[Getter-Done]

Here is another article on NPT.
Hope this helps.
Link: https://www.pegasusautoracing.com/do...%203%2F8%20NPT.

Ordering and using NPT fittings can be confusing because the nominal sizes have no relation to thread size. To make matters worse, NPT fittings do not fit together the way most other fittings do. This has led many people to believe that they have mismatched sizes. Fortunately, NPT sizes are easy to identify. The sizes are also different enough that it is impossible to even start to assemble two different sizes together.


(Background)
NPT stands for National Pipe Tapered thread. It is also sometimes called National Pipe Thread, NPTF (F for Fuel), American Standard Taper, or simply "pipe". Most other plumbing fittings have straight or parallel threads, but NPT fittings have a tapered thread profile. The outside diameter of the male thread tapers down from the base, so the end of the fitting has a smaller OD than the base of the fitting. The matching female thread has a larger OD at the opening, tapering down as it goes in.

NPT fitting sizes refer to the nominal ID of a standard cast-iron pipe. This is where the confusion comes in. Most plumbing fittings are named for the thread size, but NPT sizes are not. An NPT fitting measuring 3/8" OD thread is actually a 1/8 NPT, not 3/8 NPT. Use the chart below to positively identify NPT fitting sizes.