Teflon tape a no no?

[Skyco]

The NPT threads rely on their thread form and taper to do their sealing job.

To further confuse the issue I've done a lot of reading about NPT and NPTF since this thread started. Actually from what I gather in my research the common NPT fitting isn't really designed to seal without something applied, be it tape, or goo or whatever.
Yep thats what the real techy docs say. Sure you can overforce an NPT fitting to seal, we've probably all done it but it isn't the right way...technically, based on what I read in several places.
Now NPTF is a completely different matter and it is designed to dry seal with no application of tape or goo or whatever.

The above tid-bit should get the debates going again

 

[Wayne County Hose]

I will now make my point that npt does not have to be made to the tolerances of other fittings. I have seen, multiple times, and heard of many others, that BSPP was mistaken for npt. A BSPP fitting was then installed on an npt fitting. And, it worked. From my experience, actually held quite well. Now, BSPP is a parallel thread, NPT is taper. How on earth did a parallel thread hold to a tapered thread? Also, -6 BSPP is 3/8-19. -6 NPT is 3/8-18. So, with a 1 thread per inch difference, it still screwed on and held. 1 tpi in 19 tpi difference. That's almost a sixteenth of an inch off. So, when you're talking tolerances to a thousandth, not only is the taper way off, but the tpi is also over 5% off, and the thread angle is off by 5 degrees (15%), but it still holds. So, as far as hundredths go, we are specifically talking 5 hundredths and it still holds.

As far as the machinery handbook goes, the above just blows it all right out of the water. Ask any hose maker in the country, they will tell you that BSPP and npt get crossed up quite frequently but it still works. But, according to the book, it shouldn't. I am in no way claiming to know more than the machinery handbook. I never claimed or represented myself as an expert on anything. All I ever claimed to be was a putz stuck here in Pigsknuckle, Pa. I probably would have let it all go, until your last line in your original post telling me to stick to the facts. I know what I have seen work. I do not and never will advise the using of BSPP on an NPT fitting, but I have seen the 2 connected and not leak, also at over 2500 psi. That is why I say that npt does not have to be made to close tolerances and still work. I think everyone knew what my first post meant that you took such offense to. And yes, I am just an end user, I manufacture nothing. Never claimed to. And yes, you are right in saying that I should have stated "this is what I think". You see, I readily and openly admit when I am wrong. We are all wrong at one time or another. But reading your original post, you continually state how wrong I am and how I should stick to the facts. Knowing what I know about real world applications, what works and what doesn't, of course I will take offense to that. I fully admit that I was wrong when I stated that npt was made to tolerances of a hundredth. But, what I meant and you know it, was my point that npt could be sloppy and still work fine. And I have proven that. You know just how different BSPP and NPT threads are, yet they will still go together and work. You stated that if npt were made to tolerances in a hundredth of an inch, they would not even go together. Well BSPP and NPT are easily 5 hundredths of an inch off, yet they not only go together but work. I wonder how when the book says they won't.

If you would have worded your original post differently, we would not be here now. And yes, if I were more careful in using my words, we would not be here. But, you did know what my post you took such offense to meant. You chose to tell me I didn't know what I was doing. How was I supposed to feel?

 

[SPYDERLK]

I will now make my point that npt does not have to be made to the tolerances of other fittings. I have seen, multiple times, and heard of many others, that BSPP was mistaken for npt. A BSPP fitting was then installed on an npt fitting. And, it worked. From my experience, actually held quite well. Now, BSPP is a parallel thread, NPT is taper. How on earth did a parallel thread hold to a tapered thread? Also, -6 BSPP is 3/8-19. -6 NPT is 3/8-18. So, with a 1 thread per inch difference, it still screwed on and held. 1 tpi in 19 tpi difference. That's almost a sixteenth of an inch off. So, when you're talking tolerances to a thousandth, not only is the taper way off, but the tpi is also over 5% off, and the thread angle is off by 5 degrees (15%), but it still holds. So, as far as hundredths go, we are specifically talking 5 hundredths and it still holds.

As far as the machinery handbook goes, the above just blows it all right out of the water. Ask any hose maker in the country, they will tell you that BSPP and npt get crossed up quite frequently but it still works. But, according to the book, it shouldn't. I am in no way claiming to know more than the machinery handbook. I never claimed or represented myself as an expert on anything. All I ever claimed to be was a putz stuck here in Pigsknuckle, Pa. I probably would have let it all go, until your last line in your original post telling me to stick to the facts. I know what I have seen work. I do not and never will advise the using of BSPP on an NPT fitting, but I have seen the 2 connected and not leak, also at over 2500 psi. That is why I say that npt does not have to be made to close tolerances and still work. I think everyone knew what my first post meant that you took such offense to. And yes, I am just an end user, I manufacture nothing. Never claimed to. And yes, you are right in saying that I should have stated "this is what I think". You see, I readily and openly admit when I am wrong. We are all wrong at one time or another. But reading your original post, you continually state how wrong I am and how I should stick to the facts. Knowing what I know about real world applications, what works and what doesn't, of course I will take offense to that. I fully admit that I was wrong when I stated that npt was made to tolerances of a hundredth. But, what I meant and you know it, was my point that npt could be sloppy and still work fine. And I have proven that. You know just how different BSPP and NPT threads are, yet they will still go together and work. You stated that if npt were made to tolerances in a hundredth of an inch, they would not even go together. Well BSPP and NPT are easily 5 hundredths of an inch off, yet they not only go together but work. I wonder how when the book says they won't.

If you would have worded your original post differently, we would not be here now. And yes, if I were more careful in using my words, we would not be here. But, you did know what my post you took such offense to meant. You chose to tell me I didn't know what I was doing. How was I supposed to feel?

Sure you can get mismatched threads to hold if they go together with reasonable overlap of the major diameters. Holding is easy. The separation forces are miniscule in comparison to the strength of steel. The problem is getting a seal between mismatched threads. That is impossible without some sort of gap filler. In the cases where a seal results without a filler it must either result from tremendous distortion or a lucky metal to metal face contact at either the outer periphery or the tip, such as happens on purpose in JIC.
The book says they wont work for a reason. They just arent counting on luck or the resourceful foolishness employed by those in a situation where it has to work.
larry

 

[3RRL]

Skyco,
You are right about a thread sealant being required for NPT threads.
NPT threads are truncated both in the root and crest, so they need a sealer to fill the minute gap of a couple thousands. For a 1/2 pipe thread the truncation is .0024"-.0056" and about half that for and 1/8 pipe. NPTF threads are designed for metal to metal contact or interference at the crest and root. I should have included that in my reply but failed to do so.

Wayne, please excuse my short response...
There is no doubt what the Machinery's handbook says about the tolerances. Whether or not they are MADE to tolerance is another thing ... and whether or not you make two fittings of different type work is an entirely different matter altogether. I think Larry said it better than I could so I'll leave it at that.

 

[Wayne County Hose]

How about we just let it go altogether.

 

[Dougd]

A Teflon based past (PTFE) like THIS is much better for hydraulic connections. The small bits of tape can wreak havoc on the close tolerance parts in a hydraulic pump or spool valve.

Hey Kenny, I usually take whatever you say as "Gospel" but I'm just a little confused... Although the statement "The small bits of tape can wreak havoc on the close tolerance parts in a hydraulic pump or spool valve" is 100% accurate, I must ask what purpose the suction-side filter has in a hydaulic system if not to remove these "small bits"?
Since any fluid passing through the pump must first pass through the filter, how worried should I be about these small bits of teflon tape? Are they small enough to get through the filter?

 

[3RRL]

How about we just let it go altogether.

Yeah, agreed.

 

[kennyd]

Hey Kenny, I usually take whatever you say as "Gospel" but I'm just a little confused... Although the statement "The small bits of tape can wreak havoc on the close tolerance parts in a hydraulic pump or spool valve" is 100% accurate, I must ask what purpose the suction-side filter has in a hydaulic system if not to remove these "small bits"?
Since any fluid passing through the pump must first pass through the filter, how worried should I be about these small bits of teflon tape? Are they small enough to get through the filter?

What? You actually want to get this thread back "on topic"?

Here are my thoughts: In theory, all the fluid gets filtered before the pump...
But let's say you just replaced a hose on a FEL bucket curl cylinder and where very careless with the tape on the fittings, and/or your believe in the theory that if a little is good then a lot must be better. When you go to use the FEL, that fluid has to travel through the QD's and the FEL valve before it even gets close to the filter correct? The tape could certainly get hung up in the QD or in one of the small passages in the valve.

These are just my thoughts...they be worth what you paid for them This is just one of those topics here on TBN that spark controversy all the time like:
Welding: MIG vs. Stick
Air lines: PVC, Copper or Steel
Oil: Synthetic vs. Dino
Transmissions: Gear vs. Hydrostatic

 

[orangebluegreen]

However, these cheap Chinese npt sets sold by the truckload in Harbor freight and every other jip joint across the world can not make an npt thread with a tolerance of a thousandth of an inch. Yet, for the millions of threads they have made throughout the world, they have been extremely successful. So yes, npt is being made all over the world with tolerances of a hundredth of an inch. Even the good sets don't hold their tolerances. This is my issue, engineers not knowing what works in the real world. Anyone that believes that npt must absolutely be made with tolerances to the ten-thousandth or even the thousandth of an inch is absolutely off their rocker.

Thank You.

I've been reading this thread and the lack of China mention had me in hopes they had not yet influenced hydraulics. It's been many years since I've had to work on any hoses but I'm sure it will be soon I need to again. In the past 35 years I've done my share of home water plumbing and various machinery jobs involving NPT and years ago never had a leak, and tightness or number of turns of teflon tape wasn't a big deal. In recent years I have found most of the water fittings bought at the local big box stores are China made. I have had leaks in most water connections I've done over the past few years! Especially bad is any PVC NPT, just try keeping one from leaking, I have found every connection requires a different number of turns with the tape and often that won't even work and I resulted on a couple of PVC water fittings recently using silicon sealer which is absolutely ridiculous (I hesitate to mention that because it would be disaster in anything other than water so DON'T do it, it would get sucked into the system where it would expand and clog, don't ask about the gasoline tank I used silicon on when I was dumber than now). I can't even imagine the nightmare of being a professional plumber, they must have to seek alternate sources of materials or they would be wasting a lot of time. I swear China manufacturers are not making threads anywhere close to what they should be or used to be. I was hoping China products have not made it to hydraulics, if they can't make threads that seal water then what can we expect with higher pressures.