backbencher

80% aluminum lowers are down to $32/ea now.  Might be just as easy to do another one.
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  Paraquat

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Posted: May 13 2017 at 4:05am | IP Logged Quote Paraquat

For the cost, I'd buy a second lower.

Seal that one up behind a piece of drywall.
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  Brim

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Posted: May 13 2017 at 9:51am | IP Logged Quote Brim

backbencher wrote:
80% aluminum lowers are down to $32/ea now.  Might be just as easy to do another one.

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If it had only been a regular lower, that would be a viable option. Unfortunately this one is a $130 lower for a 9mm, specially made to take Glock magazines, with a special mag release and all. Worst case scenario, I simply leave it how it is and deal with it being ugly, since it works just fine. Well, and I give up hope of recovering my match trigger unit from it. Sure seems silly to waste a nice aftermarket match trigger kit on a 9mm plinker carbine.
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  Brim

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Posted: May 13 2017 at 10:34am | IP Logged Quote Brim

Paraquat wrote:
For the cost, I'd buy a second lower.<br><br>Seal that one up behind a piece of drywall.<br>

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I sometimes have a hard time understanding sarcasm on forums, especially since I'm new here. Is the "sealing up behind drywall" comment regarding embarrassment in making a mistake, or for "putting away for a rainy day" type of thing? Or something else?
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  backbencher

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Posted: May 14 2017 at 1:37pm | IP Logged Quote backbencher

Ah, bugger.  If it's 7075, be aware there are recognized welding techniques, but those are not generally recommended b/c of the possibilities of voids left in the weld, I believe.
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  UKBiker

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Posted: May 14 2017 at 3:03pm | IP Logged Quote UKBiker

Another option would be to drill the holes slightly larger diameter then tap a thread into them.
Find some ally bar of suitable diameter and tap the appropriate thread onto the piece of bar.
mix some chemical metal up, Belzona is by far the best stuff, though not cheap.
Cover the threads on the ally bar then wind it into the threaded hole on the 80% lower
Let the chemical metal cure and then hacksaw the protruding bar off and sand flush.
Mount the 80% lower into an appropriate drilling jig and redrill the pin holes in the correct position.
For added security you could stake the threaded ally bar in place , or even drill and tap at 90 degreees and add a small grub screw to lock it in place.

It saves TIG welding it and the attendant issues that can accompany TIG welding, bt to be fair if you have a decent TIG welding guy on hand or you are good with a TIG yourself then it can be welded easily enough.
You could even use the threaded ally rod method and then run the TIG around it to secure it rather than trying to fill a big hole with TIG which isn't always easy.
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  Paraquat

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Posted: May 15 2017 at 6:08am | IP Logged Quote Paraquat





Seal it up behind the drywall means don't destroy or sacrifice it, but to hide it where no one else knows where it is, but you for when you need it. When the sh*t hits the fan, it doesn't matter how ugly or pretty it is, so long as it works. Punch a hole in the wall and now you have a firearm.

Could always totally customize it if you made your own oversized pins.
Start with, say 5/16 stock and turn it down to .1545 and thread the end.
This is difficult to explain. Give me a minute to MS paint something.
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  Paraquat

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Posted: May 15 2017 at 7:35am | IP Logged Quote Paraquat



Ok, if you make this properly the heads of the pins will be just protruding from the outside of the receiver.
The ends are threaded, any thread you want. 8-32, 6-32, 5-40, whatever. The shoulder of the thread sets the length of the pins ensuring that you don't squeeze the receiver.

The base assembly will protrude .060", but the two bosses that protrude from it should be equal to the wall thickness.

If you can't turn a diameter that long, you can always tap the ends and put button head screws in there.
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  Paraquat

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Posted: May 15 2017 at 7:49am | IP Logged Quote Paraquat



Tap both sides for large flange button head screws if you want symmetry.

Ream diameters and cut shoulders to suit.
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  Brim

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Posted: May 15 2017 at 1:13pm | IP Logged Quote Brim

Dang, those are some good ideas. However, I only WISH I had the tooling and machinery to confidently make cool little doodads like that.

My metal-working tools: Delta floor-standing drill press, flux-core MIG welder, stick welder, 4.5" angle grinder, 6" bench grinder, Dremel. That kinda limits my cool doodad-making ability.

I have manual taps and dies, though, and with some thinking might be able to make one of those ideas work, though. I will have to puzzle over that a bit.

These are all some good ideas, though. Thank you all!
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  Brim

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Posted: May 15 2017 at 1:43pm | IP Logged Quote Brim

Paraquat wrote:
<br><br><br><br>Seal it up behind the drywall means don't destroy or sacrifice it, but to hide it where no one else knows where it is, but you for when you need it. When the sh*t hits the fan, it doesn't matter how ugly or pretty it is, so long as it works. Punch a hole in the wall and now you have a firearm.<br><br>Could always totally customize it if you made your own oversized pins.<br>Start with, say 5/16 stock and turn it down to .1545 and thread the end.<br>This is difficult to explain. Give me a minute to MS paint something.<br>

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Thank you for the clarification on the "seal it behind drywall" message. That makes much more sense now!
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  Paraquat

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Posted: May 17 2017 at 9:16am | IP Logged Quote Paraquat

https://www.midwayusa.com/product/213686/kns-hammer-and-trig ger-pin-2nd-generation-non-rotating-ar-15-large-pin-170-stai nless-steel

Here's an oversized pin kit, but you'd need to swap out the hammer and trigger.
Not sure is .170 would clean up for you but .171 is a standard drill size.
(11/64)
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  backbencher

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Posted: May 18 2017 at 7:42am | IP Logged Quote backbencher

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