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Posted: 10/28/2006 12:19:56 PM EST
so I picked up a rifle at the gunshow, I basically have a little over 500 in it right now. It's a Century Arms R1A1 Sporter as indicated on the receriver. The rifle seems to appear in good shape without any serious wear at all.

so i go to shoot the rifle. I know about the gas port adjustment and how it should function. I put in a magazine, assuming the gas port is properly adjusted by the previous user. it is turned almost all the way up, on 6. the rifle fires and now the bolt carrier group isn't shut all the way. it appears as if the bolt didn't tilt and lock after it chambers a round.

upon closer inspection, i realize that the bolt carrier group isn't closing all the way because the gas piston is in a bind and won't go all the way forward. i remove the magazine and then eject the chambered round, then let the bolt slam the gas piston back to the resting position. I fire another round, and the same thing happens.

I pull the gas piston out and clean the heck out of it and the inside of the gas piston tube, and manually move it back and forth many times. Now the rifle shoots as if it were functioning normally, but i did notice one more thing.

when i put a single round in the chamber with an empty magazine and shoot the round, the bolt stays back, but i also notice the gas piston is not back at the resting position. I'm pretty sure the spring inside the gas piston assembly should move it back to the resting position without help of the bolt carrier group. the rifle functions perfectly fine, but I wonder how I can get the gas piston to go back to resting position on it's own without use of the bolt carrier group slamming it forward. If i could get it to work right, I could juse a lower gas port setting which would cause less wear on the rifle over time.

what do you guys think?

thanks for the help,

Mike
Link Posted: 10/28/2006 2:41:55 PM EST
[Last Edit: 10/28/2006 2:42:15 PM EST by BrandonP]
Assuming you've already checked the piston "housing" (?) for obstructions/gremlins, it may be possible that your barrel is canted to the point where the piston binds?

Check for cant, but I'm by no meals a FAL expert. Just throwin' it out there.

It might just have been crapped up if you cleaned it and it works fine now.
Link Posted: 10/28/2006 4:59:02 PM EST
It is possible the barrell is over or under timed which could cause binding. With out some tool to align the barrell, one way to check this is to look down the sights and you may be able to visually check if the front sights are canted left or right. Also, are the rear sights adjusted all the way to the left or right?

2 other things to check,

CAI used a 2 peice gas pistons on some of their builds. One of mine had the threads stripped and the piston was mis-aligned. Another possiblity is that the piston is bent. To test, just lay it on a hard, flat surface and roll it, just like you were checking a pool cue. If it's bent, it'll wobble as it rolls.

The other thing to check is the gas tube it self. If it's been crimped or is out of round (like someone using vise-grips on it) it will restrict the head of the piston.

Just thought of one more thing to check. The gas nut, the piece that screws into the front of the upper receiver just above the barrel (holds the receiver end of the gas tube). Check the small hole at the small end and also check that it is aligned in the receiver properly, On some older CAI receivers, the hole was drilled offcenter.

Remove the piston return spring from the piston and with the gas plug out and the bolt to the rear (or the upper seperated from the lower) check the movement of the piston. It should move freely in the tube. If it's binding, you should be able to tell if it's binding at the head or tail of the piston.

Hope some of this helps.

Good luck and keep us posted.
Link Posted: 10/28/2006 5:00:56 PM EST
PS Could be a weak spring.
Link Posted: 10/29/2006 10:47:20 AM EST
height=8
Originally Posted By FrenchKat:
Remove the piston return spring from the piston and with the gas plug out and the bolt to the rear (or the upper seperated from the lower) check the movement of the piston. It should move freely in the tube. If it's binding, you should be able to tell if it's binding at the head or tail of the piston.


when i take the spring out, it is still in a bind. I actually tried putting in the piston assembly in from the backside while the rifle is hinged open to see where the bind is. It is at the backmost part of the gas tube assembly. I think there is some sort of washer or bushing that is out of spec or something causing everything to be in a bind.
Link Posted: 10/29/2006 12:04:32 PM EST
[Last Edit: 10/29/2006 12:06:35 PM EST by HKPDW]
May be time to take it to a smith to check the barrel timing
Link Posted: 10/29/2006 2:47:33 PM EST
height=8
Originally Posted By HKPDW:
May be time to take it to a smith to check the barrel timing hat
is this the angular position of the barrel and gas tube with respect to the upper receiver?

i would much rather troubleshoot and fix this rifle myself as the local gunsmiths are only good for wanting to take money from people instead of properly diagnosing a problem.

thanks for the help
Link Posted: 10/29/2006 4:10:22 PM EST
When the barrell is screwed into the receiver, it should come out with a line extended straight up from the front sight that is 90 degrees with the plane of the upper receiver.

It's a little hard for me to explain, but if you were to put a level across the flats of the upper receiver with the dust cover removed, and run a long straight rod thru the hole in the gas block where the hand guard screw goes, the level and the rod shold be parrell.

Not sure if I explained it well enough. I know in my mind what to look for, but can't explain any better with out pictures.

Any, that is barrel timing. If the barrell is not screwed in enough (undertimed) the front sight post will be canted to the left. If the barrell is screwed in to far (overtimeed) the front sightwill be canted to the right.

Try to set the rifle in a vise or some sort of stand. Position yourself behind the rifle and sight down the barrel. If the barrell is over or under timed, you might be able to see the cant of the front sight post.

If it's too far one way or the other, the gas piston will bind. That's not the only problem that will occur, but that's one of the symptons.

Check out www.falfiles.com/forums and go to the gunsmithing & build it yourself discussion. From there you can do a serch of the archives and there is a wealth of information there. You may have to wade thru some BS, but the files have been a great help to me in my 6 buillds.

On one of my builds, I had a gas tube that was out of round. I ended up putting the gas pistion in backwards to find out where is was binding.

I'm including a picture of the timing tool I use and how it's set up.

Keep plugging. Ken



Link Posted: 10/29/2006 4:56:13 PM EST
thanks for the info.

so if the barrel was out of time and the front sight was canted, the rifle wouldn't shoot straight either, right?

my rifle shoots dead on with what I have done with it so far.

i need to measure the piston for roundness and size, then see if it's in spec.

Im wondering if i could just use something to slightly enlarge the hole where it is binding. It is only at that one part where what seems to be a washer or bushing is that the piston binds in.
Link Posted: 10/29/2006 6:05:50 PM EST
I borrowed these pictures from the FalFiles. This shows that the piston hole in the receiver is not drilled in the center and binds the gas piston.



Bug Tussell, whose thread these pictures are borrowed from says "Check piston operation with the gas tube nut in place. If the gas piston binds up then some judicious filing and polishing is called for. You want the piston to move freely back and forth and not bind either direction. This receiver isn’t too bad – the gas piston will insert and with a little wiggling will insert fully and retract. The previous receiver wouldn’t allow a gas piston to insert at all. Some filing and polishing remedied this problem. This gas piston hole will get a little polishing to assure smooth functioning"





Lots of good information at the FalFiles. Here is the link for the thread about preparing a Century receiver for a build:

www.falfiles.com/forums/showthread.php?s=&threadid=175591
Link Posted: 10/30/2006 5:05:23 AM EST
thanks a million for the help

i've shot some more rounds through the rifle now, and now when I try to put the gas tube in backwards, it is sorta in a bind, but not as much, only on the first couple of inches. after that, it goes through just fine.

the rifle still fires correctly, but on the last round, the gas piston won't go back to it's resting position until the bolt carrier slams it forward when i release the bolt carrier.

The hole in the receiver looks to be in the center. I have a feeling that the gas tube is somehow bent or out of round. I can't tell if it's bent by rolling it around on a flat surface. I plan on measuring the diameter at a few different positions to see if the tube is out of round.

thanks again for the help, i'll keep you guys posted.
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