AR upper to lower mating

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lencac

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Hi guys :neener:
I'd like to get your thoughts on something. AR-15.
Using a feeler gauge how much clearence between your upper and lower receiver is there :scrutiny:
How much is acceptable :eek:
thanks :)
 
As long as it functions properly, the clearance should not matter much. However, loose upper to lower fit bugs the hell out of me, so I glass bed my uppers to their respective lowers. End result is rock solid.
 
gotigers said:
Some billet lowers have a tightening screw thru the bottom to take up the slack
Click to expand...

This is a very good point to bring up as mine has a tightening screw in it as well and I often wonder if people miss noticing that as it's not often advertised on lower receivers (like mine, it was a very nice bonus to an awesome lower that I already loved but did not have this bonus feature advertised).

Also, as for the 'wedge', you could just use a spare foam earplug for free instead of paying $6+.
 
Hi Guys:
USING A FEELER GAUGE how much clerence do you have?
When these forgings are machined on an EDM machine there is an "allowable" tolerences. Some say .010 in. and some say .015 in. I suppose it would depend on what kind of quality control at the machine shop is followed. So either way you can have a stack-up of tolerences.
So with the intended bearing surfaces being the parting line of the upper and lower receivers, if the tolerences of the 2 receivers matched-up perfectly. But they never do, perfectly. If the rails of both receivers at the parting line are not in contact then the bearing surface will end up being where the pins go through. The further apart the upper and lower are the more "wobble", at the upper receiver pin loops will occur as the rifle gets shot, serviced, cleaned, banged and beat up. Yes :scrutiny:
I have 3 ARs that have .003 in. clearence. I have 1 brand new AR, never shot, "matched" upper and lower receiver from the manufacturer. It has .013 in.
So hence my question.
How much clearence does your rifle have? :)
 
It is a pity that most uppers are not made a little overside, so you can custom fit it to your lower.

My JP Rifle's upper was that way. With just some minor fitting, it is very solid, even without the pins being inserted.
 
Using a feeler gauge how much clearence between your upper and lower receiver is there
Click to expand...

Don't know and have never bothered to measure. Rifles shoot just fine. After a while, the wobble bugged me so I got some accurizing wedges to "fix" the wobble.




.
 
Apperently using a feeler gauge is a bit much.
I was just looking for some measurements, not rhetorical.
Thanks anyway. :rolleyes:
 
Apperently using a feeler gauge is a bit much.
Click to expand...

It varies. I've seen anywhere from 0.004" to 0.017", and the amount of upper/lower wobble had nothing to do with it.

It is a pity that most uppers are not made a little overside, so you can custom fit it to your lower.
Click to expand...

I think the market prefers a little slop with universal interchangeability to swiss-watch fitment of mated parts that are incompatible with others.

I can't stand the wobble, so I shim the front pivot. This shim is cut from 0.010" stainless brake caliper hardware, and preloads a little when closed, which keeps everything tight:

ARShim.jpg
 
I'd suggest that any $$ contemplated for buying that feeler gauge be spent, instead, on a good supply of Paxil and a couple of years of psychotherapy for that OCD! :cool::eek:
 
lencac said:
Apperently using a feeler gauge is a bit much.
I was just looking for some measurements, not rhetorical.
Thanks anyway. :rolleyes:
Click to expand...
Then maybe you should have left out the part in the OP asking for people's thoughts.:uhoh:
 
Oh,it's on now boyz :banghead:
Rhinoh, the subject matter wasn't even addressed in the first replys :(
Hentown, I AM NOT ODC :what:
Now Madcreatebuilder is on to something. Wow, looking at what .013 in. looks like, .020 in. would be unacceptable :uhoh: .............. to me anyway :scrutiny:
And I'm totally down with what MachIVshooter is layin down, if you're pickin up what I'm layin down? Using the brake hardware is ingenius :D Being a certified Master Ford technician for 28 years I can appreciate the resourcefulness.
That also might explain the OCD :rolleyes:
 
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MACHIVSHOOTER, got any extra spacers handy? I have a rubber wedge in mine and don't care for it. The slop in my M&P seems to come from the front pin area
 
Is it possible for poor fit (too large a gap) to cause problems with the bolt stop? I've been wondering about that with the issue.
 
I was the Base Weapon Inspector for many years...routinely checked for any over .020

Most very very loose, few ever went beyond .020

In an automatic, generous tolerances are usually a good thing for reliability/function.
 
The following quote is taken from The New Competitive AR 15 written by Glen D. Zediker. It has some bearing on the subject of this thread and more information.


"Everyone" seems to have the idea, and freely espouse information regarding same, that receivers "don't matter". I think they do.

I think that the lower doesn't matter as much as the upper, from a re-design perspective. They are, after all, and as Derrick Martin characterized them in The Competitive AR 15 first-edition, "cases." The lower receiver doesn't do much work, but it holds some parts that do, and that most importantly is the trigger. Quality always matters.

Whenever I've been asked by manufacturers to pitch in a couple cents on what I'd like to see done I usually include "a better upper" on my list of recommendations. A "better" upper is one that's stronger and stiffer around the threads (longer threaded area and finer threads would be good too), has a larger ejection port with no cover, better height on the rail, no forward assist appliance, and closer tolerance where the barrel extension fits. The upper is a part where materials changes might make it even nicer.

Areas of concern (those that can make some difference) is the parity between upper and lower mounting holes, and then with one another. Folks, the "fit" is in the pins, not the receivers. It's the receivers that perhaps interfere, and no doubt destine this fit, but the gap between upper and lower is not at all the "fit". What matters here is actually that there is a gap. The upper and lower shouldn't bear on one another. That causes binding and binding can indeed hurt accuracy.

Ideally, the receivers will fit together equally if we were to compare "tensions" in the front and rear mounts assembled. I'm not sure if this is measurable but someone with enough creativity and a dial indicator could probably compare movements front and rear when only those pins were installed. There has to be some room for give in the upper and lower joints. An idea that I do know has merit is to line-bore the takedown pin holes. That's a metalworking tactic whereby the parts are set in place, where they should be of course, and then the holes are drilled through both parts at once. That would make the upper and lower "fit" as good as it could be. RND line-bores its receivers, but I don't know of any other doing it.

How does fit matter? Other builders re on record saying things to the effect that "the more they rattle the better they shoot". I think that's a way to say it's not at all critical rather than it is an unyielding truth, and I'll never buy the idea that they shoot better when they are loose enough to rock. I an tell you that unyielding truth regarding upper and lower looseness says that benching a rifle and shooting it from the mat are two distinct things, and they are accordingly more or less influential in what they reveal.

Noticeably loose fit in the receivers which, to me, means they can be twisted out of sync, will mean orientation changes from the shoulder, in the shoulder. Whether this gets transmitted to the barrel is hard to judge, but if the barrel (which is firmly attached to the upper) is that free to shift, then it may also look elsewhere. Pressures the shooter puts against the rifle are as influential on an AR15 as they are on any target rifle. That has to do with the way the whole "pulse" transmits during firing. The proactive means is to put the pressure and position the same on the gun each shot. I can tell you (and boy do I know) that altering pressures and orientations intentionally sho will move a group. Altering them unintentionally is, using reverse physiology, a huge key to escalating one's classification. Anyone who doubts is encouraged to fire three 10-shot runs using one or two notches different in sling tension or using two different buttplate orientations on the shoulder, or, debunking another "myth", more or less pressure on the piston grip or forend. It's the unintentional alteration of pressure that's my big question on receiver fit. These pressure point changes don't affect a rifle being bench fired nearly as much, if any.

Fit will get worse with rifle use before it ever gets better. I think that steps should be taken to preserve fit, whatever quality it might be, and that's primarily in the receiver pins. If there's a booster of any sort bearing upper away from lower then push very firmly down on the upper receiver when replacing the back pin. If the upper has been removed all together from the lower it's probably best to install the back pin first before putting in the front. Keep both pins greased. Heavily.
Click to expand...

The author makes some excellent points as to the fit of upper to lower and how this fit overall effects the accuracy of the rifle. The book is well illustrated and the quote represents a few pages of the total 470 pages.

Another excellent book is Black Magic The Ultra Accurate AR 15 by John Feamster.

Considering the cost of a good AR 15 rifle I find it unusual that many people will not invest a few dollars in a good book or two. While every book out there should not be considered gospel the books written by well accomplished shooters should always be considered.

Ron
 
Very good write-up. I would tend to agree.
There is no way that when the barreled upper can slightly move independent of the held lower receiver, wether or not off the bench or some other, there's no way it will be as accurate as being a solid fit. This for obvious reasons. Wobble definitely detracts from the rifle's overall appeal.
I suppose too the less the upper and lower are taken apart the better.
The rub here is I always clean the barrel from the breech end.
To a small degree also a .003 in. gap would far better resist forgeign material getting into the receivers than a .020 in. gap.

I recently was sent an upper and lower by a manufacturer as a Christmas gift. I do business with them through a second non-firearm related business.
So the owner said he had the two receivers made as a matched pair. I said what does that mean? He said that both upper and lower were mated for lengthwise alignment and then the take-down pin holes were line-bored together and the width of the take-down pin locations were machined so as to require an almost press fit as the upper pin holes slide down into the lower receiver. Super tight and perfect fit and when all put together the two feel as one. However, it does have a .013 in. gap through the entire length of the receivers. The other two I have are in the .003-.004 in. range.
So I like the shim idea. Perhaps both front and rear.


So hence my curiosity :scrutiny:
 

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I don't have a feeler gauge, but my Daniel Defense upper fits my Armalite lower so tightly, that it is a chore to push out the takedown pins.

I've had it for about 8 months now, and have probably had them apart about 30 times, but it is still a tight fit.
 
HOOfan_1 said:
I don't have a feeler gauge, but my Daniel Defense upper fits my Armalite lower so tightly, that it is a chore to push out the takedown pins.

I've had it for about 8 months now, and have probably had them apart about 30 times, but it is still a tight fit.
Click to expand...
From the article I posted "Keep both pins greased. Heavily". So do you grease the pins? With a rifle that tight (not necessarily a bad thing) every time the rifle is fired there is tension changing between the upper and lower. The pins are made of steel and the upper & lower holes are drilled in aluminum. Slowly something will begin to give and I do not see the pins giving. This leads to the possibility of the holes the pins rest in becoming elongated. That being a bad thing. If this rifle is as tight as you say I doubt you could manage to get the blade of a feeler gauge in there. Grease the pins would be my suggestion.

Ron
 
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