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1/8 NPT

J

jorogi

Ultra Member
Made a small part today which needed a 1/8 NPT thread. Stock was 3/4" of some grade of SS, machined beautifully. I cut a 1.7899 degree taper and used a cheap die to cut the threads and this was my first time doing a pipe thread. It worked out fine but it was a really fiddley bugger to get right.
Two questions,
1 ) is there a trick to this ?
2 ) why the heck is it called 1/8" ?
 
NPT is sized by the inside diameter of the cast iron pipe, i suppose back in the day a 1/8npt fitting would have been cast iron and actually had a 1/8" diameter bore, obviously that is not the case any longer, but the naming convention stuck

npt is tricky, easy to tap to far, i have made that mistake many times, i try to tap maybe 1/4 of the tap/die and test with a fitting. Im sure if you did it all the time it would be easy to figure out how far is just right on that particular tap/die
 
phaxtris said:
NPT is sized by the inside diameter of the cast iron pipe, i suppose back in the day a 1/8npt fitting would have been cast iron and actually had a 1/8" diameter bore, obviously that is not the case any longer, but the naming convention stuck

npt is tricky, easy to tap to far, i have made that mistake many times, i try to tap maybe 1/4 of the tap/die and test with a fitting. Im sure if you did it all the time it would be easy to figure out how far is just right on that particular tap/die
Click to expand...
I had the hole for it to fit into and I didn't want to fiddle with that as it worked fine with the old fitting. I guess it's like how you get to the Royal Albert Hall, practice, practice, practice.
 
jorogi said:
I had the hole for it to fit into and I didn't want to fiddle with that as it worked fine with the old fitting. I guess it's like how you get to the Royal Albert Hall, practice, practice, practice.
Click to expand...

ya and ive found that even from fitting to fitting they are not all the same, i have had holes that i have tapped, one fitting snugs up right where i want, the next fitting bury's itself, same hole, that little hair over or under makes a massive difference it seems

one tap to the next same story, tap x number of turns on tap a, perfect, tap same number of turns on tap b, to tight, or to loose

i dont know, npt is pick, that blasted taper i guess
 
NPS tolerances are very sloppy. Pipe O.D. can be WAY off from a machinist viewpoint. 1/8" NPS O.D. is allowed to be +1/64" / -1/32", I.D. is minimum -12.5% of nominal wall, with no maximum wall thickness. I have checked thousands of pipe fittings. Take 100 pipe elbows from a manufacturing batch, 40 will be undersized, 50 will be oversized, 9 will be <close> to nominal, 1 will not even be threaded, or the thread will be so off-centre the thread will be incomplete.

Plumbers needs to remember three things - payday is Friday, shit flows downhill, and it's cheaper to throw away a buggered pipe fitting than to try to fix it or jam it onto the pipe.

BC Hydro used to have a very well-trained and educated piping foreman who checked the threads on every single pipe fitting and pipe nipple they bought. Many discussions with him, I could never convince him that having a fully-quailifed P.Eng at $80/hour checking $0.50 pipe fittings was a waste of money. His response was he couldn't get $50/hour pipe fitters to understand that if the fitting was no good, throw it away, they'd take that crappy pipe fitting and thread it into a $1000 gas meter until the gas meter cracked.

Nominal Pipe Size - Wikipedia

en.wikipedia.org en.wikipedia.org
 
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Yeah, trying to get the right depth with pipe and fittings is a bit of a pain. I am not sure there is an easy way to do it. Even with bought fittings I have sometimes had to tap a bit deeper or use a different fitting instead the first one tried. At least the pipe if you are threading it, leave a bit less threaded, can cut a bit more if needed.
The white tape is thinner then the yellow tape.
 
Done 40 years of stainless steel pipe connections. Good size dollop of teflon paste on the male thread, hand tighten into the female thread, so you have some paste on both fittings. Undo. Three wraps of good-quality teflon tape - best is the nickel-impregnated tape used for stainless steel, alternate is the teflon tape certified for natural gas applications, Pipe fittings don't need to be be gronked down to 10,000 ft-lbs of torque, snug is all that's needed. If you know automobile stuff, my go-to for small fittings up to 1" is use about as much oomph on the wrench that you'd use tightening the head bolts on a small block Chev V8. Less is better.
 
whydontu said:
Done 40 years of stainless steel pipe connections. Good size dollop of teflon paste on the male thread, hand tighten into the female thread, so you have some paste on both fittings. Undo. Three wraps of good-quality teflon tape - best is the nickel-impregnated tape used for stainless steel, alternate is the teflon tape certified for natural gas applications, Pipe fittings don't need to be be gronked down to 10,000 ft-lbs of torque, snug is all that's needed. If you know automobile stuff, my go-to for small fittings up to 1" is use about as much oomph on the wrench that you'd use tightening the head bolts on a small block Chev V8. Less is better.
Click to expand...
I was told/taught that on larger pipe fittings the dope/tape actually served as a lubricant to help the two halves mate. Largest I've hand threaded was 2" I think, and you're getting into three foot pipe wrench territory fast
 
Did a medium-size piping manifold for a client. 2" Sch40 316 pipe & Class 150 threaded fittings. Nickel teflon tape, teflon paste, Big pipe wrenches. Pipe fitter and I combined have 80 years experience. Pressure test overnight, held pressure. Sat overnight in the warehouse. Still holding pressure. Customer came to inspect. Did another pressure test. Failed. Redid all the joints, very carefully torqued everything in accordance with MSS guidelines. Checked each others work. Customer comes back, we redo the pressure test. Passes with flying colours. Carefully pack assemblies for shipment. Strapped to good quality skids, braced every which way from Sunday. Gets to jobsite, fails pressure test. Credited the customer the assembly costs and never did another shop assembly that was going to be shipped farther than across a parking lot.
 
Another problem with npt connection was the orientation of the components. Lets say a pressure gauge, an elbow..... needed to face certain direction and you feel its tight but you think you can still get one more rotation and it will give you a better seal. I usually tried for the extra turn but stopped if it was too hard, I would completely un-screw the components, new teflon tape and settled for one less turn. 99% of those instances my helium leak checker wouldn't detect at 10⁹ cc/sec. Tighter or more Teflon tape doesn't get you a better seal. Just 2 or 3 wraps of Teflon tape is good enough. It's mean as a lubricant not a sealer.
 
Actually, the Teflon tape or paste IS the seal.
Bare NPT threads will not produce a gas tight seal. Gas molecules will follow the thread root all the way out. The Teflon tape and/or paste create the seal.
Stainless requires paste & tape to prevent galling. In this case, it is also acting as a lubricant, in addition to being the sealant.
 
I think all the gas fitters I worked with used tape and paste on any fitting, some of the regular plumbers too. Also seemed to allow better positioning of fittings and gauges instead of that “extra bit tighter” to get things right.
 
Correct, at no point should any metal npt fittings be assembled without lubricant, liquid or tape.
For exactly reason mentioned above.
@Bandit and @FatherWes
Ai also agreed that teflon tape seals the joint between these fittings.
Yes, Teflon (PTFE) tape provides a seal for National Pipe Thread (NPT) joints by filling micro-gaps and spiral leak paths between tapered metal threads. [1, 2, 3]

It's difficult for me to understand how a thin film of teflon tape would seal npt fittings when the material is so permeable.
I have spent days leak checking complete teflon gas systems, tubing, fittings, valve.....etc
I would have a 5 to 10 minutes window to test the system for actual leaks with helium gas before it would start to diffuse through the walls of the components, .250" wall thickness didn't matter, just a few more minutes delay. I would then stop the test, vacuum pump for an hour to clear the system of helium and start again, looking for non diffusion leaks. These plastic systems were the most difficult and frustrating for me to leak test. My blood pressure just went up. Lol. Maybe that's why I still believe teflon tape doesn't seal and it's a lubricant in my books.
What really bothers me, is when I see assembled npt fittings with a 1/8" bulge of teflon tape sticking out, meaning they have 20 to 30 wraps of tape, were 2 to 4 wraps is all what's needed.
Liquid pipe dope might be different, I don't have much experience working with it. But I often see a nice even capillaries joint oozing out between the mating parts and often it will later dry out and maybe that would help with the sealing.
Ok, I will take in the " filling the micro gap and spiral leak path" into consideration but I still think there are mostly lubricant.
 
I have had the exact same experience with poorly fitting pipe thread fittings.

Further the flats of the majority of plumbing fittings never match typical mechanics type wrenches. IMHO adjustable wrenches suck, I use them as a last resort (almost always for plumbing). The moment I see a video with some guy struggling with an adjustable wrench I think oh… it’s one of those type of videos. Most plumbing is simple enough that anyone can do it, so I think they decided to throw a monkey wrench into the process to keep the plumbers employed;-)

Am I the only one that has contempt for adjustable wrenches when a proper wrench will work?

Seems every time I need a plumbing fitting to go from size to another it requires 17 fittings to do the conversion, I have a big bin of fittings must be 100+ and it’s a miracle if I have the correct one when needed. I needed a fitting just yesterday and made one on the lathe instead of hopping in the car to try and find one.

We do have a plumbing supply place with a great selection, however the prices are over the top if you’re not in the trade, like $20 for a little fitting that you paid $1.50 for back in the day.
 
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slow-poke said:
Am I the only one that has contempt for adjustable wrenches when a proper wrench will work?
Click to expand...
I was in that camp, until I watched a senior tech change an air tank for me. Say a dozen fittings, he used an adjustable and spent zero time trying to get the right size wrench. It wasn't a cheap adjustable, but it changed my mind. Sure, there's places a wrench is better, or space is limited, but watching him change the tank efficiently was an eye opener
 
Having run a lot of rigid conduit in my time. I have a full set of taps for 1/2, 3/4, 1 inch ndp. I can not believe how many fittings are not tapped correctly. requirement is 5 full threads of engagement. Some not even a couple, some bury the threads right to the pipe. I also have a full rigid die set up to 1". I had a two inch set but sold it with my tristand. God I miss that tristand. Once you use one for some time, it hurts not to have it anymore.

I have run 1/4 and 1/8 a bit with pneumatic and hydraulic controls. There are lots of instruments using those smaller sizes.
 
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