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Degree of piston seal and thread sealant question.

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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Degree of piston seal and thread sealant question.

Unread postAuthor: boyntonstu » Thu Sep 24, 2009 6:02 pm

How long should a well designed piston hold 400 psi before it leaks down to 350 psi (if at all)?

Other than Teflon tape, what other thread sealant would you recommend that would enable the main 3/4" fitting on my cannon to be screwed and unscrewed multiple times?

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Unread postAuthor: spudtyrrant » Thu Sep 24, 2009 6:33 pm

ptfe paste as far as thread sealing goes, and a well design piston should be able to hold 400psi as long as you need it to without dropping
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Unread postAuthor: boyntonstu » Thu Sep 24, 2009 6:40 pm

spudtyrrant wrote:ptfe paste as far as thread sealing goes, and a well design piston should be able to hold 400psi as long as you need it to without dropping


PTFE Paste: Brand? Home Depot or Lowes available?
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Unread postAuthor: spudtyrrant » Thu Sep 24, 2009 6:49 pm

yes its available at lowes or home depot but its quite expensive i'm not exactly sure of the brand
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Re: Degree of piston seal and thread sealant question.

Unread postAuthor: velocity3x » Thu Sep 24, 2009 9:19 pm

boyntonstu wrote:How long should a well designed piston hold 400 psi before it leaks down to 350 psi (if at all)?


At 400psi, my 2" piston will hold almost full pressure overnight. I've never tested any longer.

Other than Teflon tape, what other thread sealant would you recommend that would enable the main 3/4" fitting on my cannon to be screwed and unscrewed multiple times?


LOCTITE 55 Pipe cord
It looks like dental floss and can go as high as 10,000psi. Very easy to use and NEVER fails. After trying it I never used teflon or other sealers again.
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Unread postAuthor: inonickname » Thu Sep 24, 2009 11:16 pm

I used teflon tape in my co-ax and it works fine. Just get the thickest variety you can, use lots of it and wrench the connections down very tight.

My piston gun will hold 400 psi for half an hour without any noticeable leakage on the gauge or underwater.
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Unread postAuthor: psycix » Fri Sep 25, 2009 9:11 am

Easy:
Preparation: Make sure both threads are undamaged, also clean them if they are dirty.
Step one: Apply LOTS of teflon tape (in the right direction)
Step two: Find a big wrench and tighten it.
Step three: Find a larger wrench and tighten it even more! :D

Step three seems like "well duh" but there are many people out there which simply do not tighten it enough, and it can make a big difference.
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Unread postAuthor: jackssmirkingrevenge » Fri Sep 25, 2009 9:14 am

Araldite works for me, it can still be dismantled if needs be with a bit of strength.
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Unread postAuthor: Gippeto » Fri Sep 25, 2009 9:47 am

I use ordinary teflon tape past 10000psi. As mentioned, clean threads, wrap it in the right direction, 4-5 turns is enough, tighten with an appropriate wrench.

Some ptfe tape you'll find IS thicker (double and triple density), and easier to use...it doesn't break as easily when you pull it snug. Otherwise the major difference is simply a color coding. (double and triple density is code for some applications thus the color)(and attendant increase in price :roll: )

I've also found blue/red loctite to work very well.
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Unread postAuthor: psycix » Fri Sep 25, 2009 10:06 am

Note that if we are talking about the fittings on the pilot side, then leaks can be very dangerous.
A leak in the chamber is a loss of pressure, a leak on the pilot makes it a timer-triggered gun, unless the leak is very small and your piston fit is loose.
So your pilot side is important to be leak-free.


How do you guys seal threads you can't tighten all the way down.
For example the blowgun on the back of a QEV: it needs to be aligned in a certain direction, or else the gun looks weird and is held awkward.
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Unread postAuthor: D_Hall » Fri Sep 25, 2009 10:08 am

Gippeto wrote:I've also found blue/red loctite to work very well.

This.

Once upon a time I used to play a lot of paintball. Leaks were always a problem. I tried many kinds of sealing compounds and such with mixed success. Then somebody recommended Loctite.

I never had a leak again. Not one.
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