View Full Version : Upper/Lower Fitment Issue
Achilles11B
12-08-18, 11:16
I recently bought a 16” LMT CQB upper. I tried to put it on a lower and the front takedown pin would be extremely tight or completely unable to be pushed through. No problems with the rear takedown pin. This has happened on two lowers, I’ve never had this problem with any other built or bought upper before. Any assistance is appreciated. Thank you.
Assistance? You'd like me to bring a bunch and hammer? If some persuasion is needed, rock on. If it simply won't slide into place with moderate impacts, then let us know.
Tight is good.
bamashooter
12-08-18, 12:27
The rear pin aside, will the front go in any easier in relation to different "pivot" angles of the upper? Is it the same for removal whether it's first or last to be popped loose?
Kind of with Grizzman on this problem.
First of all, try installing the pivot pin with the upper shotgunned open - if you haven't already.
Does it start into the upper receiver tab? If so gently tap with a plastic/leather mallet. If you don't have one of those, use a piece of wood as a buffer.
If it won't even start into the pivot pin tab, I'd check the pin to make sure it isn't over .250. Mil-spec on the charts I've seen is .248 (+/- .002)
The (+) or (-) tolerances in two pieces sometimes stack together to make a tight fit.
Tight is good, once you get it in, let the assemblies wear into each other and it'll be fine.
It's likely caused by the Cerakote, but my FDE Colt still requires a hammer and punch, and I'm okay with it.
Achilles11B
12-08-18, 14:17
I’m good with tight, not good with out of spec parts. Other uppers don’t have this issue, which caused the concern. After I get a lower built for it hopefully it’ll wear in.
I’ll share my experience with my mrp. It is a tight fit on all the lowers I own. One of the lowers its a chore to mate to the mrp, think tapping with a mallet. The other lower is tight, but it’s just the right type of tight.
Im not sure why they are uniquely tight compared to other upper/lower combos I have, but I like it.
Have you contacted LMT about the problem?? I am surprised this thread hasn't been canned yet.
I recently bought a 16” LMT CQB upper. I tried to put it on a lower and the front takedown pin would be extremely tight or completely unable to be pushed through. No problems with the rear takedown pin. This has happened on two lowers, I’ve never had this problem with any other built or bought upper before. Any assistance is appreciated. Thank you.
First, find out where the problem is.
1. Remove the pivot pin from the lower and see if is goes through the hole in the upper.
If it doesn't there's your problem. If it does, go to step 2
2. Place the upper on the lower and slide the takedown through the rear lug of the upper. Examine the holes for the pivot pin, do the line up? (remember, there is .020" play in the rear hole so slide the upper back and forth on the lower while looking through the front hole.)
3. Repeat step 2 but look from the opposite side.
If there is a step that doesn't go away with sliding back and forth, there's your problem.
There is a range of acceptable specifications for both the upper and the lower. Sometimes those specs don't allow the two of them to play nice with each other. I have two BCM uppers that will NOT go into a Noveske lower. All high quality stuff, but they just don't fit each other.
That said, if I could get the front pin in (even with a plastic or leather mallet), and the rear pin goes in easily, I would simply rock on. The rear pin is the one I use 90% of the time. I don't even take out the front pin when cleaning my rifles.
Curious to the make of lower?
AKDoug may have hit the nail on the head. I have an Aero upper and lower that are likely just a hair out with one another. They mate up with other combinations of uppers and lowers however. The thin layer of cerakote may be contributing to the issue. I used a Dremel to remove .005 or so off of the rear pin so I wouldn't need to hammer it in.
If you have tried it with 2 different lowers (although you left out some details) and you think there is an issue, then contact LMT as per the site rules.
To the OP, are you attempting the front pin hinged or flat? I bought an MRP upper last year from Fin which shipped directly from LMT complete. This upper is extremely (uncomfortably) tight on my newest and unused non-MARS lower, and fits very close fit on lowers with more time. LMT is machining pads into the upper which I believe by design are supposed to tension the pair a bit so the top and lower half are tight. Shooting teams would do something similar by taking a piece of shell casing and shimming between the two. The pads are about .75" back from the front pin, and are ~.005 thick. Pic attached:
55196
Achilles11B
12-27-18, 22:19
Lowers are both Spikes (I know, I know...). Neither have had issues with other uppers (Vltor and BCM). Front pin is still tight, but it does go through with some persuasion and rifle functions normally. Tight is indeed good.
Neither have had issues with other uppers (Vltor and BCM). Front pin is still tight, but it does go through with some persuasion and rifle functions normally. Tight is indeed good.
I've experienced tight fits with an occasional BCM UR here and there, but nothing like the MRP and that newest LMT lower. I do believe they would wear in and be fine, but the pads may explain the difference between your standard uppers (Vltor and BCM) and this MRP upper. It was just a data point I've not seen anyone else mention.
https://www.youtube.com/watch?v=Cxs-s8fLdt4
Sounds like a good problem to have! In all seriousness, I've heard these are tight from many people. And function well.
What kind of lower?
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I've seen that happen when the buffer tube has been installed one too many turns. The lower is causing the upper to sit forward just a hair because the upper is hitting the end of the buffer tube. Try backing the tube out one turn, just make sure that it still captures the buffer retainer.
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