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making a piston valve in a 1.25" galvanized tee

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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making a piston valve in a 1.25" galvanized tee

Unread postAuthor: JDP12 » Wed Mar 25, 2009 8:56 am

Hey all,

I've been inspired by the surge of sawed off guns around and I decided that I want one..

However I've run into a couple design issues I was hopin you guys could help me out with.

I plan on using this gun with unregulated CO2, so everything has to be galvanized fittings. However, I haven't figured out how to build a piston valve inside one yet for a variety of reasons. First off, I have to figure out what to seal against. I thought about a piston face which slid into a bushing, sealing it that way, but then I couldn't figure out how to get it to pilot.

The front of the piston will be a 1.25"x3/4" bushing. The back will be the same to go to a 3/4" ball valve.

Does anyone have any ideas on building a piston to fit in it? I know BTB has built pistons inside galvanized tees before, but I just haven't figured out how yet because a) there's less space and b) I can't figure out what to have it seal against


Thanks all
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Unread postAuthor: psycix » Wed Mar 25, 2009 9:07 am

a) there's less space and
b) I can't figure out what to have it seal against

a) You can also have the piston slide inside a pipe instead of the T.
b) Well duh, the barrel?! Just have it stick into the T. You may need to modify a bushing.
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Unread postAuthor: JDP12 » Wed Mar 25, 2009 9:17 am

The problem is getting the barrel inside a tee. I'm only going to be using threaded fittings, and I can't figure out how to get the barrel inside. I tried threading a pipe nipple into the opposite side of a bushing but that didn't really work. It started to turn but then locked up due to the threading.

I thought about somehow threading the bushing backwards for part of the way... but abandoned that idea due to the difficulty


I could epoxy in a nipple into that opening in the back of the bushing. The trick is finding one the right size

Also for the piston I was thinking about a cast epoxy piston. Was going to house it in a pipe in the back of the tee.

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Last edited by JDP12 on Wed Mar 25, 2009 9:50 am, edited 1 time in total.
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Unread postAuthor: psycix » Wed Mar 25, 2009 9:45 am

You can either thread both the barrel and the nipple on two sides into the bushing like you tried (why didnt that work?)
Or thread your barrel a long way so you can just keep screwing it in further and further.
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Unread postAuthor: JDP12 » Wed Mar 25, 2009 9:53 am

I'm not sure why it didn't work. Could be because they are NPT threadings and gradually tapered so then it is only a one way. NPT threadings eventually are forced to stop due to the tapering I've found... not sure what you foreigners use.

I tried to turn it into the back of the bushing that you see in the picture and it "locked" into the threads but then wouldn't turn any more. I also tried to reverse turn the nipple, but didn't thread in at all.

Could also have a crappy fitting, but it threads in fine to the front of the bushing.

When I get home I'll try to thread it in backwards again. If that does not work, I'll just have to get my hands on some epoxy and epoxy the nipple into the bushing.
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Unread postAuthor: psycix » Wed Mar 25, 2009 10:52 am

The tapering makes it nearly impossible to thread it in backwards.
Maybe get another bushing which uses non-tapered thread. (try BSP, BSPP in this case. Also noted by the letter G)
This requires an extra bushing to go from NPT to BSP though.

You could try screwing NPT on BSP, but that is not recommended.
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Unread postAuthor: Fnord » Wed Mar 25, 2009 10:58 am

Try to find a couple different types of the same bushing if it isn't too inconvenient. From what I can tell some work fine and some don't.

If you have a thread chaser of that size, try it. If you don't I'd tighten it down into a vice and muscle the nipple in with a big pipe wrench. There may just be a burr or bad spot on the threads that's keeping it from moving.

If that doesn't work, I'd try solder instead of epoxy, though brazing would be better than both.
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Unread postAuthor: JDP12 » Wed Mar 25, 2009 11:30 am

hm.. well we'll see what happens. i could just destroy the threads and force the nipple through.

Or I could weld the nipple into the bushing.
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Unread postAuthor: mark.f » Wed Mar 25, 2009 11:47 am

Why not use a copper sealing face?

Get a male adapter the same size as the tee (1.25"), get a bushing that goes down to the size of your barrel (the copper tube size that has the same I.D. or better as your barrel), solder them together, file out the bushing, and then solder a piece of copper through the bushing and into the tee. A female or male adapter can then be soldered onto the piece of tubing extending out of the tee.

Just my suggestion. Velocity made a metal barrel sealer with a 1.25" tee valve, but I think he used plastic as the sealing face... you'll have to contact him...
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Unread postAuthor: Fnord » Wed Mar 25, 2009 11:50 am

If you have a welder, why are you relying on premade fittings so much? :)

My last welded piston valve consisted of a 2" coupling, 1.5" coupling, and some threaded 1.5" and 1" pipe (All modified). It was tricky but ended up extremely compact.

edit@mark, depending on the pressure used and barrel diameter, copper pipe could really cut into a rubber sealing face because of its thin walls. Could work though

edit@jeepkahn, Yeah, galvanized is bad for the lungs. It will kill you if you breath enough of it, so you should grind off the coating or soak it in vinegar over night. Then it'll be fine to weld.
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Unread postAuthor: jeepkahn » Wed Mar 25, 2009 12:10 pm

if your going to be welding, I would recomend black iron as opposed to galvanized, when heated galvanized gives off nasty green smoke that you REALLY don't want to breath...

It won't kill you, but you will wish you were dead...
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Unread postAuthor: ramses » Wed Mar 25, 2009 12:24 pm

I just had the same difficulties. I used a drilled out reducing copper male adapter to get a piece of 1/2" copper pipe into a 3/4" tee. I will have the piston slide in a nipple that was drilled out to get rid of the weld seam thing. (won't be a problem if you use seamless pipe). EDIT: oops, I was beat!

These things will be more difficult for you because of the greater size (you will need a BIG drill bit and a BIG drill press (or a lathe). Try fitting different sizes of pipes and try to get them through the bushing. If you find one that is close, you could try to solder it, weld it, JB weld it, etc.
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