Question RE: First AR build..Fail zero upper and an issue came up

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usnavdoc

New member
I am in the middle of my first AR build and ran across an issue. Not 100% sure what to make of it. I have a fail zero upper reciever with cerro forge markings.

This upper reciever was intended to go on a Daniel Defense lower with a G@R tactical lower parts kit. When I assemble the two, the rear retaining pin goes in perfectly but the front pin will not pass through the upper reciever.

I placed the Upper reciever on my upper vise block and its front pin will not go through the upper either.

I placed the upper on a different BCM lower and it works perfectly.

Im not sure what to make of this. I thought initially the pin might be out of spec but then it didnt work on my vise so I thought the reciever hole might be out of spec. But then it works on my BCM lower.

Not sure what my options are? I ordered another pin but that doesnt tell me why my vise doesnt work either. Unless that pin and my vise pin are both out of spec. I just cant imagine I would be lucky enough to have had two separate issues like that.

I can post pics tonight when I get home if they might help
 
I am betting you are seeing the result of tolerance "stackup."

It will happen in any manufacturing environment where processes are not controlled closely enough.

If the upper is nominal and the lower is nominal, the parts should assemble.

Deviation from nominal dimensions on either the upper or lower will result in various "fits."

In the worst case, the upper either will not assemble or the fit will be so loose as to feel as though it is coming apart.

Many times the most EFFECTIVE solution, is to trial fit uppers and lowers until they pin. You CAN try undersize pins and pick up some tolerance that way. The problem with that approach is the peening effect of mis matched parts as they work against each other during recoil.

Sorry to not have better news for you.

Geetarman:(
 
I have a Daniel Defense so here's a tip:

Try inserting the front receiver pin first, then do the rear; see if that helps.

I know with my DD, the front pin is always harder to insert/remove, than the rear one. No sure why, but it's not an issue.
 
Makes sense. I always do it that way. Good idea. A lot of times I don't take the front pin out at all. I can get everything apart by just popping out the rear pin.

Geetarman:D
 
Same problem, an LAR Grizzly upper, and an AGP lower. Front pivot pin first, and the a lot of pressure with both thumbs on the rear pin. I have to drive it out with a non marring tool.

It's tight now, in a few years it'll be loose enough to rattle and still shoot just as straight. Silly AR's do that.
 
Fairly common problem when inserting the rear pin first. I've had a few that needed you to tap the rear pin in and out a few times before you could use thumb pressure to insert it. First trip to the range solves tight fits.:)
 
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