Sheared Gas Key

[MAP]

After a class last year I noticed the gas key was loose on my CMT carrier. I removed the screws, cleaned them and reinstalled with red loctite. The screws were torqued to min. spec as per the USMC manual. I staked them using a MOACK.

At the range on Thursday I fired one (1) round. This is the first time the rifle was fired since the repair.

http://i9.photobucket.com/albums/a89/MAP1911/IMG_0909.jpg

http://i9.photobucket.com/albums/a89/MAP1911/IMG_0913.jpg

http://i9.photobucket.com/albums/a89/MAP1911/IMG_0920.jpg

Both screws sheared. It took about 15 minutes for me to get the rifle apart. No damage except to the carrier. Sad to say the first CMT carrier I had wouldn't even fit in my receiver.

Problem permanently solved when Grant hooked me up with a BCM BCG.

Mike

[mark5pt56]

That stinks, to my knowledge, any time the screws are removed they are to be replaced.

It appears by the discoloration, that the one may have had a fracture in it already and the pressure was to great for the other alone.

Mark

[Robb Jensen]

I'll give you $20 bucks for that carrier! ;)

















J/K just replace the carrier key and screws with a new BCM carrier key and screws and you'll be GTG.

[C4IGrant]

My guess is that the screws were so cheap that when MAP re-tightened them that he actually cracked the screw heads (even though he used the minimum spec). Then when he shot the weapon, the pressure snapped them off.


C4

[Robb Jensen]

My guess is that the screws were so cheap that when MAP re-tightened them that he actually cracked the screw heads (even though he used the minimum spec). Then when he shot the weapon, the pressure snapped them off.


C4

I 100% agree cheap screws break quite easily, another example of when 'but this costs less than...Colt, LMT, BCM etc.'

[ADC]

Yep they were stressed when they were tightened

[Heavy Metal]

Grant, when are you going to offer the Torx bolts?

[MAP]

I'll give you $20 bucks for that carrier! ;)


I'll be in South Hill next week. Cash only!



J/K just replace the carrier key and screws with a new BCM carrier key and screws and you'll be GTG.

Replaced the whole carrier. Ya think CMT will take it back?

Mike

[C4IGrant]

Grant, when are you going to offer the Torx bolts?


Pretty soon I think. ;)



C4

[PALADIN-hgwt]

xxxxx

[Bill Alexander]

Typical grade 8 types but look for an aircraft specification and certification over a grade type. You can get grade 8 that are made in Turkey ( fastener kingdom of the world) but I would not trust them for a pressure bearing application. Stick with Allen, Holocrome and Torx but ask for certification regardless.

As an aside the fasteners in the picture have failed from cyclic fatigue. There is no ductile tearing present on the surfaces which indicates that the initial failure may have been from corrosion. Once the crack reached the critical size it prpogated in a few cycles. The thing to note is that the corrosion finish of the fastener is equally as important as the grade.

Sorry wandering, summary; don't buy cheap stuff and don't trust all the expensive stuff either.

Bill Alexander

[tactical1]

Well said Bill - can you recommend a source to obtain quality bolts until Grant gets the Torx in stock?

Thx

[45r]

About how many rounds did you have through that carrier before it failed?

[MAP]

About how many rounds did you have through that carrier before it failed?

No more than 2.5K. One after I tightened the screws.

Mike

[jackinfl]

MAP, Grant, or Bill Alexander,

MAP, did you use a torque wrench? WHat type? and if so, how many inch pounds did you apply?

Grant or Bill ALexander, Have you guys seen this with brute force Torque settings? i.e. "I don't need a torque wrench, I will just snug it up good and tight."


Thanks,
Jack

[MAP]

MAP, Grant, or Bill Alexander,

MAP, did you use a torque wrench? WHat type? and if so, how many inch pounds did you apply?


Thanks,
Jack

I was a Craftsman torque wrench. Not the cheapest one. It was min. as per the USMC manual. I don't recall it in numbers.

Mike

[dhrith]

What didn't help you was the red loctite. With a MOACKS your pretty well assured of it being done correctly, making it redundant, but also a detriment. It likely acted as a lubricant changing the torque spec. Since you used the low end I'd imagine you still had some safety margin (off hand guess), but when you add in the possibility of a questionable spec bolt and questionably calibrated torque wrench you easily could have over done it. Hence,..... and the reason I guess this, It "appears" you "fatigue cracked" it at installation due to what appears the red loctite seeping into the crack and drying prior to it's complete failure.

[PALADIN-hgwt]

xxxxx

[markm]

Any ideas how to "unstake" the gas key?


I would just replace the key. I recently had my front bolt break in a Colt M16 carrier. I had both bolts and the gas key replaced with new parts.

I should get a pic of the stake job that the guy did on the key. It's different.

[CarlosDJackal]

Boy, that sucks!!

[WS6]

This illustrates the reason for replacing the screws rather than re-tqing them.

[Robb Jensen]

This illustrates the reason for replacing the screws rather than re-tqing them.

Very true. I imagine that after the screws have been torqued once (they stretch) that they could probably only handle the TM torque spec of 35 INCH pounds to 40 INCH lbs. (with new screws I torque to 55-60 INCH LBS)