From a ‘Brit’ hydraulics engineer, BSP is parallel thread, BSPT is taper.
On the tube we measure the OD (outside diameter) and with hose / flexi we measure the ID (inside diameter) There are a ‘few’ times we measure the ID of tube, but on modern sizes it’s the OD.
As for the nut threads, yes a 1/4” tube can have a 5/16 thread, but normally they are matched. You can get BSP or METRIC in both, pre formed ends or compression olive (ferrule think you might call it in places)
SAE or flared ends as pre forming the tube ends into a flared face, like brake pipe flares etc.
All these are readily available over here if anyone has problems.
Instead of trying your Auto stores or truck places, try the smaller, longer established hydraulic places.
Hope this helps guys
