I have never had a problem with Teflon tape for water pipe joints before.
Now I did 4 joints and all 4 leaked, even after tightening them tighter
than I have ever had to tighten a joint. I ended up disassembling and using
the white pipe dope with Teflon in it. One joint seemed to leak water
through the layers of tape! Any ideas what happened? I put 3 layers of tape
on as snug as I always do but this didn't hold any of the joints.
Did you buy it at a real supply house?
I was helping a friend and he bought some at big box and the texture and
behavior was noticeably different than good tape. It didn't wrap
properly and took way too much torque. I ended up using some from a box
of stuff I brought. Made me wonder if it wasn't recycled or some other
I had the same thing happen with home cheepo solder on another occasion.
I know how to solder and the solder just wouldn't melt and wet properly.
Teflon tape is NOT designed or promoted to be a sealant - it is a lubricant
to facilitate joining pipes. (Oil will work almost as well as Teflon.)
It is the deformation of the threads themselves that act to seal the
And actually none of it has Teflon brand TFE according to DuPont.
So in a way you are correct since there is no such thing as "Teflon Pipe
Here's what I've found working with threaded pipe.
Probably nothing designed for more than 150 psi.
Includes industrial work, paid residential plumbing, and my own stuff.
High pressure water/steam stuff like I did in Navy is all welded
flanges, with a crushable metal gasket between the flanges.
Called the gaskets flexitallic, maybe a brand name.
Designed to whatever the pressure needs are.
I've cut thread in many pipes up to 1 1/2".
Good new threads don't need dope/tape to seal.
As HeyBub said, the threads deform.
I've gone without dope a few times - can't remember why.
With no dope/tape you have to crank more to deform more threads to
ensure a seal.
Water will rust up the innermost threads, so when you reuse the
fitting it's harder to get a seal, dope or not.
Both male and female will rust and lose threads, so you don't want
A doped/taped fitting can be resused numerous times.
Thread deformation/rusting is less, because the dope/tape
does help seal by filling in thread imperfections.
You'll crank it in a little harder with each use.
That's basically a "feel" you develop, but it's not rocket science.
New fittings should be easy to seal unless they're cut badly.
From watching novices work the main reason I've seen for leaks is
simply not cranking it in deep enough.
Normally for new pipe you only want 3-4 threads showing at a fitting.
Heavier drain pipe might no get cranked in as deep.
I've got a number of 50 year old galvanized fittings in this house
showing 5-7. They'll fail first.
I don't know why people don't crank them as deep as they should be.
When reusing some, I've cranked the last thread flush.
Valve threads are usually harder metal than pipe fittings, so you want
to dope/tape them well. They don't deform as well.
But you don't want to overdo the cranking as you can crack the valve
casting. You'll know when that happens.
Tony probably got badly threaded fittings, one side or the other.
I've never seen a real difference in dope. The old stuff was oil and
lead and maybe something else.
That would harden up good and the biggest disadvantage was you
had to clean it off the upper threads to reuse it.
I've been using the tape for years now and it comes in different
I wind it tight the entire length of the threads until if fills about
a third of the thread depth.
Never had a problem with it.
I will second all of the above. Vic, that was a good treatise on
threaded pipe. I am a certified pipefitter and a certified pipefitter
instructor, and that is almost exactly how I would have described it.
Perhaps but I think there is some misleading information the preceding
"Normal" / "every day" (the kind you buy at HD, hardware store, etc)
pipe threads NEED a sealant.
If the threads are cut as NPT threads they CANNOT work (ie no leaks)
without a sealant.
Only NPTF (dry seal) pipe threads are designed to work without
My experience (unpleasant) with NPTF dry seal threads was on military
hydraulic systems where tape or dope was prohibited.
On properly cut regular (NPT) pipe threads, the crest of the male
thread and the valley of the female thread
have a designed in (manufactured in) clearance.
No matter how much you tighten the thread there will always be that
small gap, the crest & the valley never meet.
This gap creates a spiral leak path from inside the fitting to the
This spiral leak path needs to be filled with sealant of some sort;
dope or tape.
NPTF are designed to have a crest / valley interference that deforms
and creates the seal.
Here is a link to further explanation of the NPT vv NPTF
Maybe the crappy NPT fittings sold today can function similarly to
NPTF threads but that is not the design intent of NPT threads. :(
Agree. It's always standard practice to dope/tape NPT.
See, I don't want to argue about this, because doping/taping is always
standard practice. You should ALWAYS dope/tape NPT.
But I've seen plenty of undoped non-leaking NPT on residential water
supplies. Maybe 50 psi or so.
And I know I've done it myself with no leaks, probably for a temporary
Sometimes you see rust spots where it seeped but stopped, sometimes
I figure minerals or oxidation fills the remaining thread gaps.
So I was wrong if I gave the impression you could get by with no dope,
because that's the wrong way to do it.
Also wrong about saying that thread deformation itself will seal.
That's all HeyBub's fault.
ALWAYS dope/tape NPT.
more once I've ignored a tiny leak (drip) on a threaded joint because
we have medium hard water.
A day later, no drips.
The main reason NPT "can" be assembled without dope or tape is that a
lot of water systems have water with calcium carbonate in it.
check out Locite HVAC thread sealer....
one of the major constituents is calcium carbonate
First, sorry for taking so long to get back to you. Just got my NG's
working again. This is garbage simply because this type of guideline
demands precision tapping and reasonable cutting standards. If one cuts
too many threads it can bottom out on any female connector that narrows
down soon after the threads. Also, the female threads would need to be
machined to a uniform standard, but they are not. Therefore one cannot
set such a standard as the number of threads showing on a fitting.
3-4 threads showing is hardly a standard. But it works.
I've threaded many pipes with machines and hand-turned dies.
Never "cut too many threads."
Threading machines have a stop, and with hand dies you thread until
the pipe is flush with the die.
Look at some threaded pipe at the store. You should see the same
number of threads on every one.
Can't speak to female fittings. Had some bad ones. Very few.
Of course with the notable exception of of anything using NPT (typically
found on anything a homeowner might encounter) which require dope/tape
whatever by design.
Succinct explanation here:
My swimming pool connectors used to leak until I
consulted expert Sam at the pool store. He advised
(1) six layers of teflon tape over the threads and
(3) another three turns at the end of the fitting (where
a large hex-shaped moulding appears.) These
connections leak no more.
When you mention ......my ongoing troubles with large PVC
Threaded PVC to threaded PVC or PVC into metal or metal into PVC?
Two practices that will greatly reduce problems:
1) Never use tape on threaded PVC to threaded PVC; the "extra
thickness" can over stress the female thread and create a spitting
Use Loctite - No More Leaks thread sealant
2) PCV into metal never metal into PVC; the tapered metal thread can
over stress the PVC part and induce a splitting failure
interesting. I can find it listed for sale, but not on the Loctite
I get mine at McMaster but it's available lots of places online
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