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Sealing face leaking at pressures above 200psi

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: jrrdw » Wed Aug 18, 2010 8:23 am

A-98 wrote:So just to clarify, you're suggesting putting a glob here, covering the bolt and contacting the washer?

Guh, that's gonna suck if I ever want/need to change part of the piston.


Remove that nut, holding washer, sealing rubber and the aluminum backing (piston). Smear the JB Weld all over the center bolt and re-assemble quickly.

When you re-assemble you should be pushing JB Weld like a plow pushing snow.

Removing JB Weld is easier then you think. A wire wheel works wonders...
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Unread postAuthor: A-98 » Wed Aug 18, 2010 8:34 am

Right, that makes more sense. I guess both would work, but that way is prettier. Thanks
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Unread postAuthor: A-98 » Wed Aug 18, 2010 11:13 am

So I applied epoxy to the nut, let it set, and brought the gun up to bike-pump pressure.

Still leaks /:

Mind you, I think this did help a bit, but it didn't solve the problem.

I hate to think the valve-side sealing face might be slightly out of alignment, or for that matter any part of the valve. And I don't see how it can leak when it's a lot more precisely constructed than some functional pistons on this site.

It's not like it's a big leak, just enough that you can hear it hiss if you ear is within 3 inches of the barrel or you cover the muzzle with your palm for 5 seconds and release.

But the simple fact that it does leak, gah it bugs me.
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Unread postAuthor: jrrdw » Wed Aug 18, 2010 3:53 pm

Did you take it step by step as I discribed or just epoxy the nut?
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Unread postAuthor: A-98 » Wed Aug 18, 2010 4:00 pm

There should be atleast some epoxy on all the threads in that area.
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Unread postAuthor: jrrdw » Wed Aug 18, 2010 4:16 pm

A-98 wrote:There should be atleast some epoxy on all the threads in that area.


Sounds to me like you didn't follow the guide I spent my precious time posting just for you! :P

Do a leak test by squirting red food coloring on the center bolt behind the piston and presurize to you hear it leaking then look at were the red comes through at. Work fast, food colorong dries quick...
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Unread postAuthor: A-98 » Wed Aug 18, 2010 7:41 pm

I couldn't get the aluminum backing off of the threaded rod because of the JB weld, so I applied epoxy at the red arrows:
Image

The nut-side I "snowplowed" into the washer, and the side on the aluminum backing I just applied epoxy. Again, I could not easily rotate the aluminum and sealing face because the JB weld layer was in contact with the rod.

As I see it, there is no place for the air to get in and no path for the air to travel.

And I don't have any food coloring, unfortunately.
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Unread postAuthor: the unit » Wed Aug 18, 2010 8:21 pm

c11man wrote:also did you do something to seal the seat into the adaptor besides just pressing it in??


This will likely be your problem if you didn't.
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Unread postAuthor: A-98 » Wed Aug 18, 2010 8:40 pm

It had a +.01 tolerance and was pushed in with a hydraulic press. I think it's fully sealed.
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Unread postAuthor: jrrdw » Wed Aug 18, 2010 9:04 pm

A-98 wrote:It had a +.01 tolerance and was pushed in with a hydraulic press. I think it's fully sealed.


Loctite make a compound used in this situation but it's been so long I can't remember the number. It's not the thread locker, Loctite makes a slue of different adhesives.

You can use any kind of colored liquid to find the leak...
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Unread postAuthor: A-98 » Wed Aug 18, 2010 9:39 pm

Well oh my, don't I look like a fool. It *was* leaking around the copper sealing face and into the barrel.

And yeah, the reason i didn't use some water earlier is because I had just finished polishing it to be all nice and sexy, and freakin moisture in the bane of my existence.

Anyone know a good way to keep it from getting tarnished? I was using gun oil, but I bought some lacquer spray and figure I'll give that a shot.
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Unread postAuthor: c11man » Wed Aug 18, 2010 10:34 pm

well good job finding the leak. and i dont know why everyone wanted you to epoxy the sealing face around the bolt, ive never had one lead in my numerous pistons.
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Unread postAuthor: A-98 » Wed Aug 18, 2010 10:42 pm

I really owe you an apology, I don't think I noticed your original suggestion. You must've been sitting there thinking I'd totally cold-shouldered your input. So really, sorry about that.

crap, now I've got epoxy all over my piston.

that sounds dirty.
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Unread postAuthor: c11man » Wed Aug 18, 2010 10:57 pm

its all good, if forgot about this thread also. if you dont like all of the epoxy redue the front half of it. you seem to have access to lathes and the like so it shouldnt be too hard to remake it. i would also suggest making the washer on the front larger and in a cone shape to hopfully reduce turbulence
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Unread postAuthor: A-98 » Thu Aug 19, 2010 12:24 am

That's a pretty good idea, but it might be a while before I manage to implement it; school is starting in a week and damned if that doesn't tend to consume my time.
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