AR-15 barrel nut torque

[lencac]

Howdy guys/gals
Who knows what the correct torque for the barrel nut on an AR-15 is.
tanks a lot

 

[Howard Roark]

Min 30 ft.lb. - max 80 ft.lb. to get it to align with the gas tube hole.

 

[rcmodel]

It is supposed to be 30 lb-ft for about three tightenings to seat the threads.
Then, final assembly is 35 lb-ft.

But it really is where the hole for the gas tube lines up perfectly with one of the scollop's on the nut.

That will very seldom turn out to be 35 lb-ft.

rc

 

[briansmithwins]

This manual has those values on page 3-40, along with many others. Feel free to share: http://dl.dropbox.com/u/9136061/Gun Stuff/Gun Manuals/usmc_m16a2_manual.pdf

BSW

 

[lencac]

rc so we meet again
Thanks for the input and I understand. That's pretty much what I had figured but thought I'd check.
Being it's looking like the days of building 1903's and A3's is about over for lack of finding usable receivers I am going into the black gun thing.

 

[MrCleanOK]

Grease the threads on the upper. Tighten the barrel nut to 30 ft-lbs and then remove. Repeat. Tighten the barrel nut past 30 ft-lbs until the next available notch in the barrel nut lines up with the hole in the upper receiver. Do not exceed 80 ft-lbs.

 

[TonyAngel]

Yeah, be sure to grease those threads really well to prevent gaulling. Although attaching a barrel is easy in theory, problems can present themselves. Getting to the 35lb torque spec is easy, lining up the next notch for the gas tube may not be so easy. Depending on the quality of your barrel wrench and the quality of the barrel nut you are using, the "teeth" on the barrel nut may give way at around 50lbs of torque. When you get to 50lbs, you're putting some pretty good pressure on the wrench.

What I usually do is to keep a few barrel nuts around. Sometimes, you can get lucky when one nut won't work. Try another and it might work. If I try a couple and it won't get to where I want it, I use an upper lapping tool with lapping compound to take a 1/1000th off at a time until I can time the gas tube. It usually doesn't take much at all. Ten or fifteen seconds with the drill. A hairs allow a lot more movement of the nut. I really don't like to torque too much and have never torqued up to 80lbs. I'm just afraid of galling the upper.

I've also found that the nuts that come with a lot of the free float tubes that are available make timing a lot easier because the timing holes are closer together than they are on a mil spec nut.

Just whatever you do, be sure to get a GOOD barrel wrench and a fixture to hold the upper while you torque the barrel nut. Don't try to skimp by going with the minimum spec or just under it. I've seen plenty of nuts come loose after firing because they weren't torqued tight enough.

 

[Fumbler]

I've got a related question.

I have a barrel nut wrench that has a half inch square hole to attach a torque wrench.
Once you attach the barrel nut wrench to the torque wrench, what position should the two wrenches be in in relation to each other?

Basically my concern is that the hole in the barrel nut wrench would be about 2" away from the center of the barrel. If the two wrenches are in line with each other and you set the torque to 30 ft-lbs, wouldn't that mean the actual torque at the barrel nut is higher because you have an extra 2 inches of leverage?

It seems to me that you'd want the two wrenches perpendicular to each other.

 

[briansmithwins]

Down load the manual I linked to earlier. It has a diagram showing the correct wrench position. BSW

 

[Fumbler]

Ah, thanks.
Looks like they should be in line. That probably does result in more than the torque you set the wrench to, but that doesn't matter as long as you measured it the same way the manual does.

 

[MrCleanOK]

You are correct. Using the wrenches in-line does give an inaccurate torque reading, but do it anyway as this is how the manual instructs.

 

[Shawn Dodson]

The barrel nut wrench should be 90 degrees, right angle in relation to the torque wrench. When it's in line it applies more torque than indicated on the wrench. The barrel nut wrench is in essence, a crowfoot extension that increases the "effective length" of the torque wrench by 2 inches when installed in line. The illustration in the tech manual depicts an incorrect method. (If I were to attach the barrel nut wrench in line on my Sturtevant Richmont CCM1200I micrometer adjust snap-action torque wrench and set the wrench to the maximum torque value of 80 ft/lbs (960 in/lbs) and apply the maximum torque to the barrel nut to align the gas tube notch, then I'd actually exceed the upper torque limit by 10 ft/lbs (120 in/lbs). I'd have to set my torque wrench at 71 ft/lbs (850 in/lbs) with the barrel nut wrench in line to ensure I don't exceed 80 ft/lbs).

USE OF TORQUE WRENCHES WITH EXTENSIONS Space limitations sometimes make it necessary to increase or decrease the length of a torque wrench or to change the angle of drive. The torque values indicated DO NOT account for modified torque wrench length or angle. The effect of using an extension or crowfoot is determined by its position on the torque wrench. When the extension or crowfoot (E) is pointed in the same direction as the torque wrench (L), the effective length of the extension or crowfoot is added to the effective length of the torque wrench (L + E). When the extension or crowfoot is pointed backward toward the handle of the torque wrench, the effective length of the extension or crowfoot is subtracted from the effective length of the torque wrench (L - E). When the extension or crowfoot is pointed at a right angle to the torque wrench or when an extender is added to the handle, the actual value does not change and no corrections are necessary. Determine reading required on torque wrench when using an extension or crowfoot as follows:

(L) is the distance from the center of the drive mechanism to the approximate center of the handgrip on the wrench or handle extender (H).

1. Multiply required torque by effective length of torque wrench (L).
2. Divide this result by (L + E) or (L - E). Result is setting or reading required on torque wrench to obtain necessary torque.

For example:
Maximum torque = 80 ft/lbs
Effective length of torque wrench = 16 inches
Effective length of crowfoot wrench extension = 2 inches
Therefore, (L + E) = 18 inches
80 x 16 = 1280
1280 ÷ 18 = 71.11
Set the torque wrench to 71 in/lbs

 

[lencac]

Hi guys
Ok, first, thanks for all your input. Much appreciated
Anyway, I'm waiting for the rifle kit to arrive. Should be Tuesday.
I do have the lower receiver. I've had a few AR's but never built one from scratch. For some weird reason I liked building and restoring back to spec 1903's. However, so far I'm pretty stoked. I must admit though for some time I took a deep disinterest in AR's. Everybody else's wife drove a Ford Taurus and every guy's hotrod has a small block Chevy 350 engine .................... and everyone has a AR15
However, this fact alone does not change another fact. The fact that this "AR15" thing is still minimum REQUIRED equipment. SOP as it were.
Anybody understand
However, I cannot deny the fact, I just plain like the AR format.
Here's a few pics of what I'm starting with.

 

[lencac]

I was looking through the manual that brian was so kind to provide. I found an interesting statement. "Torque is measured with both wrenches used together". And it is most definitly showing the position of the wrench in relation to "the other" wrench. The manual specifies 31 ft/lb to 35ft/lb of torgue. Using the formula so kindly provided by shawn I calculate the true torque is supposed to be in reality more like 35 ft/lb to 40 ft/lb., at the nut center.
So here's the plan. Get it all set up so the threads are clean and lubed. Barrel seating area to the receiver is checked with machinist's bluing. Then prep so barrel nut will draw up tight without galling the threads to the torque that will be needed. I think antiseise would be ideal as lube but caution is really needed so none of the antiseise comes in contact with any of the external surfaces. It will stain it. So, I'm going to torque it to like 35 ft/lb., then start to look for the gas tube hole to line up.
Yes?

 

[Shawn Dodson]

1. Torque barrel nut 30 ft/lbs.
2. Loosen barrel nut.
3. Torque barrel nut 30 ft/lbs.
4. Loosen barrel nut.
5. Torque barrel nut 30 ft/lbs.
6. If barrel nut gas tube notch is not aligned with gas tube hole in upper receiver then set torque wrench to 80 ft/lbs (or the corrected setting/indication as indicated by the formula).
7. Torque barrel nut until gas tube notch aligns with gas tube hole in upper receiver. Do not exceed 80 ft/lbs.
8. If 80 ft/lbs is reached before barrel nut gas tube notch aligns with gas tube hole in upper receiver, replace barrel nut and repeat steps 1-7.

I found an interesting statement. "Torque is measured with both wrenches used together".

You can't measure torque without installing the barrel nut wrench onto the torque wrench.

And it is most definitly showing the position of the wrench in relation to "the other" wrench.

Again, THE ILLUSTRATION IN THE TECH MANUAL DEPICTS AN INCORRECT METHOD OF ATTACHING THE BARREL NUT WRENCH TO THE TORQUE WRENCH!

If the manual specified a particular torque wrench to use then the illustration would probably be correct for that particular torque wrench. However the illustration is incorrect because ANY torque wrench of adequate capacity can be used. The illustration does not take into account the differences in effective length of any suitable torque wrench.

The upper torque value is the most critical as you don't want to exceed it. However, on the flip side of the coin, there's the possibility that the barrel nut notch will align at the lower torque value, but if you install the barrel nut wrench incorrectly onto the torque wrench you may turn the notch past the gas tube hole before your torque wrench indicates minimum torque (which is actually greater than what the torque wrench indicates). Then, as a result of applying too much minimum torque, you may exceed 80 ft/lbs before you can get the next notch to align.

Using the formula so kindly provided by shawn I calculate the true torque is supposed to be in reality more like 35 ft/lb to 40 ft/lb., at the nut center.

Incorrect. The minimum torque value is 30 ft/lbs. You don't need to use the formula if you attach the barrel nut wrench onto the torque wrench at a right angle. You *should* use the formula if you attach the barrel nut wrench in line with the torque wrench as shown in the illustration. The formula will indicate the correct setting/indication for the torque wrench to apply the specified torque. (The torque setting/indication value produced by the formula will always be lower than the specified value.)

Cheers!

 

[lencac]

THE ILLUSTRATION IN THE TECH MANUAL DEPICTS AN INCORRECT METHOD OF ATTACHING THE BARREL NUT WRENCH TO THE TORQUE WRENCH!

I see the manual is very specific with a very clear drawing and very clearly making the statement about useing both tools together.
I see that, I read that in the manual that is used by the U.S. military as the source for all things AR.
But shawn says it all wrong. What proofeth doith shawn haveth? We need a witness

Having a torque spec that can be anywhere within a 50 ft/lb. spread it won't even be an issure of starting at 30 or 35 ft/lb., before you start lining up the gas tube hole. When it's all there at that moment of clicky clicky, or in this case buzz buzz on the wrench, I think we will know what feels right.

 

[briansmithwins]

Interesting.

This manual also has the torque wrench setup like the Marine manual: https://docs.google.com/fileview?id...gxN2EtNTgyNjAxMzBlZmIy&hl=en&authkey=CIfWs_0O

Page 3-31, if anyone is interested. BSW

 

[Tirod]

The engineers who developed the armorer's instruction book for the Army in the 1960's weren't uneducated fools. They set up the illustration and specific instruction for basic level assemblers to torque the nut without screwing things up.

Add in using lubrication on the threads, the INDICATED 30 foot pounds is much higher at the thread root. Setting a maximum INDICATED value of 80 foot pounds also means that's not the actual value.

Most torque values aren't that precise anyway. There are too many variables, and normally, NO lubrication is allowed, threads should be DRY. In this case, the engineers figured all that out ahead of time, gave us the working INDICATED values, and the armorer gets the job done.

It's not a graduate thesis kind of thing.

 

[Onmilo]

No it isn't rocket science, no doubt.
I lubricate the receiver threads with grease to prevent galling of the disimilar metals and set the torque wrench to 40 foot pounds, put it inline with the wrench and bear up until everything lines up.
Never ever, not even once, have I had a barrel nut or a free float handguard assembly come loose from use and I have done a boatload of the damn things,.

Savage 110 bolt action rifle barrels are even easiler to install.
The Engineer who designed that system should be given a medal and a steak supper!

 

[Shawn Dodson]

I see the manual is very specific with a very clear drawing and very clearly making the statement about useing both tools together.
I see that, I read that in the manual that is used by the U.S. military as the source for all things AR.
But shawn says it all wrong. What proofeth doith shawn haveth? We need a witness

I used to write tech manuals for the military - Standard Maintenance Procedures, Fault Isolation Procedures, Standard Operating Procedures, and Maintenance and Repair Procedures. Incorrect illustrations are common and hard to get corrected once published. As a field engineer I've worked to get incorrect illustrations changed, sometimes unsuccessfully. (In one case an illustration depicted the wrong tool and the wrong use of the tool to replace a missile door fastener. The document manager agreed to change the illustration to show the correct tool but refused to correct the depiction in which the tool is used - despite vehement argument from me and my department. I finally got to say "I told you so!" a couple of years later when a weapons facility complained that they couldn't complete the procedure because they couldn't get the tool to work as depicted in the illustration. This was nearly 20 years after the original illustration was published.)

The "USE OF USE OF TORQUE WRENCHES WITH EXTENSIONS" and formula I posted are directly from a nuclear weapon maintenance manual. I work directly on nuclear weapons and ballistic missiles. The shop practices and disciplines with nuclear weapons and aerospace are formal and leave nothing to chance. (Indeed we have to "exercise" torque wrenches in accordance with published SOP before we use them.) I'm very familiar with torque wrenches of all kinds and crowfoot extensions because I've been trained in their proper use and I've used them extensively for over 30 years.

I've shown you, by way of the formula, that if you install the barrel nut wrench onto the torque wrench as depicted in the tech manual that you will exceed 80 ft/lbs maximum torque. If I attach the barrel nut wrench in-line with the torque wrench as depicted in the illustration and set my torque wrench to 80 ft/lbs then I'll end up applying 90 ft/lbs if I reach maximum torque.

It's possible that decades ago the tech manual specified a particular torque wrench to use and the illustration was correct for that torque wrench. Over time the manual may have been changed to allow use of any general-purpose torque wrench but neither the illustration nor procedure were updated. Stuff like this happens all the time.

You're welcome to ignore my advice. It won't be my upper receiver that might be damaged by applying excessive torque to the barrel nut.

Cheers!

 

[UniversalFrost]

you go by the min 35 and max 80, but i choose in the middle and make sure that gas hole lines up.

 

[ambidextrous1]

Keep plugging, Shawn! Don't give up!

 

[lencac]

Good input guys
I was a Certified ASE and Ford Master Technician for over 25 years. Spent most of that time working the line at the dealership as a driveability technician.
I'm familiar with torque wrenches and I'm well aware of the variables. And also have learned and earned the 6th sense ................ a feel for the machinery. It's an intagable but it is a factor that can and does make the difference between 2 guys looking at the same task and each having totally different results.

 

[wideym]

I tend to use German torque wrench specifications-guduntight.

 

[migkillertwo]

On a well-built upper receiver, I just tighten it until it is hand tight, and then align the next closest hole with the gas tube slot in the receiver.