1/8 NPT

[jorogi]

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" ?

 

[phaxtris]

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

 

[Golem Pens]

IIRC it's because it's 1/8" ID for the connecting pipe. Some pipe fitting standard.

Ope, pipped at the post.

 

[jorogi]

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

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.

 

[phaxtris]

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.

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

 

[whydontu]

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

 

[Bandit]

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.

 

[whydontu]

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.

 

[Chicken lights]

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.

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

 

[whydontu]

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.

 

[fixerup]

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.

 

[jorogi]

Well thanks guys, you're making me feel much better.

 

[FatherWes]

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.

 

[Bandit]

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.