I was eyeballing a project that I am going to have to retap something 1/8 NPT, grabbed my tap and die set and it dawned on me how small 1/8" actually is... when I measure a 1/8 fitting, it's actually more like 25/64 on the outside of the threads. Even measuring the inner diameter of a fitting, it's 1/4"? Perhaps this is a question, but where does the 1/8" come into play? On the same subject, what size tap would I use to tap a hole for a 1/8 fitting to screw into?
The size is loosely based off of the inside diameter of the pipe when schedule 40 pipe is used I believe. The id will vary depending on the wall thickness of the material actually being threaded.
Check this link for drill size chart:
Huh. Chart has "R" as the tap size. I realized after that there is a specific tap for this after googling it. Not sure about the 1/8 designation though as I have measured multiple fittings and none of them measure anywhere close to 1/8" ID.
The "R" is a lettered drill size. It's slightly larger than 21/64" and slightly smaller than 11/32" Here is a drill size chart that breaks all that down.
Pipe threads are tapered threads so that they form a seal when tightened together, unlike bolt threads. You cannot use a bolt thread tap here as you've discovered.
In regards to the size... with the larger NPT sizes there is a closer correlation to the inside diameter size of the pipe. It gets screwy with the smaller sizes for some reason. In other words... for 1/8" NPT it won't correspond to any physical measurement being an actual 1/8"
Originally Posted by Levi
I just read up on this stuff yesterday about thread pitch gauges etc peaks and valleys and there are like 3 measurements then based on those three you refer to a chart then you get thread pitch type
if its a macro line fitting 3/8th x 20 is what i use and it i have no issues