gas piston spring broke

Rick65Cat

Rick65Cat

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Bought a 1950 Russian brand new, and yes, I stripped and cleaned it...within 30 shots the gas piston was sticking out into the breech. Tapped it with a screwdriver and it popped back into the tube. Dis-assembled the gun when I got home and found this.....

https://photos.app.goo.gl/EtPD41zAzvcAqLDu9


I would think theres more than enough spring to kick the piston back. Why would it stick out into the breech?
 
I just took it out of the safe and pulled it apart to be 100% sure it was clean...the part with the spring was spotless. Slid thru the tube thingy into the breech flawlessly. What I did find out though was the longer gas piston was sticking when it bottomed out in the foregrip tube.
I took an old worn out 12Ga brass bore brush and ran it in that foregrip tube with a drill. I polished the spot on the piston where the shaft meets the head. No luck...Long story short it still sticks.

The two tiny relief holes in that tube...is the head of the gas piston supposed to go past those holes? Coz mine doesn't.

https://photos.app.goo.gl/uHPmEwE6DNDFP26T8
 
Roll both pistons on a flat surface - check that they are straight and true. The large piston should slide end to end in the tube without any resistance. If the piston is straight, the bind is in the tube, one end or the other. Is the tube straight?
 
Rick65Cat said:
I just took it out of the safe and pulled it apart to be 100% sure it was clean...the part with the spring was spotless. Slid thru the tube thingy into the breech flawlessly. What I did find out though was the longer gas piston was sticking when it bottomed out in the foregrip tube. I took an old worn out 12Ga brass bore brush and ran it in that foregrip tube with a drill. I polished the spot on the piston where the shaft meets the head. No luck...Long story short it still sticks.
Click to expand...

The piston only travels the first 1/4-1/2inch of the piston tube. Any resistance past that point is meaningless, as it is not a factor in firing. Any polishing past the 1/2" below that tube opening will be a wasted effort.

Rick65Cat said:
The two tiny relief holes in that tube...is the head of the gas piston supposed to go past those holes?
Click to expand...

NO

Make sure there are no burs inside the RearSightBlock or in the narrow end of the gas tube which guides the piston shaft:
Run a cotton swab inside channel in the Rear Sight Block and in the narrow end of the gas tube. If the cotton snags, you've got burs that will need to be addressed.

Also a good idea to take PinkyPlinker's advice and roll op rod and main piston to check that they are true. And also inspect them for uneven wear patterns.

(The Main piston is called the piston. The Secondary piston with the spring is called the Op Rod or piston extension.)
 
Just because it only travels 3/4” or so during firing is irrelevant. It should easily travel the entire distance of the tube without resistance when it is out of the gun. If it doesn’t something is out of kilter.
 
PinkyPlinker said:
Just because it only travels 3/4” or so during firing is irrelevant. It should easily travel the entire distance of the tube without resistance when it is out of the gun. If it doesn’t something is out of kilter.
Click to expand...

Nope.

If it travels smoothly the required 1/4-1/2 inches aft of the opening, it's good to go.

Piston head getting "stuck" at the bottom of the tube is due to build up or because of the taper in the shaft just below the piston head getting stuck in the shaft guide channel. If the piston were out of true, it would be detectable well before it bottoms out.
 
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PinkyPlinker said:
Just because it only travels 3/4” or so during firing is irrelevant. It should easily travel the entire distance of the tube without resistance when it is out of the gun. If it doesn’t something is out of kilter.
Click to expand...

good advise on the straightness till you said it should travel the entire distance out of the gun, it doesnt.
all sks travel easily for about 1.5" in the tube out of the gun then it resist.
 
Rick65Cat said:
The two tiny relief holes in that tube...is the head of the gas piston supposed to go past those holes? Coz mine doesn't.
Click to expand...

boris is correct, the piston head doesnt past those relief holes, the relief holes are there to relieve air pressure behind the head not in front of it.
 
PinkyPlinker said:
Roll both pistons on a flat surface - check that they are straight and true. The large piston should slide end to end in the tube without any resistance. If the piston is straight, the bind is in the tube, one end or the other. Is the tube straight?
Click to expand...

Rolled the large piston on my workbench which is flat. I couldn't see any discernable wobble. I took some 600 grit and polished around the piston head. Back into the tube and there is some scratchyness near the end of its travel. I cannot tell if the tube itself is bent. I can't see any dents in the tube and it seems too short to be able to bend.
Maybe I'll get some 3/8 dowel and wrap some 600 grit around it and run it in and out of the tube for a bit.
 
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PinkyPlinker said:
Just because it only travels 3/4” or so during firing is irrelevant. It should easily travel the entire distance of the tube without resistance when it is out of the gun. If it doesn’t something is out of kilter.
Click to expand...

This is a Russian Sks...I used to own a Norinco years ago. And I'm pretty sure I don't remember the piston binding in its tube when I disassembled it.
 
I'm gonna take it to the range tomorrow and put a few rnds thru it to see what happens when the last shot goes off and the bolt stays open. If the op rod still sticks out into the receiver maybe I'll just order a new spring. Maybe the original Russian spring steel quality is poor? The spring is not the wound style that I've seen, just a plain steel spring.
 
Rick65Cat said:
I took some 600 grit and polished around the piston head.

Maybe I'll get some 3/8 dowel and wrap some 600 grit around it and run it in and out of the tube for a bit.
Click to expand...

These are both very bad ideas, because each will increase the spacing between the piston head and tube surfaces and will lead to under-gassing, and other problems associated with weak recoil.

NEVER use sandpaper in the gas tube or on the the side of the piston head.

Instead, use a copper or brass bore brush for the tube. And use fine grit sand paper ONLY along the very corner edge of the piston face where burs often develop.

IMHO, the problem has nothing to do with the piston. The problem is most likely a faulty extension spring or a burr inside the RSB. It may also be or with the op rod, though I doubt it.

(The op rod and piston are not the same thing).

Use a Qtip to 'feel' for burs inside the op rod channel in the RSB. Then order a replacement extension spring. Modifying parts with sand paper will only create more problems.
 
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Boris Badinov said:
It should be pointed out that this is not a brand new gun. The un-blued RSL, and MOLOT stampings on top of the barrel indicate that it has been extensively re-built.
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Ok, I should clarify that I bought it new from a store. And yes, the finish looks more black than normal bluing.
 
Boris Badinov said:
Nope.

If it travels smoothly the required 1/4-1/2 inches aft of the opening, it's good to go.

Piston head getting "stuck" at the bottom of the tube is due to build up or because of the taper in the shaft just below the piston head getting stuck in the shaft guide channel. If the piston were out of true, it would be detectable well before it bottoms out.
Click to expand...

Here is a picture of how far the rod protrudes just it STARTS to bind

https://photos.app.goo.gl/9zMzF42vVpfZeBTJ8
 
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