I recall that I could use one of my allen bits to torque the gas key screws, but the new CMT/Stag bolt carrier group I got from globaltactical appears to use a non-standard allen size.

It's something between #4 and #3 metric (#3.5?) or some uncommon SAE-size. I tried every allen key I had just to find the size but nothing fit.

The RRA bolt carrier I have used gas key screws that took one of the bits I have. I have to go home and check on the size... but is this a common issue? Does CMT use a wildly different gas key screw from everyone else?

Different socket sizes for different manufactured bolts is quite common.  I run into it all the time on plant equipment and once or twice on AR related stuff, like carrier key screws and pistol grip screws, for example.  I'd guess that they wouldn't be metric, but you never know nowadays.  The thread in the carrier is standard thread, so I can't see why the socket head would be metric, but again, you never know nowadays.

I dug up Tweak's thread about the gas key screw staking and it looks like either 9/64" or 1/8". I found 2 bits that were really close but no cigar type fits. Since its a CMT, it may just be the screw is deformed???

Hi, I used an old 4mm key and ground it down a bit. I think you are right though as I have just tried a 3.5mm and it fits fine.

singlefire

Why did CMT use such weird gas key screws??

Mongo:

You can tell the age of stuff in the plant by the socket head size.  Older copies of "Machinery's Handbook" will list the two different sizes as "socket head screws" & "socket head screws (1960 series)".   Socket head screws were standardized in the USA in 1936, and the standard changed in 1960.  I still find screws & bolts of the old standard installed in old machinery.  Even though the standard changed in 1960, many suppliers still had the old standard in stock in the mid-1970's.

The screw problem today rests on all of the "offshore" suppliers.  Some plants are located in conutries that never adopted the 1960 standard.  Some offshore suppliers DO occasionally put metric sockets in SAE threaded fasteners.  Metric measuring tools, and sloppy manufacturing can lead to "inbetween sizes" of fractional socket heads.  Buying quality socket head screws can be a challenge sometimes.  I insist on USA made screws myself.


Metroplex:

Fractional allen keys are available in 1/64" increments, starting at 1/16th.  A full set is much bigger than what you usually see in a hardware store.  The industrial supplier I use has about 12 feet of shelf space dedicated to loose allen keys alone.  If you are looking for Apex bit type allen wrenches for a torque wrench, try McMaster-Carr - they stock a full line of apex type bits.


Lem

I do not like buying a special allen bit just for the CMT gas key screws. If that is the case, I am drilling them out and replacing with something more standardized.

Just buy one of those allen key sets that fold into a handle.

Quoted: Just buy one of those allen key sets that fold into a handle.

None of them fit the gas key screws.

Update:

Turns out that the gas key screws were deformed by the improper staking. It was 9/64". I noticed that and used a hammer to lightly tap in the allen wrench and they came off.

45 in-lb is about hand tight territory.
I lapped the gas key and applied some sealant to further prevent gas leaks.

Update #2:

You can tell I'm a busy guy if I have to split these things up over 2 days...

Well the RRA gas key screws require a 1/8" allen wrench.
The CMT gas key screws require a 9/64" allen wrench.

I do not have an allen bit for 9/64", I may have to buy a whole set (been wanting to do that anyhow).