I know this thread is very old, but I'm adding just in case someone stumbles upon it looking for information. Hopefully it will clear up a little confusion & helps someone someday:
Your G thread is the same as fittings titled BSPP
PF, used mostly in Japan, is also the same as G & BSPP
But there are other British threads that people confuse.
Huh? Which Is Which?
BSP is a technical standard called "British Standard Pipe". It covers two distinctly different threads:
Both are 55 Degree pitch, but...
A) BSPT is tapered thread. It looks almost exactly like NPT with a slightly more severe taper.
B) BSPP is parallel or straight thread. There's no taper.
How To Spot The Difference:
You can easily tell the difference between male BSPP & BSPT because BSPP used an O-Ring to seal & BSPT does not.
BSPP female fittings have a flat land for the O-Ring.
Which Is More Common?
The BSPP is excellent at the high pressures seen in hydraulics. It's often used in North American hydraulics, too.
BSPP is the standard for hydronic radiators in the UK, Europe & Asia. Most of the flat panel ones & towel warmers sold in
North America also use BSPP (G): 3/4 for the bottom & 1/2 for the sides.
(Either Myson or Runtal are NPT. I always forget which one.)
I've only noticed BSPT to NPT in hydraulic catalogs. It is used in the UK for general plumbing work, too.
Versus NPT?
BSPP (G) kinda-sorta fits 1/2" NPT, but eventually they'll leak. The pitch is too loose of a fit. The diameters are
also different. (I suppose the evil & dreaded blue Leak-Lock might work for a longer time than pipe dope or tape.)
Adapters To NPT & To Tubing via Compression:
Adapters are available at hydraulic stores, McMaster Carr and anyplace that sells flat panel radiators.
You can also get adapters to take copper tubing or PEX to a male G fitting by compression. The G3/4 size is common to almost all flat panel radiators. The beauty of those is one can remove them and re-use. Example: Simplex F11173 & it's siblings. I've seen those at a 6 degree angle & still not leak.
I hope this is helpful to someone for clearing up the confusion.
Paul