I need a bit of advice. I just had a job completed by a machine shop
that called for a 1/2" NTP and 1/4" NPT hole. The part was titled
"manifold". The parts just arrived with the 1/2" NPT hole tapped to the
bottom of a 1 1/8" deep port (will never seal)and a correctly tapped
1/4" NPT hole. The machine shop says it was my fault because I didn't
specify the thread length and I will now have to pay for a "rush run"
if I need to meet my deadline. Isn't NPT supposed to be tapped within a
certain range? Shouldn't they have asked if there was any doubt? I
think that they should have asked. Am I wrong?
I think so, but if they looked in Machinery's Handbook it
shouldn't have been necessary.
Hard to say without actually seeing your drawing, but it
doesn't sound like it. You may be guilty of not giving them
enough room to tap the hole with a standard tap, but the
shop either should have brought that to your attention or
made the part to the print.
It shouldn't be very hard to grind the end off a pipe tap
and bring the holes to size. If you really want the holes
to NPT spec, rather than just good enough to work, an NPT
plug gage is required.
The problem is not that there was not enough depth for the tap. The
hole is threaded so deep that I run out of male threads on a nipple
before the nipple seals to the manifold. If I run the nipple into the
manifold to just before the end of the male thread, I can actually
"rattle" the nipple in the tapped female NPT hole. All I really needed
here (quality wise) was "good enough to work".
Would I be out of line by asking for a re-run?
You did not need to call out thread depth. When calling out a
straight thread you just need to specify the nominal O.D., the pitch,
class, and thread depth. For example: 1/4-20 3A .500 deep. You don't
need to specify the limits of the O.D. or the pitch diameter. The
nominal size and class do that. Same for the pipe tap, except for
tapered pipe. Since the thread is tapered the depth is controlled by
P.D. (pitch diameter). So, if the P.D. is correct, the depth muist be
correct. Pipe taps are ground this way. Only if the thread is going to
deviate from standard is the depth called out. The machine shop is
lying to you. They scrapped the parts and want you to pay for their
mistake. I would be leery of sending them anymore work. I bet that if
a tap manufacturer were consulted they would be ablke to send you
written proof that the thread depth of a tapered pipe is controlled by
the tap cutting to the correct P.D. If it was me, I'd tell the shop
that they screwed up and present them with thread data from a tap
maker, or from Machinery's Handbook to prove it. Then give them a
chance to do the parts over at their cost. If they refuse, and if the
dollar amount is low enough, take 'em to small claims court. If the
dollar amount is too high then call a lawer and have a letter sent to
them from the lawer. Oftentimes this is all it takes to get some
You may have be able to save your parts though. Heli-coils may
work. Since I don't know the dimensions of your part, or the material,
I can't say. But contacting the makers of Heli-coils should get you
the info you need to decide whether your parts can be saved.
takes you to
the RECOIL web site for pipe thread repair. If the preceeding URL
doesn't work then this one takes you top their homepage
.The Helicoil site makes you
register so I tried Recoil and they don't require registration.
Eric R Snow,
E T Precision Machine
OK, the opposite of what I was imagining, they removed too
much material. If you specified a 1/2 NPT thread, they
should remake the parts.
How much pressure does this have to take? If safety isn't
an issue and you're prepared to be cussed by anyone who has
to work on this in the future, a down and dirty solution
would be to make matching nipples with oversize threads.
Another dodge is teflon lined nuts that will seal on the
nipple thread and the face of the manifold. McMaster sells
It may be cheaper to just walk away from them -- lies don't come in sets
of one. If they're going to play that sort of game once, they'll do it
again and again. Unless they're the only shop in town you should
probably tell them that you're happily paying the expedite fees -- then
tell them which one of their competitors you're paying the fees to.
Suing, or threatening to, may be indicated if they've screwed up a bunch
of parts -- but if it were me I'd my money and/or rework out of them and
then never darken their door.
A few months ago I needed to make a similar manifold and investigated
pipe taps for the project. I discovered that "NPT" can refer to either
tapered or parallel treads. A 1/2" tapered will screw into a 1/2"
parallell, but not seal. The opposite is not true. The parallel thread
requires a sealing mechanism such as an "O" ring.
The same threading problem occurs with metric pipe threads, excep that
each country with metric standards has it's own standard. That is why
there is a 1/8" pipe tap in my set of metric tape/dies. Many Asian
countries use the American pipe standard.
I wonder if the shop doing your work happened to use the parallel tap
instead of a tapered tap?
According to the 26th edition of Machinery's Handbook, pages 1849-1850
and my Starrett tap drill chart, if you called out that you wanted th
thread a 1/2-14 NPT on your print the shop should have done a
Drill hole 23/32 in Diameter
Tap 1/2-14 NPT to a depth of .5343 or 17/32 (.53125)
In the diagram on page 1849 that depth is the "Handtight Engagemen
Length (L2=.320)" plus the "Wrench Makeup Length For Internal Threa
Any reputable shop should know that if it is specified as an NPT threa
it is because a seal is required at that joint. If you would like a cop
of those pages from my Handbook if you don't have one, let me know. Als
if you happen to be in the Chicago area and need any more of this wor
done again let me know.
As far as boring the hole out and pressing in an insert, I am assumin
from what you said that this is for an intake manifold on an engine. I
that is the case then you probably won't want to do that. Aside from th
fact that there probably isn't enough wall stock to bore the hol
larger, if you did that you would have to put an insert in that has th
exact same expansion rates as the rest of the manifold, ie: Sam
material. Probably not the best idea. Go back to the shop and see wha
they say when you have proof, also review your print and make sur
everything is called out right. Hope this helps
Thanks to all for the replies! Due to the thoughts expressed here, I
was able to successfully argue my case. The job is being re-run at no
Obviously my limited machining knowledge will not allow me to pay you
folks back by contributing to this group. Take a look at the following
URL. If anything there interests you let me know via email (off group).
I'll see what I can do to work you a deal. Only those that responded
prior to this post please!
NPT does not refer to both straight and tapered threads. Straight
pipe threads have the following designations: NPTR, for railing, NPSC,
for couplings, NPSM, NPSL, and NPSH, for different types of mechanical
joints. Along with the preceeding are NPTF,PTF,NPSF, and NPSI, all
different designations for dryseal pipe threads.This information comes
from Machinery's Handbook, 26th edition, published by Industrial
Press. (gotta give credit where it's due).
Eric R Snow
Since the thread is tapered the depth is controlled by
. I bet that if
Yep------and the rule of thumb is to tap until you have 5 threads (on the
tap) above the finished surface, unless you happen to have a plug gage to
check the proper depth. The rule may not be perfect, but it will always
yield a functional thread.
Chuckle! Most of the time? That's the beauty of the shop. I can't
help but wonder if we're at the mercy of poor work turned out by others in
many cases. I've had to tap until I had 4 or fewer threads on more than one
occasion, and have kicked myself for tapping too deep at five, so what you
say certainly makes sense. If nothing else, it's playing it safe. You
can always go deeper. I have never had the luxury of gages (rarely ever
tapped pipe threads for product, only my own use) so I tend to tap to about
5½ threads and call it good.
In commercial production, NPT thread depth is measured with a gage that has
a ground notch. Unless otherwise specified on the blueprint, and with the
gage screwed in as far as it will readily go, the thread depth is correct if
the notch is within plus or minus one thread from the surface of the part.
There is no need to specify depth for a standard NPT thread.
The correct depth is plus or minus one thread from that notch,
Sounds like they had either the "new guy" or the apprentice do it,
and he ran the NPT tap right through the plate or maifold or what-ever
it is, as he would with a standard tap. Maybe they can take it out to
3/4" and use a 3/4 to 1/2 breducing bushing (standfdard stuff
The straight threads you refer to are called NPS, they are the same dia. and
pitch as NPT but use an o-ring at the neck to seal. Valve to bottle
connections are where I usually see them such as scuba tanks, fire
extinguisher CO2 paintball or beverage tanks.
I have to chime in with the majority: You called for NPT threads in
the hole, that's what you should have gotten. I might even venture as
far as "someone at that machine shop is a moron", perhaps more than
one. Forget your paying them for a rush job, they should be bending
over backwards (and staying till Midnight if necessary) to make it
again and make it right.
NPT is all you needed to say, unless you wanted something done
differently. The T stands for Tapered, and there is a set standard
for it. If there were any questions about what you were calling for
on the prints, they should have called or written you and asked for
clarification before turning the part into scrap. Especially since
they got the 1/4" NPT hole right.
It works the same way for the male half of the threaded connection -
If I walk into a wholesale house and ask for a pipe or conduit nipple,
I get one with NPT threads on each end, a standard. A nipple cut 50
years ago will thread into (and create a seal) in a fitting made last
week, that's what standards are for.
If I want running threads all the way down the piece of conduit,
that's "allthread" and it's an entirely different animal. They do not
hand you allthread when you ask for a threaded nipple, or vice versa.
I've also worked in a print shop where the same principle stands -
we always got written confirmation of the proof sheet when they wanted
a word spelled wrong, and all our dictionaries and reference materials
disagreed with the customer. That way they couldn't come back later
and ask for the entire print run to be done again for free.