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 threaded pipe fittings always leak
Author: Anonymous User


I am having a heck of a time with threaded pipe fittings on pressurized water pipe.

Seems they always leak. Im sure that I am doing something wrong but I dont know what.

I have some brass 1 1/4" brass cased ball valves coming off an underground water supply that are leaking.

The underground pipe is vinyl plastic tubing. I installed the plastic pipe nipple into the tubing, tightened the hose clamp around it, and threaded the valves onto the nipples hand tight plus a turn. I thought that the plastic nipples would have excellant sealing characteristics, so I first tried the joints with no sealant at all .

They leak. I tried tightening them more, no change.

I disassembled and installed a new nipple, assembled with teflon tape on nipple threads. They leak. Tighten more, no change.

Disassembled again. Installed a new nipple, applied a paste type joint compound with teflon in it. Reassembles, still leaks.

Disassembled again, installed a new nipple, threaded valve onto nipple only finger tight with teflon tape seal pressurized. Slowly tightened until leaking was at minimum, still dripping.

Decided that plastic threads do not seal to brass threads so I chucked the plastic nipples and got a galvanized steel nipple.

Installed galvanized steel nipple into vinyl plastic pipe, tightened hose clamp. Applied teflon tape, threaded brass valve onto nipple hand tight plus 1 turn. Leaks.

Disassembled, installed new galvanized steel nipple, assembled valve onto nipple only finger tight with teflon tape as sealant, preassurized system and slowly tightened. Still drips.

Cranked the heck out of it, as tight as God would let me, still leaks.

Gave up and mulled it over in my head for a while. Meanwhile went to help mother-in-law install a new faucet in her bath tup.

Faucet has compression fittings. The male side of the compression fitting is a female pipe thread connection to the supply pipe. Soldered copper thread to tube adapters on water pipes then threaded the compression fitting onto the adapters with paste pipe dope. Set the new faucent in place on the compression fittings, tightened the lock nuts, pressurized. Guess where it leaks?
At the threaded joint between the compression fitting and the copper pipe adapter.

Tired of messing with threaded fittings I applied flux to area and put a bead of solder all around the threaded joint.

Pressurized. Leak fixed. Melted plastic valve inside new faucet. Faucet junk. Time to start over again.

Why cant I get any threaded joints, plastic copper brass or steel, to seal pressurized water with any combination of joint compound and torque?

What am i missing?

Thanks for help

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 Re: threaded pipe fittings always leak
Author: jblanche (WI)

Try search for "hose barb" or "barb fitting" here. One tip I picked up was to use a heat gun or hair dryer judiciously to expand the pipe slightly before putting the barbed fitting in. (Maybe you could also put the brass fitting in some liquid nitrogen!) I have used all brass fittings with one hose clamp each, torqued with a cordless drill. Nothing special. No leaks.

However I have black polyethylene pipe, so I can't comment on vinyl.

Links to the State of Wisconsin Plumbing Code:
I am not a plumber.

Post Reply

 Re: threaded pipe fittings always leak
Author: Dunbar (KY)

Well system or city water?

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 Re: threaded pipe fittings always leak
Author: HytechPlumber (LA)

When ever connecting plastic threads care must be taken not to over tighten the threads. I like to use teflon tape and teflon paste. I like 5 wraps of teflon tape but with plastic threads a little more. Another common problem is not using the 3 point grip with the pipe wrenches. This will often egg shape the nipple and therefore cause a leak. And then there is that one thing all plumbers are familiar with. That one day when nothing seems to go right. GOOD LUCK

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 Re: threaded pipe fittings always leak
Author: Anonymous User

It's very easy to confuse fitting compatibility. I do it often.

There are basically two kinds of seals:

Compression relies on smooth mating surfaces (taper-to-taper, flat-to-flat, etc.) or deformable surfaces (o-rings, etc.) being pressed together. Often the surfaces will be aligned and drawn together by some kind of captive threaded fasteners, but these threads should not be expected to provide a seal. Teflon tape is useful here just to keep the threads from corroding together. ALL the seal should be on the planes of compression.

Thread seals rely on TAPERED thread, AKA "pipe thread". If both male and female parts are tapered and threaded identically (the thread cutting also forms the taper) you get a sort of compression seal running all around the threads. Unwound, the seal could be several feet long. That makes it strong. Tapes and goops are useful here because the threads will never be perfect mirror images, so unless one part is maleable (plastic), there will be a long narrow path for water to creep around. Pipe dope will fill that gap.

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 Re: threaded pipe fittings always leak
Author: Anonymous User

Well system.

Its not the barb to tube connection that leaks, its the valve to barb NPT connection that leaks.

And in the faucet connection it was the NPT adapter to fitting connection that leaked.


Post Reply

 Re: threaded pipe fittings always leak
Author: jimmy-o (CA)

I have always been told and by experience found that 2 wraps of tape is all tou should use. Too much tape "thickens" the threads and does not allow them to engage enough turn. Plastic threads especially seem to be vulnerable to being tweaked out of shape if overtightened or overtaped. Just my opinion.

Obviously you have a system which works for you and I never argue with success!

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 Re: threaded pipe fittings always leak
Author: hj (AZ)

We cannot tell just from your description, so I guess the thing you are missing is a plumber. We cannot be sure exactly what you are working with, but I have never had problems such as you are describing. Soldering the fittings was the worst idea, especially after they had leaked.

Post Reply

 Re: threaded pipe fittings always leak
Author: Anonymous User

This sounds similar to a problem I had last week.

I was putting shut off valves under my kitchen sink. I ended up with flexible braided metal pipe connected to metal screws. The flexible metal pipe had a rubber (or similar soft material) inside the metal threaded fitting. The threads were designed to bring the rubber gaskets in contact with the other surface and compress against it.

I got the cold one working fairly quickly but could not get the hot one to stop leaking. I had been told to be careful about how tight to make these connections for fear of damaging the hardware.

To make a long story short, I hired a plumber. He came and looked at my work and admitted that what I had should work. He didn't even get out his tools. Just used what I had lying around. He tightened my leaky joint, and tightened, and tightened...

He could tell that the rubber gasket had not been seated against the other surface. He got it. It works now.

My conclusion is that the hardware store I got the parts from was giving good advise (not to tighten too much). However, a plumber who does hundreds of these a week can use his experience (which I lack) to exercise his judgment in a particular situation.

Anyhow, my suggestion is, get a plumber. I was happy.

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