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Hi, how much should the piston butt extend after the gas plug is closed? . . . if its not extending according to specs will it cause FTE?
 

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From what I have been told once the gas plug is tight the piston should be touching the op rod and be what I would call preloaded, I was told an easy way to check this was with the bolt forward and the gas plug loose it will take aprox 11/2 to completly seat the plug
 

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If it is not preloaded will that cause problems like FTE and etc?. . . Like if the gas plug was 1 turn lose. .
 

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Yes, like that.
 

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With the bolt closed and the op rod foward there should be contact with the piston. The contact should be enough to prevent the piston from having any linear movement. If there is a gap between the op rod and piston the piston will strike the op rod like a hammer instead of pushing it.This will cause damage to the op rod and piston and cause undue stress on the rifle. And is not good for accuracy.
 

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If it is not preloaded will that cause problems like FTE and etc?. . . Like if the gas plug was 1 turn lose. .
"Loose" as in the plug wasn't tight to begin with? Or "Loose" as in it would take another turn to make contact but the plug is tight in the cylinder?

If the plug is loose as in not tight then gas will escape and cause FTE since there won't be enough pressure to operate the system.

If it's loose as in the plug is tight but the piston doesn't contact the op rod then you may need to rotate the lock a turn to seat the gas cylinder and align the gas port. This would also cause FTE since the system may not be getting enough gas. A quick test is to put a 1/16" allen wrench through the gas port, if it goes into the barrel you're good to go, if not, you have a problem.
 

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You've got two important and complimentary contact points when the bolt's fully in battery: 1) op rod's bolt roller recess fully forward against roller; 2) op rod's tip to gas piston's tail.
 
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