Difference between NPT and IM fitting measurement

Published Categorized as Uncategorized No Comments on Difference between NPT and IM fitting measurement

Hey! This site is reader-supported and we earn commissions if you purchase products from retailers after clicking on a link from our site.

by Jerry
(Paramus, NJ)

What is the difference between a 1/4 NPT and an 1/4 NPT I/M fitting?

Are they inter changeable?
Hi Jerry…

I do not recognize the I/M portion of the thread name. NPT is, of course, national pipe taper.

Knowing where you found the name might help.

I could guess that I/M might stand for imperial / metric, which would suggest that it is a thread designed to be used in both imperial and metric ports.

When you measure the O.D. of the thread, is it the same for both?

Guide to NPT fitting size
Guide to NPT fitting size

If so, then the next step is to thread both into a 1/4 port and see how many threads penetrate before they cannot be threaded further. If they penetrate 4+ threads, then that would suggest that they are useful in both metric and imperial port sizes.

Anyone else from the wider pneumatic community an offer advice here?


I/M couplers
by: Doug in s.d.ca

NPT is NPT, and refers to the threaded end.

“I/M style” (apparently) refers to “Industrial / Milton”, meaning the garden variety of couplers that usually come with DIY compressors.

The bump near the tip of the male I/M connector is narrow, about a 16th inch at the outside diameter.

The bump on a “T” connector (Tru-Flate) is mostly used in automotive shops, and has about an 8th inch wide flat at the end.

There are “I/M”, “T” and ‘universal’ “U” female checked couplers. The “U” connectors work with either “I/M” or “T” males, but are not truely universal.

All are commonly sold at big-box stores.

New comment? New question? Please add it here along with photos to help others help you with your compressor and equipment problem!

Notify of
Inline Feedbacks
View all comments