New observation on my upper + lower problems

[Creeky73]

in another thread, I was mentioning how the Sun Devil upper and TI lower that I have recently purchased don't seem to match up very well. I thought I might have problems when I first got the upper, and the fit with my Rock River Arms lower wasn't very good. You can see daylight through the cracks between the two, they just did not seat up well together. I thought that perhaps this was just a fluke, and decided to try buying a nice lower on the off chance that those two might match up. So I got the TI lower, and it actually fits up worse to the upper than my RRA lower did. So I tried to match the old RRA upper to the new lower, and they match up like a charm. So that leads me to believe that the Sun Devil upper is a bit out of whack, since it is the one part that doesn't seem to fit right. Is there anything I can do to remedy this, is there someone I can send the parts to for a "fitting?" You can actually slide a piece of paper into the receiver very easily, and you can clearly see light through the two halves when you hold up up to light. It is hard to justify almost $500 on these two parts and have them fit together badly.

[Bill Bryant]

From my machinist background I can understand your concern about poor upper/lower fit. I love to get out my Starrett stuff and measure everything to the nearest .0005 and think about how a part was manufactured and how tolerances are held or not held in real work environments. I've read a number of threads, however, that seem to suggest that a certain degree of slop is within the anticipated tolerance stacking involved if you want to be able to have interchangable parts sourced out of many different machine shops and all headed for a battlefield, where a bit of slop is far less of a problem than an upper/lower match up that gets stuck and can't be torn apart easily during an "intense" situation.

My Stag/CMT lower and my BCM upper had enough slop that a .006 feeler could be run between them easily. That's quite a bit of tolerance stacking all going in the same unfavorable direction.

Rather than accept my luck of the draw and a sloppier fit than I want, I fixed the "problem" totally with a couple of dabs of J B Weld. It took a little time and experimentation to determine how long to wait before "closing the lid," and it took a bit more to figure out just how and when to trim off the excess and make everything look good. But the results are worth it: a bank vault stability and total ease of pin movement.

Later, if I put another upper on the lower (the lower has the J B weld on it) I'll remove the J B Weld with Methylene Chloride (paint stripper) and everything will be good to go.

Since I don't think I'll be using my AR in a combat zone where I might have to change out parts quickly and with lives on the line, I'm comfortable with my little bedding job and how it makes the rifle feel much more solid.

On the other hand, if I hadn't figured out how to "fix" this "problem," I would be totally OK leaving it as is, understanding that my particular upper/lower combination was on the edge of the go/no go tolerances required for parts going into a battle tool.

[Creeky73]

so what you are saying is, as long as nothing gets jammed up that would make break-down impossible, I shouldn't be concerned about the gaps between the two halves? I am concerned that these gaps may allow more dust/dirt/whatever inside the receiver in harsh environments.

[Bill Bryant]

As I understand it, the gaps are very normal. Think of it this way. Going even slightly in the opposite direction past the ideal would mean an upper and lower so tight that tools would be required to punch out the pins. Not good in a battle. So think of a magic number, say .003 of gap, that's the ideal so that + or - .003 from there will still give you hand removal of the pins. If all the tolerance stacking goes in one direction, you get a bank vault lockup and pins still go in by hand. Another .001 or so and you're stuck using a hammer and punch. If all the tolerance stacking goes the other way, you've got the .006 or so of gap and a rattle--and a rifle that works just fine and is easily opened by hand.

[Bill Bryant]

http://i172.photobucket.com/albums/w36/ttkt57/gunstuff015.jpg

[Bill Bryant]

http://i172.photobucket.com/albums/w36/ttkt57/gunstuff014.jpg

[Creeky73]

thank you for the pictures and input, it is nice to see I am not the lone ranger with these issues, and I don't feel quite so bad about it anymore. I think I am going to just build two guns though, since the two "new" parts seem to go better with my "old" RRA halves, I will do two builds, each will have an old and new part instead of both new parts going together. The only thing that kinda sucks about this is that I will be mating a common RRA A2 style upper onto my beautiful new TI lower, and a nice looking Sun Devil flat top upper with the old lower. Either way, I guess I will have two relatively nice guns. I can already hear my bank account crying...