Shorten an NPT tap?

By the books yes you use a taper reamer then the tap. You can also get away with the proper drill size but then you really need a gauge to know how far to set the tap for proper engagement.
Last year I went through all of this for a pipe fitter buddy that had a big side job for me. I tapped like 70 stainless endflanges to make test flanges for a job he was doing. They couldn’t get them because of supply chain problems. Luckily I had a npt thread gauge so every went to spec.
 
Great points / Q's that I can address:

When I started working on this, I did some measuring of the fitting and the tap to give me a rough idea of how far the tap needed to run. Its not super precise, but it definitely helped give me an idea because I had wondered the same thing (how far in does the tap need to go?). But between measuring and 1 trial on scrap: it didn't take long to figure out.

My fittings install with 90% of its threads engaged (similar depth if not slightly more than any commercially available part I've used). So I'm pretty sure I'm good to go there. Its quite snug. Although it doesn't require it, I do use a 2-3 wraps of teflon tape; helps with corrosion and galling if nothing else.

I am using straight drills; 7/16" drill for 1/4"NPT, R-letter drill for 1/8" NPT (per the book). I do not have a taper taper drill or reamer. That does sound ideal, but don't have any on-hand. The straight hole does result in noticeable bit of increased resistance near the top of the tap, but it was manageable since I'm just tapping aluminum. I could see potentially up-sizing the drill in harder materials...or tapered hole if I had the means.

Anyway: the drill size was not my issue; it is the depth of hole required to get the necessary tap diameter dimension at the top of the hole. Using the stock tap, the bottom ~quarter of the threads created by the tap are never used / could never be used. In other words: if I just used the first quarter of the tap: it would be impossible to even start installing a fitting.

I agree on using quality taps; I've broken enough crappy taps :bang head: to know that $ is well spend good brands. But for my NPT I only had a couple; one a crappy brand (Hanson/Irwin), and the other is an OSG (decent).
 
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There is also an NPTF threadform, also called Dryseal. I've been using that for tapped holes, as NPTF is compatible with NPT, though both male and female have to be NPTF to form a metal to metal seal, no compound or tape required. Seems like all the above discussion points apply.
 
In 40 years being a millwright I've done hundreds of holes and then tapped NPT from 1/8 and up. Never used a tapered reamer. It's not needed. The tap is tapered and that's what cuts the taper in the hole and creates the seal. Sealing tape is a misnomer, its function is to reduce friction between the treads so the fitting tightened more and create a better seal. Threads do the sealing not the tape. If you have poorly formed threads tape or sealant will help fill in the gaps.
 
I would buy a second tap and grind the end to suit your needs. Run the first tap in until it bottoms out and follow with the modified tap. The nature of the NPT thread is that the tap continues to cut along its entire length of engagement. Depending upon the mating part, you may want to use a larger drill size.
 
I have 1/8, 1/4 and 3/8 cut down taps to finish blind holes. It's sort of a ham fisted brute force solution but, it gets the job done without $pecial tooling!
 
markba633csi said:
Hmmm- that's the first I've heard of using a tapered reamer for pipe taps, but it makes sense. No wonder going up in drill size helped
Of course on a semi-blind hole or elbow you would need a special short reamer
Got enough tools yet? LOL
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Yes they are special short reamers for NPT

 
aliva said:
In 40 years being a millwright I've done hundreds of holes and then tapped NPT from 1/8 and up. Never used a tapered reamer. It's not needed. The tap is tapered and that's what cuts the taper in the hole and creates the seal. Sealing tape is a misnomer, its function is to reduce friction between the treads so the fitting tightened more and create a better seal. Threads do the sealing not the tape. If you have poorly formed threads tape or sealant will help fill in the gaps.
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Like I said, the tapered reams aren't absolutely necessary. However, if you use them you'll be able to tap the threads much easier and get a nice proper, full depth, thread engagement, when you make-up your fittings.
 
Eddyde said:
Like I said, the tapered reams aren't absolutely necessary. However, if you use them you'll be able to tap the threads much easier and get a nice proper, full depth, thread engagement, when you make-up your fittings.
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Agreed if you have a tapered reamer by means use it, but I'd never buy one
 
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