Svt-40 piston cup coming off.

D

derr12

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Salmon Arm
Got around to dialing in my sniper, once every 15 rounds or so the piston cup and op rod fell apart. I'm guessing it's a weak spring as I was gassing at 1.3 and 1.2 caused the occasional fte. Does that seem like the right diagnosis?
 
I meant to say i think its the spring that sits back under the rear sight. Im gonna grab the spring out of my other SVT and see if that fixes it.

i nearly sh!t a brick when the top metal handguard came off.
 
They can't come off because op rod with cup do not move back far enough for cup to come off from piston. Compare how everything works on another SVT, thre's no need for shooting tests to figure out what's wrong. Could be out of specs rod, cup or piston. Spring has no play in it.
 
Horilka said:
They can't come off because op rod with cup do not move back far enough for cup to come off from piston. Compare how everything works on another SVT, thre's no need for shooting tests to figure out what's wrong. Could be out of specs rod, cup or piston. Spring has no play in it.
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the rod was off the cup too, i assume the rod came off the cup and the cup blew off the piston.
 
I can't imagine how rod could come off unless pulled back manually while holding cup in place. Have you compared parts from two rifles?
 
I had something like this happen the first time I shot my second svt40, iirc one end of the rod had a burr or deformation and it wasn't sliding back fully after firing. One end was binding and popping out at the other end, I filed the end of rod smooth while at the range and problem was solved. Have a real good look at the ends of the rod and the inner sleeve that the rod sits in at the piston and cup ends.
 
looked good when i eyeballed it, but Ill have a look with a light inside the rod end of the cup to be sure. your theory might be right on the money tho. i had it out to the range before to function test it and it shot 20 no problems. i wonder if reversing the rod during cleaning is what started this. Ill look again and see if i can spot a deformity.
 
As it's a simple system it is likely to be some irregularity or burr/rough part on the rod ends or the sleeves the rod ends slide into. Mine would function fine for one or two shots then malfunction, was pretty apparent once I pulled the handguard off and shot it with the gas piston and cups exposed. Luckily I had a small file in my range box and it didn't stop my day out.
 
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