What's just slightly smaller OD than 1/8" NPT thread????

A NPT 1/4" thread that is made too small?

It is really common to have mismatches between 1/4" NPT threaded parts. Especially pressure regulators and pressure gauges. Even quality companies like Tescom pressure regulator and Baumer pressure gauges - will only go about 1-2 turns to the 1/4" NPT thread.

Also the difference in those NPT threads from different manufacturers seems to be same, so they have a good quality system producing the same (too small/big) NPT thread consistently. They are really NPT threads too, I have verified that as they never seem to match..

So, I'd use a 1/4" NPT tap to make the thread larger, if you think it is 1/4" NPT thread really..

however as THREADED PIPE CONNECTIONS ARE TAPERED , one should be able to run a pipe tap a little more into the threaded hole if what you have does not SCREW IN..

IF THE WALL OF THE TUBE IS THICK ENOUGH , you should be able to run a pipe die onto the tube a little more making yhe end of the tube smaller.

however as THREADED PIPE CONNECTIONS ARE TAPERED , one should be able to run a pipe tap a little more into the threaded hole if what you have does not SCREW IN..

IF THE WALL OF THE TUBE IS THICK ENOUGH , you should be able to run a pipe die onto the tube a little more making yhe end of the tube smaller.

---------------------------- I don't see the context here, but to answer the question literally, the nominal OD of 1/8" pipe is 0.405". 10mm (0.394") and 3/8" (0.375") are slightly smaller standard sizes.

This is useful if you make things from pipe:

I was in line at an industrial supply store behind an inventor who was picking up 2" ID ball bearings he had ordered to go on 2" pipe, to support a solar tracking array. He was quite upset to discover that they didn't fit because the OD of 2" pipe is actually 2-3/8", and it isn't precisely round or accurately sized. I decided not to mention that I'm equipped to build inventors' ideas.

I machined the thrust bearing on my tracking array to match the actual sizes of pipe.

I forgot to mention lamp tubing, which has straight (not tapered) thread of

There is no context because its a 10 year old zombie thread. Who knows. Maybe dpb is still looking for the answer.

There is no context because its a 10 year old zombie thread. Who knows. Maybe dpb is still looking for the answer.

Bob La Londe CNC Molds N Stuff

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No age discrimination! I still research the Titanic and Amelia Earhart.

I think I've found the first JFK bullet and what occurred on the Grassy Knoll.