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Help with 3/4in QEV

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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Help with 3/4in QEV

Unread postAuthor: jim_himself » Tue Sep 21, 2010 7:57 pm

Hello, I've just started my first QEV air gun build and have run into an issue. The threads on my 3/4 to 1/2 (used for making my pilot) to fit into the QEV very well, I can only get about a turn. Both pieces are 3/4 and I don't know why they have different threads. I would appreciate any help with fixing the problem. If anyone has had this problem or knows of an adapter to fix it, that would be great.

Thanks in advance for any help
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Re: Help with 3/4in QEV

Unread postAuthor: Matt_NZ » Tue Sep 21, 2010 8:16 pm

jim_himself wrote:Hello, I've just started my first QEV air gun build and have run into an issue. The threads on my 3/4 to 1/2 (used for making my pilot) to fit into the QEV very well, I can only get about a turn. Both pieces are 3/4 and I don't know why they have different threads. I would appreciate any help with fixing the problem. If anyone has had this problem or knows of an adapter to fix it, that would be great.

Thanks in advance for any help


My guess is the two fittings have different types of thread.

The two common types of pipe thread are..

NPT American Standard Pipe Thread
BSPT British Standard Pipe Thread

So most likely the only fix is changing one of your 3/4" fittings, so they are both the same thread types.
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Unread postAuthor: jim_himself » Tue Sep 21, 2010 8:38 pm

Decided to go the "wrong" way. Covered the fitting in JB weld and twisted it in as far as i could, got it in pretty good without jb weld and it had a slow leak so I jb welded it and am waiting for it to dry then will try again.
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Re: Help with 3/4in QEV

Unread postAuthor: Ragnarok » Tue Sep 21, 2010 8:53 pm

Matt_NZ wrote:My guess is the two fittings have different types of thread.

Hmm. Usually, NPT and BSP are similar enough to be essentially interchangeable. The problem with mixing them is more likely to be slow leaks (usually fixed with generous PTFE tape) than them not fitting.

Also, one thing I should correct you on, the T in BSPT does not stand for "Thread", it stands for "Taper". This differentiates it from BSPP, where the second P stands for "Parallel".
I'm sure you can work it out from that, but for the benefit of any morons on the forum, BSPT has threads that get wider further up, BSPP remains the same diameter the whole way up.
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Re: Help with 3/4in QEV

Unread postAuthor: Matt_NZ » Tue Sep 21, 2010 9:16 pm

Ragnarok wrote:
Matt_NZ wrote:My guess is the two fittings have different types of thread.

Also, one thing I should correct you on, the T in BSPT does not stand for "Thread", it stands for "Taper". This differentiates it from BSPP, where the second P stands for "Parallel".


I stand corrected. I knew it was British standard something. :shock:
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Unread postAuthor: MrCrowley » Wed Sep 22, 2010 2:05 am

I do have a problem with the 3/4" 'American' QEVs not threading my brass BSP fittings very well. The NPT galv. iron fittings aren't much better really, perhaps they thread twice as far, which is not far at all considering the brass fittings only go about a turn and a half
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Unread postAuthor: jim_himself » Wed Sep 22, 2010 6:42 am

Well jb weld and turning the crap out of it did the trick. No more leaks :D :)
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Unread postAuthor: Crna Legija » Wed Sep 22, 2010 7:28 am

maybe its NPTF which if for fuel and high heat application so you cont need any sealant.
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Unread postAuthor: Gun Freak » Wed Sep 22, 2010 3:54 pm

@Jim, I think you went the wrong way with the JB Weld. It may have fixed the leak temporarily but it could spring another or maybe you will want to take out that fitting eventually if you make a change in the design. The better way to go would have been to find the correctly threaded fitting to use, but whatever floats your boat.
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Unread postAuthor: bighead33 » Wed Sep 22, 2010 4:25 pm

well gun freak you beat me to.
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Unread postAuthor: jim_himself » Wed Sep 22, 2010 10:23 pm

GunFreak- im still gonna be able to get it out, i just wanted i quick solution for some initial testing, i intend to properly fix it as soon as i get time to run up to lowes, though i have no idea what fitting to get, but i will play with the fittings till i find something that works.
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Unread postAuthor: Gun Freak » Thu Sep 23, 2010 4:47 pm

Yeah well wood luck with that... you are going to have a REAL hard time getting it out. Properly mixed JB Weld can be pretty impossible to get out and if your fitting was a weak metal like brass then you will probably round out the fitting before you can get it out. I suggest going to the store with the actual valve and find a fitting that goes in further.
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Unread postAuthor: jim_himself » Thu Sep 23, 2010 7:32 pm

OK well i took out the fitting, it was hard, but it came out, i went to lowes and got the same reducer in black steel and it works fine now.
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Unread postAuthor: Crna Legija » Fri Sep 24, 2010 12:50 am

At work we have a loctite for hydraulics and if you put it on 3/4inch pipe you cant get it off, you'd brake the fitting/pipe before it gives way.
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Unread postAuthor: jim_himself » Fri Sep 24, 2010 8:47 pm

U need to remember my fitting was only in a turn and a half. Also i put the pieces together and put the weld around the edges to make a shell of sorts[/list]
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