[601HP]

I found this on the Pegasus Auto Racing website. Brent - Kudos.

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. 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.

Regarding fitting the hose ends:
The adapter fitting for the hoses will be 1/2" NPT to -10 AN, if you plan to use 5/8" i.d. hose. Some folks use 3/4" i.d. hose, and the adapter fitting would be 1/2" NPT to -12 AN.

David