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New commercial Argy barrel gas port?

biyf

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Though I have built several FALs from surplus parts, I've never had to mess with a gas port. Latest build uses a brand-new commercial Argy barrel and won't cycle. Slow motion video with dust cover removed shows the piston moves less than an inch and the bolt carrier the same, gas fully closed. I've ruled out everything else (not my first rodeo) so the only thing left has to be the gas port. Haven't measured the gas port quite yet, but I have some good quality drill bits coming, as well as a brass rod to make sure I don't bugger the inside of the barrel.

So the question is: Is it common that a brand new Argy barrel would need to have the gas port enlarged?
 

tac-40

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You also need to check to make sure the gas port and gas block hole lines up. Off center gas block port can restrict gas flow. Using my last two builds (non Argy) as an example, both new barrels had issues with the gas system related to gas port size. Both of these barrels were US made.
 

biyf

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Thanks guys - will check size and orientation and report back. The barrel came assembled from Sarco IIRC and is how it left the FMAP factory, as far as I know.
 

biyf

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Orientation looks good, still waiting on drill bits to check size but a 3/32 allen wrench isn't even close to fitting. My 3/32 bit is MIA at the moment. From your suggestion and previous reading, a 3/32 bit (between #41 and #42 bit) seems like it would be safe and not likely to be too large/overgas on a standard length barrel?
 

meltblown

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a 3/32 allen is across the flats measurement. The widest portion across the hex is .01 larger than 3/32
 

gunplumber

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All of the Argy barrels I've seen over the years from SARCO did NOT have a factory-installed gas block. SARCO installed the gas blocks. And on some of them it was done badly - one with two gas ports drilled. Most had oversize pins.

I'd certainly pull the gas block and check the journal first.

nominal diameter is .098" which is 2.5mm not 3/32" (.094") but I find most barrels happiest with a 2.7mm (.106") port.
 

biyf

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Thank you gents, much appreciated! Guess I'll pull the gas block and give everything a close look before moving forward. Not sure exactly what Mark means by journal? Perhaps the barrel grooves where the gas block pins ride? Will report back once I have it disassembled.
 

gunplumber

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In a fit between a hollow part (gas block) and the solid piece that it slides over (barrel) , the section it mates to is called a journal. I don't know the word history. And I may be off in a subtle but meaningful manner.

In machinist terms, the section of a shaft on which a bearing turns, is a journal. It is usually very precise in its circumference. But also in machinest terms, it is lubricated against the bearing.

In this case, it is the section of the barrel on which the gas block interfaces.

If there is a better word to describe this more precisely-machined section of the barrel, I am open to education.
 

biyf

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Mark, I wasn't trying to be a smartass, I just didn't understand the meaning of journal in this context. I appreciate the explanation, and no argument from me on what to call it. No issues with the journal that I can see.

Drill bits arrived, and the port was a #41 (.096) which is not even Mark's "nominal". I took the port to a #39 (.0995) and will retest and adjust from there.

Thank you for the help!
 

biyf

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Put some small effort into removing the gas block to check the journal, using a hammer and a 2x4. Then I decided I didn't know what I was doing, and watched Mark's youtube video on how to remove and install a gas block. Decided I don't have the proper equipment - hell don't even have any angle iron around to use to get the gas block off.

I did enlarge the gas port to a #36 drill bit, or .1065. After double and triple checking everything else, I may have a very slightly overtimed barrel. Like very slightly. With the gas piston that came with the kit, it drops freely - but my STG-58 piston hangs up just a tiny bit. I wrapped the gas tube in thread tape, and the bolt and carrier are traveling way more than before, but not enough to cycle. Gas is fully closed. Changed out the gas piston to one from my STG-58 (even though it hangs just a bit in the drop test), and it was just a small amount better. At one point, I got four rounds to cycle in a row. Also tried an L1A1 piston and it was the same as the STG piston, slightly better than the stock piston. The more the thread tape blew out, the less the bolt and carrier moved - but they were still MUCH improved and moving almost enough to cycle. Also noticed the STG gas piston is a tiny bit shorter than the original piston and L1A1 piston but I don't know if that's part of the problem.

Will back off the barrel a tiny bit first, and re-try. Not wanting to oversize my gas port, at this point it would seem if it still doesn't work, I could just keep going on the gas port. My main concern is overdoing the gas port as I can't go backwards with it. After backing off the barrel, and changing pistons, would it be bad if I need to go larger than .110 since I seem to be pretty close? Thanks all for the help!
 
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plinker

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Way back when, I had a Argy barrel from DSA (short tube, closed ear block) that I ended up opening up the gas port on, before that, it only cycled on 1-2 IIRC. Opening the port fixed the issues I had. It was a very accurate barrel.
 

biyf

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Yeah, I had never needed to mess with a gas port yet as I'd never built a kit using a new barrel. I think I am close on this one, I just don't want to end up with an oversized gas port. Mainly my question at this point is should I be concerned if I need to take the gas port to something larger than .110 to get it reliable with the gas system between 3-6? And I wonder if there isn't a gas leak somewhere since the teflon tape helped the situation. Any tighter on the gas tube (recent order from Gunplumber) and it would not have had any back and forth play in it, so it seems to be in the correct spot.
 

Invictus77

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If there was mention of a barrel length above, I missed it. I had to open up an 18" barrel port once to .116" or so for proper function with the shorter dwell time. Personally I've never opened a 21" barrel port beyond .110" or maybe .112" that I recall, but there are a lot of variables of course in these 50 year old parts kits we all love. YMMV.
 

biyf

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Fixed barrel issue, it was actually slightly under-timed, but I don't think that was the problem. Took it to a #35 drill bit (.110) and it now runs like a champ on setting 4. I was scared to keep opening the gas port, but that's exactly what I needed to do!
 

tac-40

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You have to remember, all your parts are old except for that new receiver. Since every receiver except the IMBEL was reverse engineered to work with all the other parts, who knows what tolerance stacking there was in your particular build. My latest build (short barrel short gas) had similar issues until I put a muzzle brake on it. That increased the back pressure so it cycles okay with gas set midway.
 
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biyf

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Good point on the receiver, and I suspected and did have a couple small issues with it (lower lockup). And the problem was compounded by the barrel being brand new, commercial Argy and it had never been a functioning weapon. Every other kit I had ever built was at one time a functioning weapon, and had a gas port that had worked. Gas port on this was undersized from Sarco and just not something I had previously dealt with.
 
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