So my Nordic upper won't fit my M&P lower

michaelclm · Mar 21, 2011

LMT lower, check.
Different M&P lower, check.
My lower, that's a big fat he!! no.

So the trouble started the first day out. I swapped my upper onto my M&P lower and the rear takedown pin got bound up right at about the halfway mark. A little gentle pressure would get it to go about 2/3 of the way but it quickly became obvious that nothing short of main force was going to get it to go all the way through.

I put it on a LMT lower (Thanks, Steve) and things were just fine. I got it zeroed and the world of cheap AR plinking was good. Next I tried another M&P lower with the same results as the LMT. The takedown pins went into position with no problems.

My first thought was that the rear takedown pin was somehow out of spec. So I replaced it and still, the upper and lower will not fit together. So I'm down to trying other AR uppers on my lower and see what happens. The only thing that I can figure at this point is that somehow the hole in the lower for the rear takedown pin is out of square on one or more (most likely) axes(yes, plural spelling of axis) by just enough to cause the pin to bind when passing through the Nordic upper.

Besides pissing, moaning and complaining I guess I'm polling for additional ideas as to what could be causing this. So, thoughts?

steak · Mar 21, 2011

I don't know what would be causing this but maybe bore mop with some jewelers rouge to clean up the pin holes on your lower. Just an Idea.

michaelclm · Mar 21, 2011

I thought maybe that could be the problem but three things: The pin slides freely through both holes with no upper installed. Two different pins and no change. Same upper, three lowers & mine is the only one that it doesn't like.

The lower and upper appear to line up by the naked eye but it still binds up. Sheesh.

At this point I'm open to just about anything.

Scott Hearn · Mar 21, 2011

STEAK said:
I don't know what would be causing this but maybe bore mop with some jewelers rouge to clean up the pin holes on your lower. Just an Idea.

Maybe do this except on the upper?

Mine is a little tight on my M&P lower but it works. On mine it's a snug fit between the two halves at the front pin.

michaelclm · Mar 21, 2011

Is jeweler's rouge something I can pick up at a local gun shop or is it readily available at a specialty store? Please pardon the ignorance and thanks for the assist here, guys.

Scott Hearn · Mar 21, 2011

Mike Ray said:
Is jeweler's rouge something I can pick up at a local gun shop or is it readily available at a specialty store?

I'd probably use some valve lapping compound, jeweler's rouge is awfully fine, though you can get some at Lowes or Home Depot
most likely. I know that a little plastic tub of it comes with the polishing kit for a Dremel tool. It will work but it might take awhile.

Have you tried to put a pin in just the upper? If it goes through like it should, the pin hole location is off on it, or on your lower.
If the pin goes through the first hole in the lower AND most of the way through the hole in the upper, a light polish on the upper's pin hole is what I'd try.
But if it goes through the first hole in the lower and all the way through the upper and just won't line up with the 2nd hole in the lower, then I'd call Nordic
and get some advise.

jtischauser · Mar 22, 2011

I'd call Nordic. They machine uppers/lower for several companies and would probably be able to help you out.

michaelclm · Mar 22, 2011

So I spoke with the good folks from Nordic and was suggested a solution: One of their staff has an M&P, as well, and he had the same issue. It appears as though I might need to put a slight chamfer on the vertical edges on the back side of the rear pin stud on the upper. There appears to be a clearance issue with these corners and the inside edges of some M&P lowers. I have to check my lower to see if there are wear marks here. These will indicate that this is, in fact, the issue. I will keep you posted. Thanks, again, for the brainstorming.