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New to pistons

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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New to pistons

Unread postAuthor: Mooglebass » Tue Aug 30, 2011 5:22 pm

Ok so before I have made a combustion, and a pneumatic with a sprinkler valve. So i looked into pistons, and followed MRR video. I made it out of a 1 inch tee, used a brass nipple to for the piston and had to make my own barb nipple. The hot glue piston moved fine by my fingers pushing it. Not as smooth as his though. I lubed it with some wheel bearing grease(all i have) still moves tight but moves.

Now for the tricky part. I assembly it to a 1" sprinkler valve to discharge( need to fix that probably) and then a chamber. I start to air it up and the piston moves forward and seats some what. If i hold my hand over the barrel or where the barrel will go, it finally seats. Which is my problem, why wont it seat all the way till i cover the barrel?
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Unread postAuthor: Technician1002 » Tue Aug 30, 2011 5:47 pm

The piston and barrel surfaces are not perfect. This permits air to leak out the barrel before enough pressure builds to press them together. Sealing the barrel permits adding pressure that seals the barrel when your hand is removed. A high flow rate air source such as a shop compressor can exceed the leak rate and fill the chamber. Hot glue and brass don't lap well together due to the huge difference in hardness.
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Unread postAuthor: Mooglebass » Tue Aug 30, 2011 6:02 pm

So should I try a bolt and washer pistion?
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Unread postAuthor: Technician1002 » Tue Aug 30, 2011 6:33 pm

First is to get the piston face polished perfectly flat and true. I'm not sure of the best way to do that on a hot glue piston.

Second is to make sure the valve seat is true and flat. This is not hard. Use a wood dowel that barely fits in the T in place of the piston. Glue a piece of 600 grit sandpaper on the end and polish the valve seat. Check to see if the wear is uneven. Keep sanding until the wear is even on the entire valve seat.


Example photo. Piston is a molded PVC pipe cap. It was sanded on the face until it is true and flat.
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My machined pistons are trimmed on the face to be true and flat.
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Unread postAuthor: jsefcik » Tue Aug 30, 2011 7:28 pm

Try bolt and washer, i have one and works great!!!! :D :D

take a look at it here
http://imageshack.us/g/842/dscf8720l.jpg/
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Unread postAuthor: Mooglebass » Tue Aug 30, 2011 7:31 pm

Ok so the goal here is to have a tight fit on the piston, but not super tight. enough for ease of movement. I know i saw somewhere on here about using a dowel rod and sand paper to polish the inside. Guess back to the drawing board. Sucks not having a garage to work on this stuff.
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Unread postAuthor: Gun Freak » Tue Aug 30, 2011 7:36 pm

A bolt washer piston with floating o-rings has barely any friction, the ones I make don't even need lubrication.
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Unread postAuthor: mattyzip77 » Tue Aug 30, 2011 7:56 pm

yes gunfreak, you do make a mean bolt/washer piston if I do say so my self, best I have seen on here!! :D :D
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Unread postAuthor: Mooglebass » Tue Aug 30, 2011 8:01 pm

On the bolt piston though. Will it make a difference if its stainless or not? Lock nuts I assume. 1/4 20 a good thickness? Time to go through the how to section on bolt pistons, or if you have a link. Thanks for the info, more info i have the better my build become.
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Unread postAuthor: mattyzip77 » Tue Aug 30, 2011 8:06 pm

I dont know the sizes, and this is gunfreaks piston, I am just posting the picture, if he wants to post the sizes, that is on him!!
http://i979.photobucket.com/albums/ae27 ... M_1098.jpg
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Unread postAuthor: jsefcik » Tue Aug 30, 2011 9:04 pm

just have gunfreak build you one!!!!!!!!!! save all the time and heachaches !!!! :D :D
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Unread postAuthor: Mooglebass » Tue Aug 30, 2011 9:08 pm

Only downside to having him build one is that how will i learn? Yeah I can copy his, but need to do the trial and error myself. Why i am here now lol
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Unread postAuthor: Gun Freak » Tue Aug 30, 2011 9:10 pm

:roll:
Guys please.

It's 5/16 SS on mine. 2" and 1.5" washers. I have plans, if you'd like.
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Unread postAuthor: Mooglebass » Tue Aug 30, 2011 9:14 pm

Sure, can never hurt. Just scale it down for my needs and then make a 2" one for another gun :lol: Thanks a bunch Gun Freak
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Last edited by Mooglebass on Tue Aug 30, 2011 9:15 pm, edited 1 time in total.

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Unread postAuthor: mattyzip77 » Tue Aug 30, 2011 9:15 pm

Mooglebass wrote:Only downside to having him build one is that how will i learn? Yeah I can copy his, but need to do the trial and error myself. Why i am here now lol

from that comment, you probably earned alot of respect from people on this site, unlike others that ask questions that fail to do any research on their own> Good luck and if you do need help that you cannot find the answers to, then ask!
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