Shorten an NPT tap?

keeena

keeena

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So, its my first time having to make some NPT threads (1/8" and 1/4" NPT) to make an air distribution block. I found that the taps required a significant amount of depth in order to allow the fitting to thread in with enough engagement. For example, the 1/4" NPT tap required a 3/4" deep hole. In most fittings and fixtures (like air regulators) that I have seen, the 1/4" NPT threads only seem to run in about 1/2" deep. In my application: I need to keep the threaded depth to about 1/2"

The NPT taps I have seem to need to be threaded in about 3-4 threads shy of the full tap in order for the part to install at a normal depth/thread engagement.

I was planning on removing about 3-4 threads from the end of the tap (and yes: I would add a starting taper to the first 2-3 courses). I just wanted to see if my experience with NPT tapping depth is normal and if I'm going to be creating any issues by shortening the tap.
 
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I've noted the same with both 1/8" and 1/4" NPTs- making the same sort of part- an air distribution block
Grinding the tap would help (although I didn't), and go up by maybe one or two drill sizes
-Mark
PS I also found Morse brand pipe taps cut better than cheap Irwin taps
 
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Well, regarding this, I have never seen a spec on how deep the tap is expected to go in. Anybody have a reference for this? I could not find anything in Machinery's Handbook, though there is a lot of info on NPT stuff.

I just did a 1"-11.5tpi in stainless, about killed me off.
 
Pretty common experience for me. Seems to vary from tap to tap. I would go the larger drill route, but try it in a scrap piece first. Mike
 
It is not uncommon for NPT taps to require significant depth to achieve proper thread engagement. However, care must be taken to ensure that the fitting is screwed deep enough to prevent leaks and ensure proper function. Shortening the tap can affect the thread quality and cause problems with the fitting. We recommend consulting a professional or experienced machinist on how to achieve the correct depth without affecting thread quality.
 
You can get away with just a straight hole. However, to tap an NPT hole properly, you must first use a tapered reamer to get the proper taper inside the hole, then tap it.
 
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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