upper and lower fitting issues

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08-06-13, 10:34
grnamin

upper and lower fitting issues

Here's what I have:
3 uppers, all the same model and from the same manufacturer
3 lowers, all the same model and from the same manufacturer (different maker from the uppers)

2 of the uppers fit properly with all 3 lowers.
1 upper won't fit properly with any of the 3 lowers. I can slide the pivot pin in fine, but the takedown pin won't slide in all the way unless it is forced (hammered in) and I don't want to have to do that. If I reverse the process, I can slide the takedown pin in first without issues, but now the pivot pin won't slide in with finger pressure only. This happens with that particular upper and all 3 lowers.

I sent the problematic upper back to the manufacturer with an RMA. Got an email back saying that they tried the upper with several other manufacturers lowers (except for a lower from the same company as the ones I have) and they all fit fine so they sent the upper back to me (awaiting delivery on my end).

What least destructive means can I resort to in order to get this upper to fit with any of my lowers?
08-06-13, 11:09
NeoNeanderthal

It does fit, it's just tight. Just tap the pin in! It will loosen up eventually. Most people would rather have an upper/lower fight that is extra tight vs extra loose.

What's the manufacturer of the upper?
08-06-13, 11:27
grnamin

Thank you. Mega lower with Vltor MUR-1A upper. You're right about better off starting tight and loose.
08-06-13, 12:39
Iraqgunz

If you have to beat the takedown pin in place then something is out of spec. We just had an upper received in my San Diego class and it was out of spec at the rear where it meets with the lower at the top of the lower receiver extension. The company was contacted and it was replaced. In your case either the MUR is wrong or the lower is or you have a case of tolerance stacking.
08-06-13, 12:41
mtdawg169

Re: upper and lower fitting issues

Quote:

Originally Posted by grnamin

Thank you. Mega lower with Vltor MUR-1A upper. You're right about better off starting tight and loose.

It will get easier as you shoot the gun and as the finish wears at the lugs. It will probably always be tight, but should get easier to break down. You could try to polish the inside of the lug with a fine abrasive like a scotch Brite pad or something similar if it's really driving you crazy.
08-06-13, 14:18
grnamin

Iraqgunz: Since the two other uppers fit all 3 lowers, I'm inclined to think that the non-fitting upper is a little out of spec, but not by much. Since Vltor won't repair or replace it, I'll go with the advice here and use it. :)
08-06-13, 15:15
Iraqgunz

That's your choice. But, I will tell you that repeated beating of the rear pin in and out is going to cause wear inside the rear lug hole.

I would try and fit it to the lower that has the least amount of issue.

Quote:

Originally Posted by grnamin

Iraqgunz: Since the two other uppers fit all 3 lowers, I'm inclined to think that the non-fitting upper is a little out of spec, but not by much. Since Vltor won't repair or replace it, I'll go with the advice here and use it. :)
08-06-13, 15:27
grnamin

Quote:

Originally Posted by Iraqgunz

That's your choice. But, I will tell you that repeated beating of the rear pin in and out is going to cause wear inside the rear lug hole.

I would try and fit it to the lower that has the least amount of issue.

Good call. I will try that first.
08-06-13, 15:34
T2C

If I have a new carbine with tight upper to lower fit, I place the bottom of the pistol grip on my workbench and press down on top of the rear of the carry handle to get the rear take down pin to go in. A little dab of lightweight grease on the rear take down pin helps. I push down on top of the carry handle and use a delrin pin to push out the take down pin for disassembly.

I would never pound on anything with a hammer to get the rear take down pin to go in.
08-06-13, 16:30
WS6

My advise is sell the offending part and don't buy from that company anymore. Been there, done that. It's what it finally took to get me to stop drinking Noveske Kool-Aid. :cool:
08-06-13, 16:51
STONE-YARDER

Quote:

Originally Posted by grnamin

Iraqgunz: Since the two other uppers fit all 3 lowers, I'm inclined to think that the non-fitting upper is a little out of spec, but not by much. Since Vltor won't repair or replace it, I'll go with the advice here and use it. :)

Why wont Vltor fix the issue?
08-06-13, 17:15
Iraqgunz

Possibly because it is within THEIR specs. They have no control over the lowers or the possibility of tolerance stacking.

I agree that I would contact them again and CLEARLY explain the issue.

Quote:

Originally Posted by STONE-YARDER

Why wont Vltor fix the issue?
08-06-13, 19:08
grnamin

Quote:

Originally Posted by STONE-YARDER

Why wont Vltor fix the issue?

They tested the upper with several other manufacturer's lowers (but not Mega) and the upper fit fine. The upper was sent back to me (waiting for it to arrive) before I could respond to their email reply regarding the tests.

I would stress again that I have 2 other MUR-1A's that fit all 3 Mega lowers properly. Maybe I should have sent the Mega lower as well so that they could see the issue firsthand, but I didn't know if the lower would require FFL transfers.
08-07-13, 02:31
WS6

Quote:

Originally Posted by grnamin

They tested the upper with several other manufacturer's lowers (but not Mega) and the upper fit fine. The upper was sent back to me (waiting for it to arrive) before I could respond to their email reply regarding the tests.

I would stress again that I have 2 other MUR-1A's that fit all 3 Mega lowers properly. Maybe I should have sent the Mega lower as well so that they could see the issue firsthand, but I didn't know if the lower would require FFL transfers.

Who cares what it fits? You mic the lugs and if it's within mil-spec, it's you're problem. If it's not, it's a manufacturing defect.
08-08-13, 17:11
AlphaKoncepts

Just to state the obvious... You don't have one of those little red wedges do you?
08-08-13, 18:36
grnamin

Quote:

Originally Posted by CGS

Just to state the obvious... You don't have one of those little red wedges do you?

No red wedges.
08-09-13, 16:00
dwkfym

The only way to 100% avoid tolerance stacking issue is to buy a fitted upper and lower from the same factory. It would usually be advertised as fitted.

If it fits, even if it requires some force, but you can take the rear pin in and out at the field, I wouldn't worry about it.
08-09-13, 21:17
uniform64

What is tolerence stacking?

Thanks
08-09-13, 21:42
Tzed250

Quote:

Originally Posted by uniform64

What is tolerence stacking?

Thanks

Tolerance stacking is a characteristic of mass production. If the pivot/takedown hole distance in the upper is at the far end of the tolerance and the holes in the lower are at the near end, along with all 4 bores being at their maximum material condition(MMC, small), then it may be that the pins will not slide into place(slip fit). Pounding the pins into place indicates an interference fit, an undesirable condition.
08-09-13, 21:47
dwkfym

Here is an example. S I want to machine my own AR-15 hammer, but I don't want to use a blueprint. So I ask a friend to mold his hammer part and send it to me. Then I take a pair of calipers and take the measurements, and do my own drawing. Then I get on a CNC machine and make the part.

Say the mold was only accurate to +-0.02" (Just example numbers here)
The calipers were calibrated and rated to be accurate to +- 0.01"
Then say my CNC machine will machine parts to 0.02" accuracy.
Then the dimensions of my finished part may be up to 0.05" off. Because the tolerances were stacked.

Theoretically that doesn't mean the part will be off by whatever the total sum comes out to be (in this case, 0.05), but it can be that much off.

I don't really know what the industry does to manufacture these receivers, but potentially something like this can happen:
Brand A and B are AR-15 receiver manufacturers. They each use a different point in the part to measure the dimensions. So Brand A has holes that tend to be off towards the forward part of the rifle, but brand B tends to go other way. Even though both manufacturers made both parts within milspec tolerances, the upper may be out of spec with the lower.

In reality I think most manufacturers generally machine things the same way; there may be differences in material, overall tolerances, quality control, etc. But an example like above where tolerances can stack isn't going to happen between most manufacturers.

My personal experience is the same. I've also not had many customers complaining about parts not fitting between different manufacturers. But I still may be wrong, so please take this bit with a grain of salt.
08-09-13, 21:57
Tzed250

Quote:

Originally Posted by dwkfym

Here is an example.

Say the mold was only accurate to +-0.02" (Just example numbers here)
The calipers were calibrated and rated to be accurate to +- 0.01"
Then say my CNC machine will machine parts to 0.02" accuracy.
Then the dimensions of my finished part may be up to 0.05" off. Because the tolerances were stacked.

Theoretically that doesn't mean the part will be off by whatever the total sum comes out to be (in this case, 0.05), but it can be that much off.

/snip.

That is actually a precision and accuracy issue. Tolerances are set in the engineering print and the part is either pass/fail per the drawing.
08-09-13, 22:14
dwkfym

Quote:

Originally Posted by Tzed250

That is actually a precision and accuracy issue. Tolerances are set in the engineering print and the part is either pass/fail per the drawing.

Actually you are right. Apologies. That was a very bad and incorrect example. When I get back from the bar tonight, I will think of a better and correct one. lol
08-09-13, 22:25
Tzed250

No biggie, I deal with the issues daily ;).