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Fixing a bad threaded PVC coupling

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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Fixing a bad threaded PVC coupling

Unread postAuthor: microman171 » Thu Nov 24, 2011 8:05 pm

Hey Guys,

I recently added a sprinkler valve to pilot my 50mm piston cannon. It made an awesome difference, and I was looking forward to getting some epic shots out of it.

Unfortunatly, when I first screwed the new 50mm reducer in, I over tightened. This partially broke the threads inside the PVC female coupling, resulting in a small leak. This was no big deal at first, but as I was trying to fix it, I had to undo and tighten the reducer a whole lot. The threads took a hammering, and now they are so bad the coupler goes in crooked.

The expensive part at one end was the 50mm to 40mm reducer followed by the 40mm to 32mm bushing. The plumber ordered me two, and I never got around to returning the other one. So I actually have a spare there. I just need a length of 32mm and a length of 50mm pipe and a 50mm threaded pressure coupling in order to get the cannon working again.

However, actually finding this pipe is getting tricky.

Leaving me with this question; Is there any way I can fix the threads on the female coupling? It is solvent welded to the pipe, so there is no way to remove it short of carefully cutting it off, and then getting the pipe perfect for a new coupling.


Cheers,

MM
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Unread postAuthor: Gun Freak » Thu Nov 24, 2011 8:16 pm

If it's cracked why the heck are you using it?? Mattyzip managed to carefully sand the pipe smooth enough to put a new fitting on when his female adapter brole from high piston impact.
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Unread postAuthor: microman171 » Thu Nov 24, 2011 8:23 pm

It's not cracked, the threads are a little broken. There is perhaps 80% of good thread holding it on, but there are really bad flat / double threaded spots on either side.

However, unless I can get it to thread back into the original threads somehow, it's not coming back from this one...
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Unread postAuthor: Lockednloaded » Thu Nov 24, 2011 8:26 pm

Do you need to be able to unscrew the male fitting? If not, I suppose you could solvent weld the threads to stop any leakage
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Unread postAuthor: microman171 » Thu Nov 24, 2011 8:45 pm

Unscrewing the coupling is important to be able to service the piston.

I managed to fix the threads enough to be able to smoothly screw and unscrew the reducer bushing into the coupling. I did this with an angled piece of steel which I forced through the channels in order to press the bad threads out, and emphasise the good threads.

However, it still does not seal. This is probably due to the less than perfect threads. Is there any kind of material I can use that will fill the gaps? I am running low on thread tape, but it's easy enough to get a whole lot of that will actually work? Normally I would just put two or three turns on, but this will ned quite a layer.
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Unread postAuthor: Gun Freak » Thu Nov 24, 2011 9:15 pm

Try putting a crap ton of teflon tape.
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Unread postAuthor: mark.f » Thu Nov 24, 2011 10:14 pm

If all else fails, you can saw the coupling completely off, leaving a piece of 50mm in your tee for the piston to ride in cut flush with the tee.

If 50mm is anything like 2" american SCH-40, 32mm socket fittings should fit well in it. IF this is the case, you can easily fabricate an o-ringed back plug to screw into the tee.

Here's a picture of how I did it. Only thing I would suggest differently (which I am doing on a valve I'm making now) is to simply cut another piece of 1.25"/32mm socket to form the back of the o-ring groove, as the washer epoxied on in the picture popped off due to excessive o-ring compression when inserted into the 2"/50mm pipe.

<div align="center">Image
Click for larger image...</div>

Best of luck getting it all sorted out.
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Unread postAuthor: microman171 » Thu Nov 24, 2011 10:18 pm

The cannon is actually a coaxial: http://www.spudfiles.com/forums/blowgun ... 23595.html

After fixing it with the piece of steel, it seems the threads are in a condition they are reusable.

It is possible I could put a large O-ring at the end of the screw fitting, and then tighten it down so that the seal is against the top of the coupler and reducer connection?
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Unread postAuthor: Gun Freak » Thu Nov 24, 2011 10:48 pm

Anything's possible. I'd make a groove in the male part for the o-ring. Go for it.
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