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How to build a 3" piston

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How to build a 3" piston

Unread postAuthor: jmccalip » Sat Apr 25, 2009 6:52 pm

Materials needed:

2 1/2" PVC coupling
A couple inches of 2 1/2" PVC pipe
(two) 3" cut off wheels(substitute metal disks if you have them, or cut some out of sheet metal with a hole saw. The more rigid, the better.
1/2" x 6" nipple
1/4" x 4 1/2" screw with lock nut
Fender washer for each side of piston
large neoprene fender washer
small neoprene fender washer

Oring

I know 2 1/2" PVC is hard to find, so I just used conduit found at Lowes.

And one Lathe... I know some of you don't have access to one, so you might be able to substitute a drill press.










Cut two small sections of pipe to fit all the way into the coupling.
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Glue them in and let dry. Mine stuck out a little, but I just faced it off later, no biggie.

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Since the coupling is not the right diameter, we have to turn it down. This is why we put the pieces of pvc in there, to strengthen it.


We are aiming for 3.030-40". 3" PVC pipe averages 3.050 ID.
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Over 2 tenths of an inch too big.
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Nice fit.

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Next, you need to grind the grinding wheels( I know...haha) down to fit inside the pvc, cut the nipple to the right length, and cut grooves into the sides of the coupling to hold the disks.

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And yes I know I put the rubber fender washer on the wrong side...I'll take the nut off and switch it around later tonight.
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I left out the oring groove, but it's pretty self explanatory.
Total weight, 11oz.

Hope ya'll find it useful.
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Last edited by jmccalip on Fri May 29, 2009 4:15 pm, edited 1 time in total.
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Unread postAuthor: explsvcookie257 » Fri May 29, 2009 2:43 pm

Hey I was wondering how deep did you make the o-ring grooves and what size o-rings. Thanks because I am going to use your how-to for the piston in my gun. I don't get why no one else commented on your how-to I think its great and a very good desighn.

Also sorry for the bump I think good how-to's like this need more attention to them
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Unread postAuthor: jmccalip » Fri May 29, 2009 4:18 pm

explsvcookie257 wrote:Hey I was wondering how deep did you make the o-ring grooves and what size o-rings. Thanks because I am going to use your how-to for the piston in my gun. I don't get why no one else commented on your how-to I think its great and a very good desighn.

Also sorry for the bump I think good how-to's like this need more attention to them


I'm measure them for you later tonight.


If you're going to follow this design, I recommend you use something stronger than the cut off disks. They lasted a good while, but they finally cracked .
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Unread postAuthor: explsvcookie257 » Fri May 29, 2009 5:09 pm

Alright thanks so much and Yeah I was thinking they might break eventually considering im dumping a 4" x 4 foot air tank at between 100 and 150psi against it. I will probably use a flat pvc disk or a 4" fender washer turned down to fit in a groove. Oh and if you could Ide like to know the length because Im thinking it might not be long enough but that can be easily fixed.
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Unread postAuthor: jmccalip » Fri May 29, 2009 5:18 pm

150PSI? That's a LOT of pressure. To be honest, I probably wouldn't go above maybe 80PSI with this piston. It might do fine, but it's too much work to have it break.

If you don't mind waiting, I suggest building a solid HDPE piston. It will handle very high pressure easily.
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Unread postAuthor: Technician1002 » Fri May 29, 2009 7:34 pm

jmccalip wrote:150PSI? That's a LOT of pressure. To be honest, I probably wouldn't go above maybe 80PSI with this piston. It might do fine, but it's too much work to have it break.

If you don't mind waiting, I suggest building a solid HDPE piston. It will handle very high pressure easily.


The piston isn't the only concern.
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Image borrowed from this instructable. There are some very serious forces and impact energies in larger pistons at higher pressure. :shock:
http://www.instructables.com/id/S67VTL2F5LD8J8D/

My cannon worked fine for a while with no problems, but when I decided to improve the seal, the valve worked too well in essence, opened too fast, and smashed the valve. The first time it just broke the fill system, that was an easy fix. The second time snapped the valve in half. This is serious power, people.


That has been my experience with PVC. It is good for a few beatings and then pow, it fails just when you feel it is safe.
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Unread postAuthor: explsvcookie257 » Sat May 30, 2009 1:04 am

but is hdpe heavy or really light i coukld gie you specifications and you can make me one lol
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