Teflon tape is not good pipe thread sealant

[soldar]

I have been doing plumbing repairs for many decades now. Long before Teflon tape existed I, we, used hemp and I would put a few drops of paint on it and those threads sealed reliably every single time.

Now we have this newfangled Teflon tape which is supposed to be better but it just does not work well for me. It does not seal as well and even if I use a lot of tape I very often get a micro drip. Not much, a drop every hour or less but still unacceptable.

I guess the very slippery nature of Teflon is what lets the water leak past it.

Using a drop of paint with hemp assures a good seal but the paint part is a bit of a hassle.

Instead of hemp I have often used strips of non-woven material, NWM, like the sheets that go in the clothes dryer, some napkins and bags, etc. I cut strips about 10-12-15 mm wide and they seal really well.

But I do not use NWM if I will later be applying heat because I do not know if the NWM will stand the heat. In cases where i will be soldering the pipe or fitting I use hemp.

I have just installed a water pressure gauge in the main pipe, using Teflon, and I am quite frustrated that I am going to have to do it all over again.

The gauge is 1/4" BSP male and needs to go into 3/4" BSP female so there are three threads: 3/4", 1/2", 1/4".

They are as tight as can be and yet still drip minimally. I am also suspicious that the threads should be tapered but in fact are straight, cylindrical. I think that would explain a lot. Tapered threads will tighten and seal but straight threads will not tighten.

If I put some container to catch the drip the water would probably evaporate and it would solve the problem. Some times after days or weeks the drip might stop because the seal improves, maybe the calcium in the water seals it. 

Very frustrating.

[negativ3]

BSPT are tapered, BSPP need a sealing washer to work.

[Simmed]

i changed 2 taps
used teflon
no problems since 7 years ago
i remember back then
i have to use the really thin version
 

[Analog Kid]

What is "BSP"? (or "BSPP"?)

Is that pipe thread (tapered) or a flare fitting where the seal is formed by a gasket or two mating surfaces instead of the threads?

If it's the latter (flare connection), then you shouldn't be using any sealant on the threads, since they're not what creates the seal.

If it's pipe thread, then you're doing something wrong.
I've used Teflon tape all over the place and never had it leak.
Maybe you're not tightening the connection enough.

[NE666]

What is "BSP"? (or "BSPP"?)

"British Standard Pipe" - https://en.wikipedia.org/wiki/British_Standard_Pipe

[Analog Kid]

What is "BSP"? (or "BSPP"?)

"British Standard Pipe" - https://en.wikipedia.org/wiki/British_Standard_Pipe

So tapered threads, right?

[NE666]

Qualifier: Enthusiastic DIY-er, with several home's worth of new radiators, bathrooms and kitchens over the years. Not a professional plumber.

+1 for trusting Teflon tape *where appropriate* and *used correctly*
Biggest mistake I've seen and made is using tape on compression fittings, the type that uses 'olive' rings to make the seal. Tape goes around the olive NOT the threads of the fittings, which aren't contributing to the seal

On tapered and parallel BSP fittings, I've used both Teflon tape and Loctite's "Type 55 Pipe Sealing Cord". The latter is fine as-is, the Teflon benefits (imho) from a smear of Fernox LS-X Leak Sealer before assembly.

[NE666]

So tapered threads, right?

Can be either. "BSPT" denotes tapered, all other BSP designations are variations on parallel.

[Analog Kid]

Biggest mistake I've seen and made is using tape on compression fittings, the type that uses 'olive' rings to make the seal. Tape goes around the olive NOT the threads of the fittings, which aren't contributing to the seal

Thanks; "compression fitting" was the term I was looking for but missed.

Except that you shouldn't be using anything around the "olive rings" (aren't those just O-rings?), should you?. No tape, no dope.

[soldar]

BSPT are tapered, BSPP need a sealing washer to work.

Well, yes, of course I know that. But normal plumbing threaded bushings are supposed to be tapered and seal that way. They have no way or place to fit a washer.

A union works by having two flat surfaces, seats, and possibly a washer between them, pressing against each other. This is not what I am using.

I am using bushings where the thread itself needs to seal and therefore needs to be tapered.

[Analog Kid]

I am using bushings where the thread itself needs to seal and therefore needs to be tapered.

Pardon my ignorance, but what do you mean when you say you're "using bushings"?
Are you referring to the pipe threads themselves as "bushings", or are you actually using a bushing--that is, a separate threaded piece--to join two pieces of pipe, or a piece of pipe with a fitting?

[NE666]

Except that you shouldn't be using anything around the "olive rings" (aren't those just O-rings?), should you?. No tape, no dope.

That is supposed to be the case, yes. Mechanical deformation of the softer olive around the pipe forms the seal. Which is why, if you take such a joint apart, you're also supposed to replace the olive(s).

However, in my experience, when I've used them without tape, a few always need an extra hard crank to stop the drips when the system is filled. If I apply tape, >99% of the time, they seal first time. I suspect, though have no evidence to support my assertion, that modern olives and fitting just aren't as well made as they used to be, and potentially from lower grade material.

[Analog Kid]

If I apply tape, >99% of the time, they seal first time. I suspect, though have no evidence to support my assertion, that modern olives and fitting just aren't as well made as they used to be, and potentially from lower grade material.

Well, I suppose there's no harm in that, despite what the current conventional wisdom is ("don't use tape here").
If the joint seals and doesn't drip, that's what matters.

[soldar]

Pardon my ignorance, but what do you mean when you say you're "using bushings"?
Are you referring to the pipe threads themselves as "bushings", or are you actually using a bushing--that is, a separate threaded piece--to join two pieces of pipe, or a piece of pipe with a fitting?

Bushing:

[soldar]

After giving it some thought I believe the threads on these fittings and bushings, which should be tapered, are, in fact, parallel and because of that do not tighten further as they are screwed in as they would if they were tapered.

That is why Teflon does not seal as well as hemp with paint. Hemp has the added advantage that it will swell if it gets wet.

I think from now on I should just stick to using hemp or NWM, with a dab of paint, with this type of fittings and avoid using Teflon tape.

Teflon should work fine in places where it is designed for, like tapered threads, but it is not working in the fittings I am using and I have had this problem for years now. And we are talking quite low pressures, under 3 or 4 bar.

[Tation]

Another DIYer here. I've used Teflon tape with success many times, but found that some threads need a not too high number of turns (say 5-7) whereas others require a much higher number (20+). Also used such Teflon cord… this time without success at all, apparently it must need some kind of technique I'm  not aware of.

[nali]

Never really been a fan of Teflon tape myself, I just find it fiddly and hard to gauge the right amount. Last time I had to fit some radiators I used this TruBlue stuff which is a mix of sealant and strands of PTFE which worked great.

[eutectique]

I tried teflon tape before, but had the same microdrips you are describing. Switched to Loctite 55, had no problems ever since.

[nctnico]

After giving it some thought I believe the threads on these fittings and bushings, which should be tapered, are, in fact, parallel and because of that do not tighten further as they are screwed in as they would if they were tapered.

That is why Teflon does not seal as well as hemp with paint. Hemp has the added advantage that it will swell if it gets wet.

I think from now on I should just stick to using hemp or NWM, with a dab of paint, with this type of fittings and avoid using Teflon tape.

Teflon should work fine in places where it is designed for, like tapered threads, but it is not working in the fittings I am using and I have had this problem for years now. And we are talking quite low pressures, under 3 or 4 bar.

What kind of teflon are you using? It comes in several thicknesses and I've found the thin tapes don't work very well. I'm using thick teflon tape for compressed air on non-tapered threads and this works just fine.

[soldar]

Another DIYer here. I've used Teflon tape with success many times, but found that some threads need a not too high number of turns (say 5-7) whereas others require a much higher number (20+). Also used such Teflon cord… this time without success at all, apparently it must need some kind of technique I'm  not aware of.

Never really been a fan of Teflon tape myself, I just find it fiddly and hard to gauge the right amount. Last time I had to fit some radiators I used this TruBlue stuff which is a mix of sealant and strands of PTFE which worked great.

Teflon tape might work very well for some specific threads, like tapered threads, but I think it stands to reason that, in general, any paste or liquid sealant, especially if combined with fibers, would seal better than Teflon tape.

I have never had a problem with the traditional hemp with a few drops of paint or sealant. I have sometimes run into problems when using solely Teflon tape.

As I say, I think the threads I was assuming to be tapered are, in fact, parallel and therein lies the cause of the problem. Teflon will not seal them as efectively as something sticky.

Saying in very general terms "I have used Teflon with no problem" is not helpful. It is important to understand the different kinds of seals and how they work.

[soldar]

What kind of teflon are you using? It comes in several thicknesses and I've found the thin tapes don't work very well. I'm using thick teflon tape for compressed air on non-tapered threads and this works just fine.

This might be a factor. I have thought about this in the past and bought a roll of the thicker Teflon but I couldn't find it at first sight and I have rolls of the thinner Teflon everywhere so I just used the thinner stuff which I had at hand.

Still, I think probably the best way to be sure a threaded joint will seal well is to use some kind of dope, paste, paint, etc. in conjunction with fibers, not with Teflon.

[langwadt]

the biggest advantage to hemp and paste is that you can adjust it to align things. With teflon tape you only get one chance

[SteveThackery]

Perhaps contrary to other people's experience, I have always had success with Teflon tape on the threads of plumbing fittings, but I put it on much thicker than "looks" correct. So thick enough to blur the shape of the thread substantially.

I realise that is a rubbish description, so my advice would simply be to try applying the Teflon tape much thicker (more turns) than you have been doing.

[jpanhalt]

@soldar

I've done plumbing for a long time too as a DIY'er (circa 1960 to present).  I've never done cast iron pipers with poured lead, but I have
watched it being done with old-fashioned white gas blow torches. I've done just sweated water joints to up to 2" and tapered threaded joints for gas (up to 1") and water.  For gas, I would not trust Teflon and use a thick paste like RectorSeal specified for gas.   

For any threaded joint, I examine the thread and try a dry fit.  If the threads are decent and fit well, I will use skived Teflon laid neatly on the threads such that tightening will tend to pull the tape tighter (looking at the male end, that would be clockwise). Use at least 3 turns.  I have both a thicker version from McMaster-Carr and the usual name branded stuff from a big box store.  Both seem to work about equally.  I do not use generic, non-branded tape.  The savings is not worth it to me.

I examine what I buy carefully.  If I happen to be forced to use an imported piece of junk with horrible threads, I use RectorSeal.  Some people say to use both.  I don't see the purpose in that.  If Teflon alone works, that's fine.  if it doesn't work (or is not expected to work) using it with the pipe dope will likely interfere with that sealant.

[mendip_discovery]

Looks at Dead Weight Tester at 700 bar, ptfe tape works fine for that.  Let me just check the air pump at 100 bar....yup still ok.

I prefer bonded seals but all depends on your connectors. Right tool for the job.

BSP aka Gas Thread is a straight whitworth thread often marked as G1", it isnt 1" on the major, but bigger as its for Gas pipe of 1" diameter. BSPT is the tapered variety.

I would blame the user that the other end of the spanner.

Sorry not read all the replies, only 30 mins for lunch.