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Best piston design for a semi auto valve

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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Best piston design for a semi auto valve

Unread postAuthor: Gun Freak » Tue Jun 29, 2010 4:01 pm

I am building a semi auto and I need to know what type of piston would be best. EQ hole, check valve, o-rings or what, it will be 3/4 with 1/2 in porting. Thanks, Gun Freak.
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 29, 2010 4:21 pm

Are you feeding through the chamber or pilot side?
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Unread postAuthor: Gun Freak » Tue Jun 29, 2010 4:24 pm

Pilot, through a homemade 3 way with 1/4" ports probably with around 140 psi.
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 29, 2010 4:28 pm

A tight fitting piston will suffice, no need for an equalisation hole if you haven't fitted o-rings.
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Unread postAuthor: Gun Freak » Tue Jun 29, 2010 4:32 pm

Alright, I'll try a wooden dowel sanded down to size, the chamber is only about 3 inches of half inch pvc so it should fill fast right?
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Unread postAuthor: clemsonguy1125 » Tue Jun 29, 2010 4:33 pm

Remember Techs said his sealed without orings so you may still need a EQ hole
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Unread postAuthor: Gun Freak » Tue Jun 29, 2010 4:34 pm

He made his to tight tolerances with his drill press, I will make mine a less tight fit than his so it can equilize, or maybe a tight fit like his with an EQ.
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Unread postAuthor: clemsonguy1125 » Tue Jun 29, 2010 4:48 pm

Yeah Id make one like his if you can
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Unread postAuthor: Gun Freak » Tue Jun 29, 2010 4:50 pm

I think one with a looser fit withough an EQ would be more efficient than a tight fit with an EQ because the EQ has a higher flow than the space around the loose piston.
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Unread postAuthor: clemsonguy1125 » Tue Jun 29, 2010 4:51 pm

I disagree, Tech piloted a 2inch chamber, 1inch port, with a blowgun, and had great performance
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Unread postAuthor: ramses » Tue Jun 29, 2010 4:58 pm

An O-Ringed piston with check valve might be the best option. It would open the piston quickly, and allow fast flow to the chamber to fill. A 1/4" check valve would provide pretty much maximum flow.
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Unread postAuthor: Technician1002 » Tue Jun 29, 2010 6:09 pm

clemsonguy1125 wrote:Remember Techs said his sealed without orings so you may still need a EQ hole


Mine sealed when it had a small amount of grease on it. This prevented filling from the chamber without the EQ port. The rising chamber pressure opens the valve without the port.

The piston was close tolerance, but not tight. It closed by gravity for filling. No spring needed. It did need standing on end to start filling.

Close fit, but not tight.

The valve seat:OD ratio is a close ratio. Piston OD is 1.5 inch. Valve seat seal is 1.25 inches. There is only 1/8 inch difference in diameters.

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