Problem with 80% lower rear take down pin

My Toy · Jul 23, 2018

Just finished 1st 80% lower build; it is an 80% Arms 6061 anodized billet lower using their Easy Jig Gen2. I followed instructions exactly; took me about 3 hours but I was trying to be careful as it was my first attempt. I'm confident that after the first I could probably do the machining in an hour. Anyway it went beautifully. I installed a Bushmaster LPK and the only problem I encountered was when I tried to install my Bushmaster 16" HB upper receiver the rear take down pin was so tight it would have had to be driven in. With emory cloth I reduced the diameter of the take down pin by a couple of thousands and got it to be able to be pushed in to place (although it is still so tight that you need a bullet tip to push it out). I tried a second Bushmaster upper and there must be slight mfg. tolerance so with this receiver you could push the rear take down pin out with your finger.

I purchased a couple of 7075 lowers (produced by Cerro Forge) to try next so I thought I better try to put them together with the Bushmaster uppers I have to check the pivot pin/rear take down pin fit since I encountered a problem with the 1st build. After installing the pivot pin through the upper in to the 7075 lowers the rear take down pin won't pass through the rear lug of the upper. The rear take down pin holes in the lower receiver appear to be slightly off of alignment with the hole in the upper receiver rear lug. Looks like the rear lug of the upper receiver should go a little further in to the lower receiver to get the holes to line up. I believe Cerro Forge makes uppers and lower receivers for major manufacturers so I was surprised with this problem (in fact one of the Bushmaster uppers I have also has a Cerro Forge key hole marking).

So what would the proper way to deal with this rear take down pin problem? Do you think I would not have this problem with a different manufacture upper receiver?

Allen65 · Jul 23, 2018

Sounds like the rear pocket needs to be milled a few thousandths deeper.

My Toy · Jul 23, 2018

Allen65 said:
Sounds like the rear pocket needs to be milled a few thousandths deeper.

Thanks for the reply; I measured the depth of the pocket vs. the depth of the lug and the pocket is considerably deeper than the lug.

303_enfield · Jul 23, 2018

Tight is good. I would measure the self, lug an walls again.

DavidA · Jul 23, 2018

I had a similar problem with an AR10. Got a Juggernaut Tactical 80 loser and the build was with a Aero Precision M5 upper. It seemed the pins lined up in terms of dimension however the upper would not close deep enough to allow the pin to go through. The solution was I took a fine file and flattened down the radius on the upper receiver. I would imagine 3 to 5 thousands. It goes in now but it is tight. When I called Juggernaut Tactical the claimed that there was no standardized tolerance and if you brought their upper set it would fit. I have no idea if it was bs or not. But I know a little file will do you.

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jrumann59 · Jul 24, 2018

the issue could be two fold. A lot of new takedown pins can be mothers until they get "worn" in as the détente can too tight, and sounds like something is out of spec. I would mike both the take down pin and the hole in the upper. It could be tolerance stacking. Your pin could be to the largest part of the allowable spec and the hole in the receivers is to the smallest part of the spec.

DavidA · Jul 24, 2018

The upper and lower visibly did not match up. You could see where the upper radius near assist button was a shorter radius than the lower radius which was longer. So going forward buying matching sets would be preferable. Mixing manufacturers can bring issues

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GTOGUNNER · Jul 24, 2018

Surely sounds as an upper problem.
Get another.

Sent from the 3rd Rock

GuitarmanNick · Jul 24, 2018

I had this same issue recently. I carefully measured and found my lower was fine but the hole through the rear lug on the upper was not perfectly round. A small round file took care of issue.

If your take down pin works fine until you try to put the upper on,... check the upper.

I would also like to add that any upper should fit on any lower. There are specifications that manufacturers use and anything sold as "mil-spec" should meet those tolerances. If a manufacturer tells you that you must buy a matching upper and lower, find another manufacturer!

DavidA · Jul 24, 2018

That’s what I figured as well. No more 80,s from Juggernaut

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outrider58 · Jul 24, 2018

GuitarmanNick said:
I had this same issue recently. I carefully measured and found my lower was fine but the hole through the rear lug on the lower was not perfectly round. A small round file took care of issue.

If your take down pin works fine until you try to put the upper on,... check the upper.

I would also like to add that any upper should fit on any lower. There are specifications that manufacturers use and anything sold as "mil-spec" should meet those tolerances. If a manufacturer tells you that you must buy a matching upper and lower, find another manufacturer!

Not supposed to be round. Supposed to be ovate. I recently had an upper that had the same problem. File or Dremel will solve the problem. Better to replace the upper, eventually.

DavidA · Jul 24, 2018

No need to replace the upper once I files the radius down it fit like a glove. Really snug no play so I am happy with it the way it is.

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GuitarmanNick · Jul 24, 2018

outrider58 said:
Not supposed to be round. Supposed to be ovate. I recently had an upper that had the same problem. File or Dremel will solve the problem. Better to replace the upper, eventually.

According to the mechanical drawings, the holes are supposed to be .250" and round!

The one that I had wasn't round because in shipping the rear lug took a hit and was dented on one side. This flattened the rear of the hole and the pin would not go all of the way through the lug until after I opened it to the required dimension. On one side the hole was 1/4", on the other it was not large enough causing the takedown pin to get stuck in the lug. By looking through the hole in the left side of the lower, it was easy to see the cause.

I generally don't pull out my files until I check things against the specifications. This is especially true on a new build. Not wanting deal with returning it for a replacement, I just fixed it myself in about 10 minutes. It fits on any lower and has shown no signs of loosening.

I also try to avoid beating anything into place on my firearms. Aluminum really does not react well to any misplaced hits. A slight tap with a light plastic hammer is all one should ever need to persuade the new pivot or takedown pins on an AR.

outrider58 · Jul 25, 2018

My Toy · Jul 26, 2018

GTOGUNNER said:
Surely sounds as an upper problem.
Get another.

Sent from the 3rd Rock

Actually I think you are right. I tried 3 different Bushmaster uppers on the 7075 80% lower and one of the bushy uppers allowed me to seat the rear take down pin. Of the 3 complete Bushmaster rifles I have (ie. bushmaster uppers and Bushmaster lowers) all of the Bushy uppers will interchange with any of the Bushy lowers. What I did was take a very thin piece of paper, insert it and slide it completely around the juncture of the lower and the upper. In each case there was a tight spot between the upper and the lower below the ejection port. It was just a matter of degree of tightness depending on which lower/upper combination it was. So trying the upper with the most out of alignment of the take down pin hole in the upper and the 80% lower I couldn't slide the paper in at that point below the ejection port at all. I'm sure it is that spot between the upper and 80% lower that has the "hinge" bound so it won't close all of the way. So if I would remove probably several thousandths or maybe a little more the upper would close down more at the rear take down pin end and probably be able to insert the take down pin without much effort.
I don't plan on filing on an upper that works fine with the lower it was married to (although you would think mil spec would mean any upper should work with any lower). I opted to order a new "mil spec" upper to use with the 80% lower and hopefully it will fit and pins will line up -- if not I'll probably alter as necessary to get it to fit.

GTOGUNNER · Jul 26, 2018

I worked for a company that would send machined parts requests to vendors. Alot of those would have a + or - leway of a few thousands of an inch. With todays technology, vendors didn't need the leway. So they would just pick a number with the spec. Low and behold, one vendor would pick the biggest + measurement the other picked the smallest - measurement. We had problems..

Sent from the 3rd Rock

jrumann59 · Jul 27, 2018

GTOGUNNER said:
I worked for a company that would send machined parts requests to vendors. Alot of those would have a + or - leway of a few thousands of an inch. With todays technology, vendors didn't need the leway. So they would just pick a number with the spec. Low and behold, one vendor would pick the biggest + measurement the other picked the smallest - measurement. We had problems..

Sent from the 3rd Rock

called tolerance stacking.

GTOGUNNER · Jul 27, 2018

jrumann59 said:
called tolerance stacking.

Didn't know that,, Makes since. Thanks!

treasurehunter · Aug 1, 2018

outrider58 said:
.

Mine looks just like your picture, pin wont go in. I thought it was slightly smashed, it's only on one side though.

outrider58 · Aug 1, 2018

treasurehunter said:
Mine looks just like your picture, pin wont go in. I thought it was slightly smashed, it's only on one side though.

It needs to be ovate on both sides. I just put one together and the rear pin wouldn't go through. In my case, the hole wasn't low enough to allow the pin to easily slide through. A little dremeling and a little polishing and now I have a very tight fitting upper and lower. The rear hole is always ovate much like a race track, not like an egg.

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Problem with 80% lower rear take down pin

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