Hmmm. Ok. So the water tight seal comes from the design of the inner tube of the SS hose? Maybe that's what decaf_Ed was telling me up above too, but it went over my head.

You certainly don't necessarily need to visit the hardware store for this project, but the next time duty calls, visit the humongous array of SS hoses these stores have. You will easily see what Ed is describing and what I am describing. Unfortunately, 1/2" NPT and various compression sizes seem to be the size de jour. A few years back you could find 3/8" NPT as best I remember.

The BSP to NPT thread difference is only one thread per inch. You can get about a half inch worth of thread before they bind up. My two cents, I would use the NPT with 5 or 6 wraps of Teflon tape to get up and running then order the proper 3/8 BSP to NPT adapter and replace the connection when it arrives. I have done that a couple of times. Just dont screw them together with a pipe wrench, just snug them up and you should be in good shape for now.

That's a great suggestion, Dave. A half inch of thread is nearly the whole threaded portion of the connector. Now that I comprehend what Eric and Ed were saying, I am no longer reluctant to give that a try.

I didn't have any pics handy earlier, but here's one now.
Here's the common (U.S.) compression fitting on the end of a hose on the left, the "real" compression fitting next to it (with the tube and ferrule). They look nearly identical when viewed from the end, especially in low light. The male receptacle that both mate to is on the right.

They use straight threads, and as EricS mentioned, the threads are for clamping force, not for sealing. The sealing surface on the male receptacle is the taper on the inside (green arrow). This mates with the ferrule on the tube, or with the taper on the outside of the faux tube in the hose end (red arrows). This is often a soft material in the case of the hose end, so hand tightening is sometimes good enough for a seal, and another half turn or two to prevent loosening (or for pushing the pressure limit).
Of course with the tube and ferrule, you need to torque down pretty good to compress the ferrule onto the tube so that that surface seals as well.
-Ed

Thank you for the pictures Ed. I was at Home Depot today, and I think all the hoses I looked at had an end like the one on the far left in your pictures, the one you called the faux tube, rather than an actual ferrule like the hose on the right.

Are the straight threads on these hoses also known as NPS or NPSM (American Standard Straight Mechanical Pipe Thread)? A 3/8th NPT fitting and a 3/8th NPSM fitting both have 18 threads per inch. They are evidently compatible enough for achieving the mechanical connection that provides the clamping force necessary to seal the ferrule or flared tube end of a flexible water hose to the inner sealing surface of an NPT fitting. Or so I assume, since this is commonly done.

With regard to the 3/8th inch BSPP or BSPT (whichever they are) male fittings on my water softener, as long as an NPSM hose will make a sufficiently tight mechanical connection to a BSPP fitting to seal the ferrule or flared tube end against the sealing surface of the fitting, then it won't leak despite the difference in threads per inch.

Hopefully I've got all that right now.

I'm sure there are more different types of hose and pipe fittings than I'm aware of. There are two basic diameter schemes, "tube" and "pipe", where typically a given nominal size pipe is larger (O.D.) than a tube of that same nominal size. Some hose fittings follow the "tube" size scheme, others follow the "pipe" size scheme. In my small sample, those following the tube size scheme had finer threads than those following the pipe size scheme (for the same thread outer diameter). Add to that the myriad of different joint-sealing schemes, like several different flare schemes, the tube/ferrule compression scheme, and the garden hose scheme (with a flat sealing surface perpendicular to the bore axis) to name a few, and it's hard to guess what specific thread is in use without an experienced eye or good measurement. EricS would be more useful here.
This site lists some thread sizes for their compression fittings.
http://www.brasscatalog.com/catalog_bro...no&page=73
Most of these appear to be of a finer pitch than a similar O.D. pipe thread (NP or BSP).
On my Expobar Office, the fittings on the thermosiphon lines are somewhat akin (conceptually) to a U.S. compression fitting, but use the coarser pitch of pipe threads, but no taper (BSPP). But the dispensing lines (steam and water) use a flare fitting (IIRC). Being a pour-over, I don't have an inlet fitting.
-Ed