NPT=National Pipe Thread is a TAPERED THREAD 1/8" pipe, 1/4" pipe, 3/8" pipe, 1/2" pipe, etc. The taper rate for all NPT threads is 1⁄16 (¾ inch per foot) measured by the change of diameter (of the pipe thread) over distance. The taper divided by a center line yields an angle 1° 47' 24" or 1.7899° as measured from the center axis. Commonly-used sizes are ⅛, ¼, ⅜, ½, ¾, 1, 1¼, 1½, and 2 inch, appearing on pipes and fittings by most U.S. suppliers. Smaller sizes than ⅛ are occasionally used for compressed air.
Most older Fuel Pump regulators and fuel pumps were a NPT thread.
Newer Fuel Pumps and Regulators can have a "Straight A/N thread".
The AN thread is a particular type of fitting used to connect flexible hoses and rigid metal tubing that carry fluid. It is a US military-derived specification stemming from a joint standard agreed upon by the Army and Navy, hence AN. The standard is sometimes mistakenly referred to as "Air Force-Navy", but it dates back to World War Two, before there was a separate US Air Force.
AN sizes range from -2 (dash two) to -32 in irregular steps, with each step equating to the OD (outside diameter) of the tubing in 1/16" increments. Therefore, a -8 AN size would be equal to 1/2" OD tube (8 x 1/16 = 1/2). However, this system does not specify the ID (inside diameter) of the tubing because the tube wall can vary in thickness. Each AN size also uses its own standard thread size.
(THE DEAL IS THAT A -8 A/N LINE DOES NOT TRULY HAVE A .500" INSIDE DIAMETER. MORE LIKE .437" INSIDE DIAMETER.) the statement above is close but not accurate if you are calculating actual fuel flow capability. Same deal when doing oil lines for coolers, remote filters, etc. If the hole in the block is a drilled 1/2" hole go one size bigger on the A/N hoses.
The Aeromotive web site is a very good source of info on fuel lines:
http://aeromotiveinc.com/2010/01/an-vs-npt/
Tom Vaught