Once in a while my chinese military sks jams on closing. During cleaning, one of the things I noticed is that even after cleaning, the piston would be very sticky in the gas tube when rotated to a certain position. Basically, it needed to be rammed/pushed out with a pencil, etc. Today, I did some fooling around with it and want to share my results:
But first, a few comment:
* due to the limited stroke of the op-rod that it bears up against, the end of the piston rod only needs a travel of 3/4". So sticking is only an issue for the 3/4" of travel.
* the small diameter part of the piston has got lots of clearance inside the gas tube. I tested each fit separately. There should not be a problem if everything was straight.
* I noticed that my piston was out of round by about 1/8" when chucked up in a drill at the small diameter end. I don't recall ever damaging it, so can only assume that it was made this way. Therefore this may be a common issue. I marked the extreme point with a Sharpie so I could see what was going on when it was assembled. This was easily done by turning the drill slowly and slowly approaching the side of the piston with the sharpie, till it left a trail on the piston. Then I transferred the center part of the trail to the head of the piston with the Sharpie.
* I also noticed that when assembled, in some positions, the piston rod moved freely inside the gas tube, meaning, the gas tube was not perfectly true either.
Ok, solving the problem was very simple, here it is:
1. I put gas tube in a padded vice.
2. I taped the non-forearm end of the gas tube to allow it to be tweaked with a 1" box end wrench without marking the blueing.
3. I inserted the piston into it's operating position and extended the small end by 3/4" past the gas tube end.
4. I slowly rotated the piston and found the positions it grabbed at.
5. I applied hand pressure to the taped area and found that with a bit of steady pressure, the piston rotated freely. It just so happens that I needed side pressure to the tube, and didn't have to re-clamp the gas tube.
6. I used the 1" box end wrench to tweak the gas tube. I didn't need a lot of pressure to do this, maybe 10-15 lbs., plus the leverage of a 14" wrench.
One try, sticky problem was solved. I didn't have to straighten the piston at all, just the gas tube. This is just as well, as all of my efforts in the past to do this kind of straightening have not met with much success, usually it bends too much the other way when the material yields.
But first, a few comment:
* due to the limited stroke of the op-rod that it bears up against, the end of the piston rod only needs a travel of 3/4". So sticking is only an issue for the 3/4" of travel.
* the small diameter part of the piston has got lots of clearance inside the gas tube. I tested each fit separately. There should not be a problem if everything was straight.
* I noticed that my piston was out of round by about 1/8" when chucked up in a drill at the small diameter end. I don't recall ever damaging it, so can only assume that it was made this way. Therefore this may be a common issue. I marked the extreme point with a Sharpie so I could see what was going on when it was assembled. This was easily done by turning the drill slowly and slowly approaching the side of the piston with the sharpie, till it left a trail on the piston. Then I transferred the center part of the trail to the head of the piston with the Sharpie.
* I also noticed that when assembled, in some positions, the piston rod moved freely inside the gas tube, meaning, the gas tube was not perfectly true either.
Ok, solving the problem was very simple, here it is:
1. I put gas tube in a padded vice.
2. I taped the non-forearm end of the gas tube to allow it to be tweaked with a 1" box end wrench without marking the blueing.
3. I inserted the piston into it's operating position and extended the small end by 3/4" past the gas tube end.
4. I slowly rotated the piston and found the positions it grabbed at.
5. I applied hand pressure to the taped area and found that with a bit of steady pressure, the piston rotated freely. It just so happens that I needed side pressure to the tube, and didn't have to re-clamp the gas tube.
6. I used the 1" box end wrench to tweak the gas tube. I didn't need a lot of pressure to do this, maybe 10-15 lbs., plus the leverage of a 14" wrench.
One try, sticky problem was solved. I didn't have to straighten the piston at all, just the gas tube. This is just as well, as all of my efforts in the past to do this kind of straightening have not met with much success, usually it bends too much the other way when the material yields.
