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9/22/2017 12:11:25 AM
Posted: 8/9/2005 6:20:20 AM EDT
A neighbor bought a Bushmaster AR-15 "as-is" at a recent gunshow. 20" HBAR barrel, A2 configuration. He's asked me to get it running for him.

He discovered that takedown pins don't stay in place when the spring and detent are both missing. Easy fix. Parts are on order.

Also, it was shortstroking badly. Turns out the carrier key was so loose it would literally rattle on the carrier.

According to the manual, you're supposed to tighten the cap screws to 35-40 inch-pounds. I don't have the proper tools to measure the amount of force. Is it critical for safe operation to have that measured exactly, or is "really hand-tight" good enough?

Also, the cap screws were not staked properly. According to the manual they're supposed to be staked in three places evenly spread out. These were only staked twice. Is that normal for Bushmaster, or is it a sign of an aftermarket change?
Link Posted: 8/9/2005 7:03:30 AM EDT
Link Posted: 8/9/2005 7:32:05 AM EDT
Roger that. Thanks, Tweak.

Should I simply punch the current stake marks to reset them, or should I stake "fresh?"

(The above is what I did for testing over the prior weekend - hand-tightened the cap screws and staked by punching the existing marks. Worked fine with USGI mags but had FTF and failure to lock open with British mags. But it may have been the gas key loosening again? Will do more testing this weekend.)
Link Posted: 8/9/2005 8:44:16 AM EDT
Link Posted: 8/9/2005 9:50:24 AM EDT
Yeah, I'm trying to salvage the current carrier key and cap screws. If they still won't play nice I'll try your "lapping trick" I heard about (light cutting oil on the carrier, stroke across 30 times to help mate the surfaces..hmm, it's not what it sounds like!). If that still doesn't work and it is still the gas key leaking, I'll tell him he needs a new one (and new screws). I can get those from Brownells.
Link Posted: 8/9/2005 11:38:58 PM EDT
Link Posted: 8/10/2005 4:01:42 AM EDT
Yikes. A DPMS carrier key is $10 from Brownells. A Colt carrier key is $50 from Brownells. At the risk of starting yet another "Colt rules/all else sucks" thread, is it worth the extra $$$ for the Colt carrier key?
Link Posted: 8/10/2005 4:18:19 AM EDT
Link Posted: 8/10/2005 4:40:01 AM EDT
[Last Edit: 8/10/2005 4:45:45 AM EDT by QUIB]

Originally Posted By Tweak:
used parts just require more torque since the screws have been stretched a little.




Tweak, I am in no way doubting your expertise in AR maintenance but I can not see how 30-40 in lbs of torque will stretch a bolt! That’s practically hand tight!

I’ve been in aircraft maintenance for 20+ years. I torque hardware on a daily basis and have never replaced hardware because of stretching, and the torques I deal with are A LOT more than 30-40 in lbs!

If the bolt carrier, screws and gas key all are in serviceable condition a simple re-torque and re-stake should suffice. I can’t see the justification in buying new parts for this.

Added note:

A buddy of mine who just so happens to also be an aircraft mech had the same problem with his new from the mfgr Bushmaster.......the gas key came loose, it was never staked to begin with! He tightened the bolts by hand, staked them and as far as I know has had no further problems.
Link Posted: 8/10/2005 4:57:09 AM EDT

depends on why there is a $40 difference


That's why I'm asking the expert. Do you know of any subtantial differences to the tune of $40? :D
Link Posted: 8/10/2005 5:02:36 AM EDT
Link Posted: 8/10/2005 5:14:28 AM EDT

Originally Posted By Tweak:

Originally Posted By QUIB:
That’s practically hand tight!



Right up until you force all the high pressure gas through there. Stretching's the wrong term but the one that gets used most around here. More akin to lapping, sloppy fit between the screws and the carrier threads.

macros,

I can think of lots of differences but not having the two parts in my hands I can't tell which ones apply.



I could see the bolts possibly starting to shear if the carrier was loose and the weapon was fired in this condition. The lateral movement between the gas key and the bolt carrier could possibly elongate the mounting holes and possibly start to shear the bolts. I think a good inspection of the bolt holes would be in order. At the least I’d replace the screws if they showed the wear patterns associated with shearing.

I think I know where your going now with the stretching i.e. “shearing“.

Just my thoughts!
Link Posted: 8/10/2005 3:27:29 PM EDT
Bolts are case harden. Go past the 40 inch pounds, combining the staking process, and you start to surface crack the surface case hardening, which leaves the softer inner metal to retain the correct torque. Add heat, and with only the softer inner metal retaining the torque, the bolts elongate, and the hotter the parts, the bigger the gap/leak becomes.

As for Bushmaster problem with keys, not really sure if it is (was) the lack of knowledge to read a torque wrench, the problem accruing when the key is stated (impact to the bolt, instead of just penning the softer key metal into the side ribs of the bolts to retain them in place), or burs left on the matting surface that is causing the problems.
Link Posted: 8/10/2005 3:34:28 PM EDT
To add, I believe the point that Tweak was making was to replace the Allen bolts only, unless the key was worn out (read opening for the gas tube). If the bolts were over torqued/streached (read surface micro cracked) then re-installing them will just put you back in the same position very shortly, or at worst, digging out a snap bolt from the carrier if one of the old ones do let loose (read way too fun).
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