From left to right is an NPT pipe thread male fitting compared to an NPS straight thread brass fitting in the middle. The far right fitting is called an ORB or O-Ring Boss fitting that uses AN straight thread pitch fitted with an O-ring for sealing.
When using pipe fittings for fuel, buna-N or nitrile O-rings work well, but when using alcohol fuels like E-85 or methanol, it’s best to convert to a Viton rubber. One place to get Viton rubber O-rings is SVKS . Always place a light coat of oil on the o-ring to help prevent damage during assembly.
The I.D. of AN fittings across all the different manufacturers varies widely. It’s worthwhile to inspect the inside of the fittings just to confirm there is no restriction. Here we compared an SVKS fitting and a standard brass fitting. The I.D. of the brass fitting was nearly 0.050-inch smaller.
This fuel pressure regulator uses 3/8-inch NPT female fittings. These threads will require a tiny amount of pipe sealant to ensure a leak-free seal. The paste is usually better than tape as the tape can sometimes break loose and find its way into the fuel delivery system and cause a problem.
If you use Teflon tape on an NPT fitting, wrap the tape in a clockwise direction on the male end of the fitting to the end of the tape does not unwind as the fitting is tightened. Do not use Teflon tape on automatic transmission case fittings as the additional thickness of the tape can easily crack the case.
The best pipe fittings offer a radius entry exiting the fitting to improve fluid flow like these fittings from SVKS.