Cothran valve leak?

[tkerrigan]

I have one of Don's .308 rifles built like a Disco.  I have been chasing an air leak on it and I am wondering if the small piston o-ring can leak to atmosphere.  I thought I had it fixed when I replaced the plasticized o-ring between the valve halves.  But the leak is still there and that o-ring did look rough from what I could see.  Regards, Tom

[screwwork]

That should be an easy leak to find in a water bath with the breech, trigger, hammer & stock removed. See if it leaks to ATM.

[rsterne]

There is a vent hole through the poppet from the balance chamber to the exhaust port.... If the O-ring on the front "piston" (inside the poppet sleeve) leaks, the HPA from inside the valve will end up leaking out through the exhaust port....

Bob

[tkerrigan]

Thank you, I know what to do now.  I'm guessing that the nut on the balance piston shaft is locktited on.  Next time apart I will try some gentle heat to get the nut off.  Regards, Tom

[rsterne]

I don't think you have to remove the balance piston to replace the O-ring, do you?.... Once you unscrew the valve and slide it out of the poppet sleeve, you should be able to reach inside the front housing and remove the O-ring with a pick.... even a straight pin should work....

If you do have to remove the piston, when you replace it, it should move a bit radially , but not axially (or at least not much axially)…. The radial movement is to allow it to self-align with the poppet....

Bob

[WyoMan]

Tom, none of mine had thread-locker but they were torqued pretty tight.
If you have a deep-well 1/4" socket (for the nut/piston), it's a simple removal. The screw (mine anyway) was just a socket head hex. And yes Bob is correct about the piston needing to self-align so the threads should bottom out before the screw and the piston clamp to the housing... all of mine did that by design.

Bob, I tried that first and I've got pretty steady hands, but it wasn't long before I made a trip to the hardware store for the extended socket,,, lol

Wyo

[rsterne]

Been a while since I had one apart.... *LOL*….

Bob

[WyoMan]

Same here... three years for me but I remembered buying the socket - and even found it 



Wyo