Explain "npt" to me (ie 1/8 npt, etc...)

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For those of us who never learned (you know, the guys who by 3 or 4 fittings which look close and hope that 1 will fit!), someone please explain the whole measuring terminology to me. I'm ready to drill & tap my manifold, but everyone tells me I have a 1/8 npt fitting, but this thing is a hair over a 1/4" thick (i measured it). Does the 1/8th mean inner diameter (as in a pipe) or what?

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'01 Eth 3500 dually, no mods planned except maybe an amsoil filter & guages. See My Truck and also be sure to read What tools & methods to use for an easy 1st oil change!!!
"Be happy you don't get all the government you pay for. " -W. C. Fields

 

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Yes! NPT means National Pipe Taper. The size is the inside pipe diameter.

 

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OK, so my fitting is around 1/4" thick, the inner diameter is 1/8". Do I need an adapter (the vote is 50/50 on this so far).

My plan so far is a 7/16 drill bit, 1/8 npt tap, and no adapter...

How far off am I?

[This message has been edited by dmurdock (edited 01-13-2001). ]

 

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Originally posted by dmurdock:
OK, so my fitting is around 1/4" thick, the inner diameter is 1/8". Do I need an adapter (the vote is 50/50 on this so far).

My plan so far is a 7/16 drill bit, 1/8 npt tap, and no adapter...

How far off am I?

[This message has been edited by dmurdock (edited 01-13-2001). ]

It is my understanding that a 7/16 drill bit is used for a 1/4 NPT tap. I just bought a 1/4 tap at Home Depot and it said on the package and on the tap to use a 7/16 drill bit. Not sure what is used for 1/8 NPT. I was confused about the NPT stuff also. The 1/4 NPT tap looks like about 1/2".

Bobby

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2000, Driftwood, 2500, automatic, 3. 54, 2WD, LWB, SLT, Quad Cab, most of the options, air horns, tow 5500lb 28'x8' Aerolite TT and two small boys.

 

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dmurdock,
1/8" pipe measures more than a hair over 1/4"
it is about 0. 40 or a little over 3/8", don't ask me why. You must not have the tap yet, am I correct. If you were to drill a 7/16" hole you would have no material left to cut threads into, the tap would fall into the hole. I used a bit a little under 3/8" when I drilled and tapped for my thermocouple. When you buy the tap, buy the corresdonding bit to go with it. Make sure it is the correct size bit! When drilling, center punch the correct postion to the rear or in front of the manifold divider, if you are drilling the exhaust manifold that is. If you plunge the bit into a can of wheel bearing grease before you drill you should end with little or no shavings in the manifold. Do the same with the tap and then insert a pencil magnet into the hole to check for shavings. The NPT tap is tappered so be sure to run the tap deep enough to ensure the thermocouple (probe) will not be to tight and crack the manifold if over tightened. If you tap a 1/8" NPT hole for a 1/8" thermocouple NO ADAPTER will be needed.
Good luck
Arlan

[This message has been edited by arlan (edited 01-13-2001). ]

 

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Your right bdaniel, my mistake on the drill bit size. I will have to buy a new tap, dug around for mine today and they are all rusted (see what I get for keeping them in their case!). Thanks arlan, this all makes sense now.

I plan on tapping a little, then screwing the fitting in a little (by hand) until the fitting gets about 3 threads deep. Does this sound right or should I just tap all the way through?

[This message has been edited by dmurdock (edited 01-13-2001). ]

 

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The only reason I can think that they would have supplied an adapter in your gauge kit would be for people replacing a thermocouple
that was already drilled 1/4" NPT. When I purchased my gauge they asked if I want 1/8" or 1/4" I choose 1/8" because I didn't want the large hole 1/4" would have required. When you tap the hole run the tap about 3/4's of the way in.
Arlan

[This message has been edited by arlan (edited 01-13-2001). ]

 

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Use some high temp anti-seize compound on the threads. Use plenty but do not get any on anything but the threads. If you don't use anti-seize you will not get it out without damaging the manifold. Thermocouples can go bad. You can buy the tap and the drill in the same package at most hardware stores. Take your time to get the tap started perfectly straight or it will not seal. If you don't have a tap handle buy one. Use a heavy grease on the tap to lube and catch most of the chips. Tap down far enough to leave about 5-6 threads exposed on your sensor when you screw it in by hand. Don't overtighten, the taper will seal it for you. Good luck

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99 2500 QC 4X4 AUTO SB 24V LARAMIE SLT 3. 5 LSD 285-75-16'S

 

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LSMITH,
Good advise and well said.

 

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<pre><font size=+0>Pipe Dimensions

------ -------- -------
Pipe Threads Outside
Size per Inch Dia.
------ -------- -------
1/8" 27 0. 405"
1/4" 18 0. 540"
3/8" 18 0. 675"
1/2" 14 0. 840"
3/4" 14 1. 050"
1" 11-1/2 1. 315"
1-1/4" 11-1/2 1. 660"
1-1/2" 11-1/2 1. 900"
2" 11-1/2 2. 375"
2-1/2" 8 2. 875"
3" 8 3. 500"
3-1/2" 8 4. 000"
4" 8 4. 500"
------ -------- -------


TAPER PIPE SIZES - NPT

----------------------------
TAP DRILL
SIZE SIZE
----------------------------
1/8-27 R
1/4-18 7/16
3/8-18 37/64
1/2-14 23/32
3/4-14 59/64
1"-11-1/2 1-5/32
1-1/4-11-1/2 1-1/2
1-1/2-11-1/2 1-47/64
2"-11-1/2 2-7/32
2-1/2-8 2-5/8
3"-8 3-1/4
3-1/2-8 3-3/4
4"-8 4-1/4
</pre></font>

 

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Like so many things, there are many different "standards". National Pipe Threads is just one of many - more reading at http://www.panamengineers.com/information_meterictube.htm http://www.unilok.co. kr/304.htm
In U. S. , if you don't know the thread type, and the joint is intended to hold pressure without a gasket (in other words, the threads form the seal), then NPT would be a great first guess...


[This message has been edited by HC (edited 01-14-2001). ]

 

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Here's a funny one #ad


I didn't know about NPT either. I ordered my pyrometer/thermocouple from TST. Well, being the knowledgeable guy I am, I sent Mark Chapple an e-mail informing him that his instruction were incorrect. Then, being the great guy he is, proceeded to educate me on NPT. I proceeded to my nearest Sears, where one of the older sale reps, who didn't know NPT, told me I could us standard tap since they didn't have the correct NPT. I decided to believe Mark and proceded to Home Depot where their sales rep took me to the NPT tap/drill bit combo. I then followed Marks printed instructions to a "T" and got a perfect installation. #ad


Learn somethin new everyday!

Wiredawg



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2000 4X4 SLT QC 8' bed; 6 spd, 3. 54 ; BD exhaust brake and 4" exhaust syst; TST PM2; Airlift Super Duty rear air bags; RS 9000 shocks; pyro/boost gauges in 2 gauge a-pillar mount; back preasure gauge mounted under dash; KN air filter; Brown's Welding brush/grill guard, rear bumper & headache rack; nerf bars and aluminum roll top rear cover, Warn front receiver and Hadley air horns, 20K Reese hitch.

 

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Get one of those little charts that tell you drill bit size for the tap size you want to tap out.
Like you I was a little confused at first. You should be able to find a chart at a hardware store or plumbing and heating outfit.

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95 Dodge Ram reg cab,2500,5spd,Tweaked & Peaked Cummins Diesel, MODS, K/N filter,dual exhaust 5'' tips,Lost muffler,turbo silencer, ACCESORIES,ISSPRO turbo & pyro. gauges. Shell Rotella in pan. Fleatgaurd filters. 133k. on the OD. IF nothing is to blame. Blame the oil then.

 

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Most of the time the tap will say what size drill to use on the side,that is you can still read it.

 

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Everyone has given you good information, except they didn't tell you that NPT is a tapered thread. Therefore it is depth sensitve, if tapped too deep the fitting will not seal and be loose. To get the correct depth suggest that you drill and tap a scrap piece of steel and then mark the depth on the tap with tape. The proper thread engagement is 2 to 2. 5 turns hand tight. Also since the manifold is cast iron, drill and tap it dry no lubricant. The TST web site has instuctions for the position of the hole in the manifold.

Neil #ad


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Neil Copeman 92 D250,LE,limited slip, 16cm turbo housing, Isspro gauges, factory tach option, Lear cap, DC crankdown tire carrier
Hadley air horns, Stealhorse leather seats, e-mail (home): bombedcummins@sympatico. ca

 

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This is all good information, but I don't believe you have to tap the hole dry. If you do you will have a lot of shavings enter the manifold. I feal removal of the turbo would be necessary if the hole were to be dry tapped. Numerous members have tapped the hole with the tap well greased and have not had a problem. Neil is correct about the tap being depth sensitive, so take care that it is done correctly. Do use the anti-seize compound so removal could be done if necessary, but once it is installed there is no reason to remove the thermocouple just to check it. I did that and it wasn't one of my better decessions. Like several had said, run the tap part way in and check the fit, you can always tap the hole deeper if necessary, and do not over tighten. All this sound confusing and frightening? It is really not bad once you get started, and it's great when you have finished.
Arlan

 

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Well, i have everything for the egt probe, but now am having trouble finding a reducer for the boost guage. Only thing I've found so far is a galvanized one, which I don't think would work very well. Off to the parts store tomorrow to continue the search...

 

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dmurdock,
I'm sure 3/4" to 1/8" reducers exsist but I wasn't able to find one. What I did was buy a brass 3/4' plug then I drilled and tapped it. Might be a thing to consider as you would gain some experience before drilling and tapping the manifold.
Arlan

 

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Arlan has the answer. Brass is good practice, it's harder to tap well than cast iron. Otherwise go to a hydraulics jobber shop. 3/4" to 1/8" should be easy to find if you get to the right store.

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99 2500 QC 4X4 AUTO SB 24V LARAMIE SLT 3. 5 LSD 285-75-16'S

 

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I have the galvanized 3/4" to 1/4" NPT reducer that I got at Home Depot that I plan to use for the boost guage. I plan to use it since the heat is not nearly as hot as on the exhaust manifold. Is there any reason why I should not?

Thanks,
Bobby



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2000, Driftwood, 2500, automatic, 3. 54, 2WD, LWB, SLT, Quad Cab, most of the options, Grover air horns, tow 5500lb 28'x8' Aerolite TT and two small boys.