Okay, I'm using SUN brand fittings with a 1/2" barb to 3/8" BSP threads.
Not sure how BSP differs from NPT.
I can very much assure you that the barbed section of the fittings have an inner diameter of 10.5mm (roughly 13/32")
The threaded section of the fittings have an inner diameter of just over 13mm (about 13.1mm or roughly 33/64")
Then again, I've also seen 1/2" barb to 3/8" BSP fittings with just 9mm ID on the barbs (3/8") and 11mm ID on the inner thread diameter (7/16").
I have yet to see any 3/8" BSP thread fitting have an inner diameter of the threaded section less than about 11mm.
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