1/4 NPT(F) is APPROXIMATELY 1/2" OD, and has 18 threads per inch. But it's NOT designated 1/2-18 NPT. I think the confusion was trying to designate it based on APPROXIMATE OD.
First pic is NPTF (National Pipe Taper Fuel) dimensions, which is a dry seal thread pattern.
Second pic is NPT (National Pipe Taper) dimensions, slightly different than NPTF in that it requires a sealant.

There's also 1/4" NPSM that's roughly 1/2" OD (0.54") with 18 TPI, but normally has an internal 30 deg chamfer for sealing with a female fitting that has the matching chamfer inside.
Third pic is NPSM (National Pipe Straight Mechanical)
Attachments
NPTF Threads.jpg (55.39 KB, 68 downloads)
NPT threads.jpg (45.37 KB, 68 downloads)
NPSM Threads.jpg (51.46 KB, 68 downloads)

Kevin
1951 Chevy 3100 work truck
Follow this saga in Project Journal
Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.