View Full Version : Technical: My StG-58 (or other FrankenFAL) won't cycle!

Farmer from Hell
November 20, 2000, 11:10
Originally posted by abacus:
Something I forgot to mention in my original post is that if I watch the action while someone else files I can see almost the entire length of the empty brass before the bolt closes on it.

This sounds like a bent gas piston.
Take off the cover close the gun back up and operate the action by hand and see if you can tell where its binding. The action wouldnt cycle at all, I dont think, if the gas plug was turned the wrong way so it seems the piston is driving the carrier back but only until something stops it. You say its this way with the gas set on 0? The first thing I would do is roll the gas piston and then progress on to more involved problems.


November 20, 2000, 11:18
That brings up an interesting question for me. Mt FAL shoots fine, but the gas setting has to be on 3 before it functions okay. Well, I figure that isn't exactly right, so I took the original gas piston out and put another one in. I haven't had the time to test fire it yet, but I did take the old gas piston and rolled it on the table as was suggested in this thread and did notice a "slight" bend. But it is very very slight, even though it can clearly be seen with the naked eye. Is a small imperfection enough to cause problems? I will find out next time I take the FAL out and shoot it with the new gas piston, which is straight as an arrow so far as I can tell.

November 20, 2000, 16:36
Warning, I am a total FAL newbie.

I recently purchased an STG 58 that just won't cycle. I have followed the instructions in the manual and have the line on the gas plug lined up with the axis of the rifle. Even with my gas regulator on 0 the empty will not eject and the action won't engage the hold open on the empty magazine. I am using NATO standard ammunition that works on my friend's FAL and my M1A.

Info about my rifle:
STG 58 built on IMBEL receiver by Centurion (I believe)

Do I have a defective rifle or am I doing something wrong.

November 20, 2000, 16:39
Take your gas piston out and roll it on a flat surface to see if it's straight. If it isn't, order a new one from DSA. That's where I'd start.
Take your handguards off and see if your gas tube is straight.
Of course make sure you have the gas plug on the right setting, I believe A is what you want, not G.

November 20, 2000, 16:44

I'll check out my piston and tube.

Something I forgot to mention in my original post is that if I watch the action while someone else files I can see almost the entire length of the empty brass before the bolt closes on it.

My gas plug does not have an A or a G it just has a line "|" on one side and is flat on the other.

November 20, 2000, 16:49
With the rifle pointed straight up and looking at the top of the rifle, the gas plug plunger (spring loaded part that you push in to rotate the gas plug) should be on the left side.

November 20, 2000, 18:56
Okay, I am at home now and have taken out the gas piston and rolled it on a flat surface. It is bent ever so slightly (the end of the rod never gets more than about 1mm off of the table)

What kind of tolerances are involved here??


Farmer from Hell
November 20, 2000, 19:12
Well you have to remmeber that the rod must slide through the hole in the reseiver to move the carrier and if there is any bend then it could possible bind slightly, although how much I cant say. One thing that I havent heard anyone debate here is how efficient is the aftermarket gas pistons in using the gas to move it. Are the US ones as efficiant as the original. Or basically is there more bypass with the US ones vs the originals. I suppose you could try a surplus original and see if the gas setting changes between it and the US one. I am not a smith but just someone who has a good mechanical aptitude and understand how things interact. These are just my opinions and thoughts and maybe a real smith can elaborate on them. I have wondered this as you hear everyone say that it should run at 4-5 but mine needs about 3.5 and I have always wondered if the piston was bypassing more gas then it should and if its because its a CAI one. It would seem to me that the great minds at FN would give you more room to boost up the gas if things get dirty. If you are already on 3.5 you dont have much to work with you know.


November 20, 2000, 19:20
I just had the same problem. Mine was bent about .003 it seemed, but would not cycle the bolt.

Got a new one from DSA and it is humming now. Works great.

Check the hold open pin for grinding marks, it may have been modified. You can get a new one of those also from DSA.



w. scott
November 21, 2000, 21:32
I too had this problem when I first got mine. I was using surplus "Radway green" and with the gas regulator closed would not cycle. I found out about "the gunplumber" here, and after I determined I was using weak ammo by buying new winchester and fireing it with no problem I decided to enlarge the gasport according to the Gunplumbers instructions. This worked perfectly! Now I can continue to buy the surplus ammo which is LOTS CHEAPER. Thanks Gunplumber!! peace

November 21, 2000, 22:50
Bent piston is a common problem on the Century guns for some reason. I changed mine out with a DSA and have had no problems since. Never had to do anything else to it. It works without a hitch set on 4-1/2 with Radway Green.

November 23, 2000, 10:39
The way I found out I had a bent piston for sure was to take it out, remove the spring from it, and drop it back in. With the slide and dust cover removed, and with the barrel pointed straight up, I used my fingers to move the piston up and down in its tube from the receiver end. At the same time I rotated it. I found that at certain points in its travel, and at certain points of rotation, the piston would bind up tightly.
I think the binding at certain points of rotation pointed out that the gas system was not PERFECTLY aligned, which I think could be expected, but the tolerance stack-up as the piston rotated finally got it around to the point where the clearances were exceeded.
I got a DSA piston. I found also that the DSA piston was SLIGHTLY larger in diameter than the original (probably just machining tolerances), but it probably helps with the gas seal.
Using the no-spring technique makes it easy to 'feel' if the movement is smooth and non-binding.
I probably bent the original piston myself, as once when I was taking the thing apart I experienced a case of brain-fade and let it get away from me. The spring launced it across the garage where it pinged off the concrete wall and bounced across the concrete floor a few times. Never was the same, after that! BTW, the bend was too slight to detect with the 'roll it on the table technique'.

[This message has been edited by Chief351 (edited November 23, 2000).]

November 23, 2000, 21:14

November 24, 2000, 14:54
Originally posted by gary.jeter:

I saw a couple Century FAL's at a gun show yesterday. I practically cried.

Chances are everything that could be wrong with the gas system is wrong.
Loose joint at gas block / gas tube.
Loose gas plug.
Oversize gas tube inside diameter.
Undersize/bent gas piston.
Gas tube out of alignment with receiver.

The headspace is probably out of whack too.

Was this at the Bealton Show? I didn't go but a friend of mine who was looking for a 308 rifle said all the STGs he saw at the show looked pretty crappy.

I ordered one from AIM last month and it seems to be put together pretty well with the exeception of a slightly bent piston
(which I expected to have to replace). Barrel is Steyr and might be new. AIM must be hand picking the best of the Century guns.