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Mummichog
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Posted: 6/26/2012 4:56:44 PM

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When tightening a standard barrel nut, it is my understanding that the ft lbs are to be between 30 & 40.

Is there any relation towards accuracy if you end up closer to 30 ft lbs or 40 ft lbs at the final tightening?
Just curious as I will be putting together my first upper in the next few weeks. Thanks.
Mike_314
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Posted: 6/26/2012 5:04:55 PM
35-80 is reccomended.

Whatever torque you end up at you need to get the barrel nut lined up so the gas tube passes through it
with a little room for movement.

Put the barrel on and take it off 3 times to help get the treading right.
Use some grease on the threads too.

I don't think staying in spec will effect accuracy, I'm sure I'll be corrected if I'm wrong.
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Mummichog
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Posted: 6/26/2012 5:07:11 PM
Thank you Mike.
Obo2
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Posted: 6/26/2012 5:08:40 PM
it's 30 - 80

tighten to 30 and then align to the next hole.

It's a good idea to do the tighten loosen thing but if you are real close to alignment at 30 and you tighten loosen then you may not have enough torque and have to go to the next hole and then possibly exceed 80...

the torque may affect poi but not accuracy.
Mummichog
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Posted: 6/26/2012 5:44:50 PM
Thanks for the help!
Littlerascal56
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Posted: 6/26/2012 11:58:20 PM
I actually use the Permatex silver anti-seize on the threads and the tighten 3 times method. Then I tighten until gas tube goes thru the hole with nothing touching. Seems to work great as far as accuracy.
Sc0tt
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Posted: 6/27/2012 5:53:23 AM
permatex may contain graphite, which is bad after time. it may cause corrosion on the upper, especially if you're using a steel barrel nut. the be-all end-all grease to use is aeroshell 33MS. contains plenty of moly (molybdenum disulfide), and no graphite. helps everything snug up smooth and keeps it from corroding.
madcratebuilder
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Posted: 6/27/2012 8:37:17 AM
The recommended barrel nut torque is 30-50lbft, not to exceed 80lbft.

Anti-seize may cause corrosion on the barrel nut threads, use only a quality molybdenum disulfide grease. You torque the nut three times to 30lbsft to seat the threads, then do the finial tightening.

Personally if I do not have gas tube alignment by 50lbsft I well hone the end of the upper about .001 and try again. You don't need to remove much material to get the torque number down to 50lbsft. Tightening the barrel nut to 80lbdft of torque puts undue stress on the upper. I think optimum accuracy would be in the 40-50lbsft of torque, IMHO.
Direct-Drive
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Posted: 6/27/2012 9:41:12 AM
[Last Edit: 6/27/2012 9:46:44 AM by Direct-Drive]
Originally Posted By madcratebuilder:
The recommended barrel nut torque is 30-50lbft, not to exceed 80lbft.

I agree with this.
Where does this recommendation originate from ?


Anti-seize may cause corrosion on the barrel nut threads, use only a quality molybdenum disulfide grease. You torque the nut three times to 30lbsft to seat the threads, then do the finial tightening.

Agree.
The Permatex product mentioned above uses aluminum powder which of course is a conductor.
That would be a no-no for the barrel nut.
The idea is to isolate dissimilar metals.

In practice with all manner of products on the barrel nut, including nothing, light civilian duty allows it.


Personally if I do not have gas tube alignment by 50lbsft I well hone the end of the upper about .001 and try again. You don't need to remove much material to get the torque number down to 50lbsft. Tightening the barrel nut to 80lbdft of torque puts undue stress on the upper. I think optimum accuracy would be in the 40-50lbsft of torque, IMHO.

Brownells states that optimum is under 40.


mrraley
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Posted: 6/27/2012 10:14:23 AM
Originally Posted By Direct-Drive:
Originally Posted By madcratebuilder:
The recommended barrel nut torque is 30-50lbft, not to exceed 80lbft.

I agree with this.
Where does this recommendation originate from ?


Directly from the ARMY TM...

CAUTION
Do not torque over 80 ft-lb (108 N-m) while tightening barrel nut assembly to next
hole, to allow for proper alignment of gas tube.
It's not called "Double tapping"... It's called "Controlled Pairs!"

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Direct-Drive
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Posted: 6/27/2012 10:21:07 AM
Originally Posted By mrraley:
Originally Posted By Direct-Drive:
Originally Posted By madcratebuilder:
The recommended barrel nut torque is 30-50lbft, not to exceed 80lbft.

I agree with this.
Where does this recommendation originate from ?


Directly from the ARMY TM...

CAUTION
Do not torque over 80 ft-lb (108 N-m) while tightening barrel nut assembly to next
hole, to allow for proper alignment of gas tube.

No, that's the upper limit.

I'm asking about the 30-50 recommendation.
I've never seen that until now.
mrraley
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Posted: 6/27/2012 10:49:45 AM
I'm not sure where the "50" came from. Probably somebody second hand hear say.

I teach in my class 30 to 80... just like in the ARMY TM.
It's not called "Double tapping"... It's called "Controlled Pairs!"

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labatt
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Posted: 6/27/2012 11:16:06 AM
It has been my understanding that anti-seize is meant to prevent corrosion, etc. That's why we use it on spark plugs and lugnuts. Here is what Permatex says abiut their product from their website.

.. For Copper antiseize....A premium quality copper anti-seize and thread lubricant that may be used to prevent seizing, corrosion and galling where high temperature conditions exist.

...For the regular silver antiseize....A highly refined blend of aluminum, copper and graphite lubricants. Use during assembly to prevent galling, corrosion and seizing and to assure easier disassembly. Temperature range: -60°F to 1600°F (-51°Cto 871°C). Salt, corrosion and moisture resistant .

Could someone explain further about how aluminum and graphite lead to corrosion, when Permatex specifically says it is to prevent galling, corrosion and seizing.
Direct-Drive
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Posted: 6/27/2012 11:50:30 AM
[Last Edit: 6/27/2012 11:58:40 AM by Direct-Drive]
Originally Posted By labatt:
It has been my understanding that anti-seize is meant to prevent corrosion, etc. That's why we use it on spark plugs and lugnuts. Here is what Permatex says abiut their product from their website.

.. For Copper antiseize....A premium quality copper anti-seize and thread lubricant that may be used to prevent seizing, corrosion and galling where high temperature conditions exist.

...For the regular silver antiseize....A highly refined blend of aluminum, copper and graphite lubricants. Use during assembly to prevent galling, corrosion and seizing and to assure easier disassembly. Temperature range: -60°F to 1600°F (-51°Cto 871°C). Salt, corrosion and moisture resistant .

Could someone explain further about how aluminum and graphite lead to corrosion, when Permatex specifically says it is to prevent galling, corrosion and seizing.

Whenever you have dissimilar metals in contact the potential for corrosion (electrolysis) increases.
With our barrel nuts we have steel and aluminum.

That's why the spec calls for no graphite or metals, which are both conductors, in the product used to lube the barrel nut.
It's thought that since special moly grease, designed for aircraft lubrication was already in military inventories, it was the natural choice to put in the spec. Both applications had the same requirements of lubrication with corrosion protection.

So the idea is to isolate electrically, the two parts.
Introducing a conductor is contrary to that, although a weapon doing light duty in a civilian role would probably be OK not following the spec.
Exposure to salt water or a coastal environment might change that.

I built an aquatic center (indoor swim center) which is an extremely corrosive environment.
The architects and engineers were very adamant about isolating dissimilar metals and with good reason.

Edit:
In our civvie AR building culture here, the terms "anti-seize" and "anti-seize grease" have been made up and tossed around freely.
It's actually a lubrication requirement because our torque values are "wet torque" values.
On top of that we need to protect against galling and corrosion.

Sometimes the new builder latches onto the "anti-seize" term and runs with it.