AR upper pivot pin bushing

In a series of 80% lower builds , I'm working with a "standard" 5.56 upper. Think "purchase based on price".

The purchased upper is sloppy beyond acceptable at the pivot pin. It might be .003 over sized. That needs fixed!

Over sized take down pins are available, but not that large, and reaming the lower would be needed even if undesired.

So I'm thinking "bushing" the upper hole.

Metal filled epoxy?

Ream to fit an aluminum (or steel) bushing, then redrill?
What would be the acceptable OD of the bushing?

What is the "best" clearance between upper and lower?

Sure this is all AR "gun plumbing", but someone must have an opinion. Better yet, have you re-bushed an AR upper?

You can just drill and tap the front lug for a setscrew from the bottom if you think it moves too much. With a scoped upper, the loose fit doesn't really make much of a difference in accuracy, it is a shooters aid only. With iron sights it is important as there needs to be a solid cheek weld, and no movement in relationship to the rear sight. We have used various methods to fix this, rubber inserts, bedding, springs, hidden bolts, setscrews, etc. We test uppers for accuracy in a mount with the only the uppers float tube clamped. This is similar to what we use.

MilGunsmith said:

You can just drill and tap the front lug for a setscrew from the bottom if you think it moves too much. With a scoped upper, the loose fit doesn't really make much of a difference in accuracy, it is a shooters aid only. With iron sights it is important as there needs to be a solid cheek weld, and no movement in relationship to the rear sight. We have used various methods to fix this, rubber inserts, bedding, springs, hidden bolts, setscrews, etc. We test uppers for accuracy in a mount with the only the uppers float tube clamped. This is similar to what we use. View attachment 234435

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I like the set screw suggestion! Thanks. I'll give that a go.
I just don't like the rifle parts slopping around. The others don't and this one shouldn't and won't!

I've got take up screws at the rear take down lug, and as this is an 80% lower, the fit of the lug in the pocket is machined "close".

I run irons, so the way things feel makes a big difference to me.
Thanks for the pic of your test rig. 'must be nice!

Here's the old die maker's trick for a too loose pin - works 90% of the time: take a ball bearing a little larger than the hole that is oversize. Place said ball bearing on the top of the hole - rap it hard with a ball peen hammer - no more loose pin! I would do that before going through any more trouble until it becomes loose again. Then it's time to bush. Being aluminum it will mush plenty and give you enough meat to press the pin back in. DON'T ream it again. Simply tap the pin back into place. The pin will serve to displace the "peened" material and give you back a nice fit. Works like a charm most of the time.

Joe Miranda said:

Here's the old die maker's trick for a too loose pin - works 90% of the time: take a ball bearing a little larger than the hole that is oversize. Place said ball bearing on the top of the hole - rap it hard with a ball peen hammer - no more loose pin! I would do that before going through any more trouble until it becomes loose again. Then it's time to bush. Being aluminum it will mush plenty and give you enough meat to press the pin back in. DON'T ream it again. Simply tap the pin back into place. The pin will serve to displace the "peened" material and give you back a nice fit. Works like a charm most of the time.

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Thanks for the "ball peening" reminder. I've used the method on static pins etc. and had considered as much in this case.
But..... ;-)

It would be nice to pull the upper down into the lower, and the contact area of a peened ridge suggested a short lifetime for the fix.

But considering how easy it will be to perform the ball peening, I will likely add the process to the fitting of the set screw.
The lug on the upper has quite a substantial countersink at the pin hole entrances. I'm not sure if that will make things better or more difficult.

The two combined should be about as good as it gets!

If I was going to make a "long term" repair I would install drill jig bushings and that will serve you for the rest of the life of the gun (or yours).

Joe Miranda said:

If I was going to make a "long term" repair I would install drill jig bushings and that will serve you for the rest of the life of the gun (or yours).

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I would worry about leaving enough "meat" on the tabs to fit drill bushings.

I did consider Valve Guide restoration sleeves. however ;-)

Hmmmm..
Pondering this myself.
I am chasing a thou or two slop in trigger and takedown pins.
These guys have various oversized pins:

Tactical Innovations

And

https://m.youtube.com/watch?v=FyAL5CoQ1D0


Anyone have the spec for the rear pin hole size in the upper?
This upper in front of me is .275” for rear hole and .252” for front.

I have both holes at .252” in lower.


Edit- Correction: the rear hole in upper is oblate: .255” high x .275” long.
I have to assume this is intended (mil spec?) to aid fit up/thermal changes?

The takedown pins in the LPK I have are .247” so leave just a hair too much slop for my liking..

The slop doesn't affect anything. We have had upper and lowers with .125" of movement that still shot well as long as the barrel is good. Tight fit is for the shooter to feel good. Accuracy is all in the upper. Sights are all on upper. When we use a test mount, we only grab the float tube or upper rail.

MilGunsmith said:

The slop doesn't affect anything. We have had upper and lowers with .125" of movement that still shot well as long as the barrel is good. Tight fit is for the shooter to feel good. Accuracy is all in the upper. Sights are all on upper. When we use a test mount, we only grab the float tube or upper rail.

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You could say all those same things about a bolt gun also.

But no one does.

eta

Think about it. with the AR construction, the first thing that happens after the sear is let go, is that the hammer in the lower, smashes into the firing pin in the upper. That impact has got to jostle the POA.
Plus, slop is sloppy. There are enough sloppy things in the world. my firearm doesn't need to be included./

We shoot thousands of rounds a day in testing and it doesn't make a difference.

MilGunsmith said:

We shoot thousands of rounds a day in testing and it doesn't make a difference.

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Machine rests don't have a need to feel "right".

Next time, have the machine rest hold the fire arm as gently as a feather with the trigger pull, and Just snug against the butt plate stop (whatever that might be).
Make sure the machine is breathing right, holding right, and following through. Then do the groups.

You see, there is no way a human shooter can hold a rifle like a machine does. And that makes a difference.
Sleds are an entirely different kettle of fish. Machine rests must be also, by their very nature.

Do you get a chance to shoot bolt guns with 1/8" of slop so you can tell all the dedicated target shooters they are all wet and have been doing it wrong all these many years? You will need to show up at the line and show everyone what fools they have been.

I do not mean you any disrespect, It's just that your methods and experience are not mine, and I think I speak for thousands of other shooters.
If it doesn't feel right, accuracy suffers! It's the bottom line

The machine rest we use doesn't touch the lower or the butt. This slop issue only applies to AR type rifles. I am a International Distinguished Shooter so I know how a rifle needs to be shot.

MilGunsmith said:

The machine rest we use doesn't touch the lower or the butt. This slop issue only applies to AR type rifles. I am a International Distinguished Shooter so I know how a rifle needs to be shot.

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Right. The machine only holds the barrel. A shooter holds the Upper in only one hand, while the lower gets a hand, a cheek, and a shoulder.

I wonder what the results would be if the machine only held the lower.

I've seen the "proof testing" of bolt guns. The barreled action is held in the machine SANS any stock or woodwork. The holes are punched, the card signed, and then the purchaser wonders how to get those results in real life.

We use the same method of testing as the U.S. Army Marksmanship Team and the USMC Team, they all win at Perry.

MilGunsmith said:

We use the same method of testing as the U.S. Army Marksmanship Team and the USMC Team, they all win at Perry.

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Do those teams accept a sloppy fit? Not from my understanding.
There are "methods" that the armourers use to take out the slop. Selection, Fitted horse shoe shims around the take down pin. shims at the pivot etc.

Maybe it is all "in your head", but at any level, shooting is "all in your head". A sloppy fit will "f%*k with your head like nothing I know. Worse than a gritty trigger. (well maybe not worse ;-)

Besides, The thread is about how to remove the slop, not if it is needed according to your preference.
If the thread was "just because I want to", have you got anything to contribute?

From your daily activities and experience, I would say you do. You test barrelled actions objectively and with as few variables as possible (I deduce from you pics and commentary) In reality, marksmanship is a combination of mechanics and "human intangibles". Fit and feel go a long way.
An example of what I am driving at is that NONE of those teams you mention shoot rifles with no stocks or fore grips. Stocks are fit at the highest levels. Sight heights are optimized for each shooter. Rifle weights and trigger pulls are "customized" to each shooter's preference. None of these accommodations are required of a barrel in a machine rest. Can you really say there are top level teams that desire a sloppy fit between upper and lower because "it just doesn't make a difference"?

I think not.

So far, I feel you have missed the point, since the machine rests hold the barrel, You dismiss the lower as irrelevant. I agree!.... If a man could hold a rifle the way a machine rest does.

Alas, we can not.

eta

a clip from the wide whyrled of web

"That’s what the “National Match” takedown pins our reader is talking about do. They’re slightly larger in diameter, which increases the friction between the upper and lower receiver and keep eliminates the shimmy."

Excuse the grammar usage and mechanics. The issue is well known.

As I said in an earlier post, removing the slop is a "Shooters Aid" it has nothing to do with how accurate the rifle is. An M14 or a bolt gun need good bedding to be accurate.

A comment "from the internet" take it as you will.

Some National Match builders are actually epoxy bedding the upper and lower.
Some just put a release agent like wax on one part, spread epoxy on the other, close them gently and let the epoxy cure.

A more involved technique is to drill shallow holes and pack epoxy in the holes. I've seen one where a builder used a milling machine to machine shallow grooves which were packed with epoxy.
I've wondered if drilling holes in key spots and installing Allen head set screws might be ever better, and they could be adjusted for a perfect fit.

Looseness has no effect on mechanical accuracy, but movement between the upper and lower can be disturbing to a Match shooter.

[End quote]

ETA there are precious few places on the upper and lower interface where set screws could be installed.

A few , but the real estate is limited.

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MilGunsmith said:

As I said in an earlier post, removing the slop is a "Shooters Aid" it has nothing to do with how accurate the rifle is. An M14 or a bolt gun need good bedding to be accurate.

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Can you explain the separation of function?

Well and ANOTHER thing...lol

I don’t like undersized pins in anything that loads the pins- it just accelerates wear.
It seems the firing impulse in the ar platform mostly transfers to the lower through the rear fit but still- a loose pin is starting on the wrong foot.