UPS delivered my stripped Aero Precision Upper and I tried connecting it to my Delaware Machinery Lower. Front take down pin fits perfectly but the rear won’t go into the lower.
I have another upper coming in within the next few weeks. Am I SOL or do I need a new lower? DPMS LPK is installed in lower.
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Is it the width of the rear lug holding you up? Or is the lug not in the right place?
Width is too big 
Do I need to file it down some? :moil:
Yeah just shave it down slightly and it will be fine.
Make sure its not the receiver extension hitting before shaving the upper rear lug.
Reason I say it is because my mega uppers won’t close on my aero lowers unless I back the receiver extension out one turn. Not sure why, as they fit perfectly on mega lowers, but it is what it is for some reason.
Please use a “slick sided” bastard file. ( a file with smooth untoothed edges. can be found at harbor freight in the jewelry tool section)
Secondly… one stroke per side in an alternating fashion. This, hopefully, will keep your material removal even.
Ah yes, AP. Now I remember why we do not use their parts any more. 
C4
Is it AP’s upper or Delaware Machine lower?
My guess is that it is the lug on the upper.
C4
OP, before you file down the lug, would you take calipers to it for measure please?
This is an opportunity to document.
Is the lug as a whole cut too wide? Or is there flash/raised material on the edge of a corner from cutting?
Grant, Who do you feel offers more consistent or better lowers/uppers? I personally like mega products, but they are hard to come by right now.
And the flood of garbage into the AR world has officially begun. 
According to some drawings, the recess the rear lug of the upper fits in should be 0.500 + .000/- .004 wide, 0.950 long, and 0.630 deep.
Might give you a better idea what’s out of spec. I’ve got 3 rifles built with AP uppers and they all fit well.
Before filing or defiling anything, check the RE and check the buffer & BCG for interference. On my ARs, sometimes the upper & lower cannot be closed without the BCG hitting the buffer.
Also make sure the pin isn’t sticking in too far & causing interference
BCM. Colt. Noveske. DD.
C4
Not exactly easy brands to get right now.
Unless my calipers are off:
Width
Lower width
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Before setting mind to purpose, is there anything I should keep in mind? Should I contact Aero first and see what they say?
As for the metal, is there anything I can apply to the receiver once it has been filed?
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so .003 off. I would chalk it up to tolerance stacking and not junk parts like so many like to claim right off the bat. If it was .03 off than I would suspect junk parts, but .003, tolerance stacking IMO. Probably one swipe with a file on each side will get it to fit.
Although if what LeviTX posts as the “spec” for the inside of the rear pin area than both parts are WAY off, but I find it hard to believe that both parts would be .05 off from the spec.
ETA: before you blame Aero do you have other uppers you can check? Or other lowers? I never thought Aero turned out junk, but I would be more suspect of the lower since I personally have never heard of Delaware Machine.
I have a long history of dealing with AO. We have learned painful lessons and is why we and companies like BCM no longer do business with them.
C4