... I can't beleive this happened.  I hardly even torqued the thing.  Snapped the bolt in half and ruined the torque wrench.



I got my new piston MRP a few weeks ago, and due to finals/work/holidays/subzero weather I have only gotten to shoot it once (~250 rounds, flawless).  On Christmas day I decided to do a full takedown of the MRP, check out how it worked, clean and lube it, etc.  The bolts and barrel came out with ease, and I am trully impressed with how slick this setup is.

Long story short though, I replaced the barrel, inserted the bolts, finger tightened them most of the way, then went and got the LMT supplied torque wrench to torque the bolts down to 140 ... big mistake apparently.  This was the first time I had used the LMT torque wrench, so I dialed it down to 100 so I could feel the breaking point to test out the wrench ... everything checked out fine, slight flex in the wrench at 100, good to go.  I dialed the wrench back up to 140, started torquing with ease, and a few seconds later there was a sickening SNAP sound and the wrench almost flew out of my hands.  I sat there dumbfounded, what the fuck, I hardly even torqued this thing ... I took out the bolt with a sinking feeling, I knew what I was going to see.

Sure enough, the bolt I pulled out was missing the majority of the threading, and that missing part was stuck in the threads.  Fuck.  Luckily though, the LMT bolts are drilled on their leading faces, so it was reletively easy to pound in a screw extractor and twist the piece out.  What you see in the picture is what I have.

At that time, I also noticed that my torque wrench had gotten nearly impossible to adjust.  It had felt weird before, but it had still moved ... now it just didnt want to budge.  When I finally did get it to move, what you see in the picture is what fell out of the back of the torque wrench ... the thing is shot.   I wonder if a bad torque wrench lead to me somehow overtorquing the bolt and causing it to snap ... but I wasn't cranking down hard at all, I was twisting the wrench with ease when it snapped.

I inspected my threads ...



They all appear to be fine, they look the same as before, no damage to them ... thank god.

It would appear all I need is a new bolt and I'm back in business ... it's just disheartening to see this happen on the first use.  

Has anyone else seen/heard of this happening? I'm kind of still in shock ... and my new $1500 upper is sitting in pieces in my build box



I'm going to call LMT CS tomorrow and see what they say about this.  I'll keep you guys posted.

Please do, mine comes in this week!

Damn!

is it ft/lbs or in/lbs? if its in/lbs it would be amazingly easy to over torque. I can't imagine it being ft/lbs, although I have no idea what I'm talking about, it looks like the bolts are used to secure the FF rail.

The bolts actually secure the barrel into the upper, that's what allows the extremely quick barrel swaps. The rail and upper are all one piece.

I'm sure LMT will make it right - they've always done good by me and everyone I know.

-Glenn

It's 140 in/lbs, which is about 11 ft/lbs. That is a very low torque, there is no way the wrench should break at that setting. I'm guessing that your wrench was defective and you overtorqued the bolt. I have the same set-up, did your torque wrench come with a calibration tag? The good news is that all you need is two new bolts (you undoubtedly stretched the other bolt) and a new torque wrench.

I have an MRP and I have yet to remove the bbl. A few people have bitched about the supplied torque wrench. I have me own tq wenches that are used in my shop (Snapon/MAC) and would never use the Harbor (china) freight one that LMT supplies with the mrp. I think they would be better off not giving you a wrench when you buy the gun if it is going to cause this kind of problem.

Do your self a favor and invest in a good torque wrench and throw the one lmt gives you away.

I do not think the LMT supplied torque wrench is meant to be adjusted since
it comes calibrated from the factory.

-1

Duggan, buy a real tq wrench man! Snapon or something.

140 what?  Inch-pounds?

Those look like shoulder bolts.  That means it concentrates the stress into the threaded portion.

Yes, INCH pounds.

Yes, 140 inch lbs.

Really sorry to see that happen to you.  Glad you mentioned it so when I get my MRP, I'll stick with a quality torque wrench.

Please do keep us informed.. I expect LMT to make it right.  They are a good company, at least from what I know so far.

that sucks.

sweet upper, though

You are correct.  It's not supposed to be played with.  They even wrap tape around the bottom so people don't try to adjust/mess up the calibration.

Yeah, 140 ft lbs is way too much. To put it in perspective, Chevrolet pickups call for 140 ft lbs on their lugnuts

He said it was in lbs.. not ft lbs.

It is a torque wrench - nothing more.  It is not a magic LMT stick (like their guns are )  If the supplied torque wrench is of such poor quality that it must not be adjusted then, they need to SAY that on the tape.  Since they do not, there is no reason to suppose that it is anything but a cheap torque wrench... that is adjustable.  

In FACT... small tidbit for y'all... torque wrenches are supposed to be stored backed all the way off of the spring.  
Matt

Just wondering, what torque are you suppose to put on the screws?

140

in-lbs.
Matt

now those anti-piston guys have proof of a broken bolt! too bad it's the wrong bolt.

We are not "anti-piston".  We are pro-engineering.  And having to use a CALIBRATED torque wrench smells of bad engineering.  Why?  Torque is highly influenced by friction.  It would have been far better to use torque to yield fasteners and a fastener without the liability of a shoulder bolt.

To me I rather have a torque value to target.  I don't consider it bad engineering.  Engineering is usually precise with acceptable tolerances.  Some people just don't know the amount of force they require to torque the screw in.  They may over tighten it and the bolt might snap due to the tightening or snap from the vibration of firing the MRP.

Think of car lug nuts.  When I was kid, I didn't realize that I can over torque the lug nuts when changing the tire.  I snapped the bolt right off and had to change it out.  That was a time where I didn't realize over tightening is bad.  

On the other hand, I've seen a tire come right off a truck due to the lug nuts not being tighten enough.. and this was right on the freeway.  That guy must have shit in his pants when his back left end dropped.

IMHO

While you obviously have some sort of engineering background, it appears that most of your comments are directed towards things you have no first hand experience with. I'd be willing to bet that you don't own an MRP rifle, or a piston AR for that matter.

I won't own something with obivious design deficiencies.  The ONLY way a piston upper can be made with aluminum upper receiver is a double piston, one on each side.  Anything else will cause off-axis rotation of the bolt and subsequent wear of an aluminum receiver/buffer tube.

I can see the function of the bolt that broke.  I can see the problem.  And why the manufacturer supplied a torque wrench as a bandage to the real problem.

The two bolts function as pinch bolts to secure the barrel extention in the upper.  Because they used a semi-shoulder bolt, the majority of the torque is borne by the threaded portion.  This is also where the majority of the friction and resistance to torque arises, the rest being in the head of the bolt.  But it is the combination of the torsion shear, CONCENTRATED in the threaded portion AND the relatively stiff shank portion of the bolt only added to the problem.

The lack of ductile failure shows this bolt is highly hardended.  Another engineering mistake.

Now what happens when a highly hardended fastener, pinching an aluminum component, experiences a rise in temperature?  As it sustained fire?  Stress on the bolts now INCREASES as the thermal expansion of aluminum is about three times that of steel.

Duggan - that really sucks.  I took mine apart the first time I cleaned it too.  I noticed that the LMT supplied torque wrench was not the greatest and I was very gentle with it.  I'm going to replace it with a decent Craftsman or Snap-On wrench.

Is this an ongoing problem? I mean the standard MRP has been out for awhile now and I have not seen any other broken bolts. Has anyone had any problems with the standard MRP bolts? Or is the suggestion that this is only related to the new piston MRP's?

Thanks!

Completely agree



You must have a good eye... because I can't see that from the pictures... nor would you be able to unless you are looking at the actual point of shear (which is 90deg off in the picture, and, as such, not visible).  I'll admit, though, that the odds are in your favor for that being a hardened (but perhaps not "highly hardened") bolt.



That part of the upper receiver has never gotten too hot to touch on mine.  Then again, I don't do full-auto stuff.  

I agree with you that the gas-piston gun is completely retarded on the the M-16/AR-15/M-4 platform.  I do not agree that the MRP is a bad design... it is a design that might could use some tweaking, but it is is not fatally flawed in the manner you seem to suggest.

My first question upon pulling my bolts out for the first time was... is this a wet or a dry torque value?  Then... How often are these bolts supposed to be replaced?  Then, I remembered it isn't an airplane, and it really doesn't matter.

I think LMT puts the tape and the "calibration" thing on their torque wrench is to keep too many people from bitching about the piss-poor quality of the torque wrench supplied with the kit... I don't think the MRP itself gives two shits whether you use a snap-on or the supplied torque wrench.  
Matt

Steer away from the Craftsman - the only part they warranty for life (like the rest of their tools) is the ratchet head - not the torque-wrench part of the torque-wrench.  Ask my how I know....
Matt

That is why often times torque requirements are "above rundown" i.e., the amount of torque from the friction of the bolt engaging the threads.  No need to get so technical, it is a method that is good enough for NASA, so I would say no need for any other method.

Well, if torque is sufficient for this application, why did it fail?  

Torque past 10% of the bolt yield has wild ranges in ultimate fastener tension.  Torque to yield is far more accurate but this means the fastener is a one-time use piece.  Ok for things like rod end caps.  The same technique used for TTY fasterner installation can also be used to effect a more precise ultimate fastener tension.

If those bolts were oiled before torquing would that have caused this?  Reason I ask is because I have seen torque charts which refer to a wet and a dry torque.  I think the wet torque limits were lower.

Edit to add: That would not explain the wrench breaking, would it?

wet torque values are lower because the friction of thread against thread is a lower portion of the overall torque (stretching of bolt + rotational friction = total torque).  The goal of torquing the bolt is to stretch the fastener a predetermined amount.  If the friction is reduced, the bolt is stretched more for a given torque... hope that helps.  

To be honest, you really have to crank down on a hardened bolt WELL past 140in-lbs to get it to shear like that... unless the bolt itself is faulty.
Matt

I would never use a China brand torque wrench, most of their tools are junk, get a Snap-on or Mac, We have a man that comes by our shop once a year and checks the calibration of ours. I would go to a bolt vendor and buy two new screws from the looks of the size, would cost only a couple of bucks, and put the upper back yo use. For what they charge for that upper they could furnish a decent wrench.

And if the bolt did have a defect that would not cause the wrench to break, would it? Doesn't seem likley to me that both would break.

I agree.  I think the wrench failed and the OP romped on the handle in an effort to get it to pop, and the bolt sheared.  
Matt

That's it. Harbor(made in China). Those Commie bastards are coming for us.

This.

Clicker torque wrenches are nearly impossible to calibrate over their entire range.  

I calibrate mine for the torque I need.  Doesn't take long at all nor does it take anything but a good scale, a combo wrench that fits the square drive end (3/8" or 1/2") and a gallon milk jug or two.  Plus water, lead shot/bullets/wheel weights.

First, set the torque to what you need.  For this case, 140 inch pounds.  Then set the wrench in a vise to hold it and allow adjustment.  Now put the open end of the wrench on the torque wrench and measure center to center distance.  Say it is 4 inches...140 inch-pounds divided by 4 is 35 so you need 35 pounds of weight.  Hang the weight by a wire and make sure the wrench is about level, small differences are insignificant.  But consider the angle offset of the wrench and adjust accordingly.

Now adjust the wrench so it just clicks when the weight is applied.  That will be 140 inch-pounds.  Note, longer wrenches make for more accurate measurements.  Why?  Because there is less bearing load on the wrench.

*Files away for future use*
Matt

You can even CALIBRATE it so the scale is more correct.  Most use a single point calibration, usually on the mid-scale.  But at either end of the scale, the wrench will have greater error.

How to do it:

Like above, only take the end nut off.  It is standard right hand threads.  LOCK the barrel with the tailscrew.  Unscrew the nut and remove.  Now unlock the tailscrew and turn the threaded insert WHILE HOLDING THE BARREL.  Turn in to increase the actual torque, back out to reduce.  Once calibrated, LOCK the tailscrew, then put the nut back on.  Test and use, knowing you have a more accurate torque wrench.

The OP's LMT supplied torque wrench looks to have stripped the treads on the retainer.  A small screw retains the tailscrew locking mechanism, the scrw probably backed out.

Threads like this make me rofl at ppl who think anyone who owns a gun is a "stupid hick" I see more ingenuity and doctors lawers and engineers(me included after graduation) here than anywhere else on the webnets.  I work at a restaurant currently so intelligent discussions are few and far between.  Actually my managers think im stupid cause ill try to explain something to them and they have no clue what im talkin about[>:/].  Sorry, had to vent a little - back on topic.

LMT has a good rep around here so im sure the'll get ya fixed up.

Improvise, Adapt, Overcome.

I know what your talking about when you say the manager doesn't understand what can and can not be done.  Feel the same way about marketing.

Hmm, very good information ... I woke up too late today to call CS (damn night job), but I promise I will do so tomorrow.  

Also, please link an "acceptable" quality torque wrench ... all the ones I see are like ~$200 and that's a bit much for something I'm going to use like, 3 times ... but by the same token I don't want to snap another bolt.

As far as piston setups being inferior ... I dunno.  I'm an economist not an engineer, but I've owned a few ARs now and wanted to try something new and cutting edge.  I've always wanted an MRP, had heard great things about piston reliability and cleanliness, had heard piston kit guys rave about their guns ... I decided to try it out.  I figure worst case scenario I would sell the piston barrel and BCG for potentially profit, as they are still hard to come by, and I'd get a proven DI setup.  However, for the ~250 rounds I have thus far, it has been flawless, clean as hell, and an overall sick setup, my favorite AR thus far I've built.

I just wish this stupid bolt hadn't snapped Ah well, to bed, cheers.

This type of torque wrench uses a heavy coil spring and varying pretension on it to vary the torque setting .  Even temperature will change the torque setting.  They are by no means calibrated over the entire range and can be as much as 10% off at the extreme range of indicated settings.  140 inch-pounds is at the low end of the scale, my cheapie goes up to 960 inch-pounds so its mid-point calibration is around 420 inch-pounds, hence the need for specific calibration.

We are talking about these bolts.  Torquing them to 140 inch-pounds indeed stressess them.  The bolt that failed had a stress concentration factor of over 2 at the threads because of the design in using shoulder bolts to effect a pinch condition and stress in the threaded region was well over 75.000 at the 140 inch-pounds of torque.

Keith_J is pretty dang smart.

Was the piece of broken bolt hard to extract?  Seems like it's a very small. What kind of tool did you finally use to extract it?

My first CQB16 comes home tonight... I'll keep an eye on this thread to see where this all goes.



FWIW, we're only talking about 11.67 FOOT - POUNDS of torque on these bolts. You could do that with a SCREWDRIVER. At least I could, and do on a regular basis. From what I've see thus far, two things are apparent: the bolt failed AND the wrench failed. There's an inverted bell-curve for failure on most anything man-made. The highest failure rates are seen (1) very early in life, and (2) at the end of the 'design-life'.



Not knowing what the capacity of the wrench [provided by LMT] is, 'locking' it in at a set value may not be such a bad thing. The best way to store an adjustable torque wrench is to 'dial' the torque value to ~10% of max. This keeps some tension on the spring, and in turn prevents the 'adjustable part' from unthreading completely while in storage/transit.



Wet torque vs. dry torque... In THIS particular instance, I don't believe it matters. LMT had to come up with a number. 'Tight' is relative unless it is somehow defined. The 'number' keeps people from going Gorilla on the hardware, and maintains a factor of safety at the same time. One bolt engages a 'notch' in the barrel, effectively keying it in place and preventing rotation, the other is there to add clamp force. Between the two, when they're fully engaged / torqued to spec, the barrel is effectively retained in the upper. Brilliant, simple design IMHO.



BTW, it would be difficult to bugger up the threads on / in the receiver... The part that the screw itself engages is an insert - like a Helicoil. Easily replaced if necessary. The screws are likely an LMT exclusive as well... I've never seen a shoulder bolt like the one's pictured; the major thread diameter to shoulder diameter ratio is almost 1:1, and the head to shoulder ratio is more like a standard SHCS (socket head cap screw). But even then, there's no knurl around the head - like there would be on a SHCS.