Connect 3/4" NPT (National Pipe Taper) to 3/4" BSP parallel (G3/4 / BSPP)
How to join 3/4-14 NPT and G3/4, and what stops it leaking.
Not a direct fit. Joining 3/4" NPT (National Pipe Taper) to 3/4" BSP parallel (G3/4 / BSPP) needs a 3/4 NPT to 3/4 BSP adapter.
The adapter to buy
NPT and BSP are both called 3/4 inch but are not compatible: NPT is a 60 degree tapered thread, BSP parallel is 55 degree. They engage a turn or two then jam without sealing. A machined NPT-to-BSP adapter is required; PTFE tape alone will not fix the thread-form mismatch.
Ignore advice to just wrap more tape. The 60 versus 55 degree flank angle means the threads never fully mate. Use the correct adapter.
Related connections
Sources: ANSI/ASME B1.20.1 (NPT pipe threads) · NPT thread dimensions chart (ASME B1.20.1), Machining Doctor · ISO 228-1 (G / BSPP parallel pipe threads) · BSP thread dimensions chart (ISO 228 / ISO 7), AmesWeb · Washing-machine / dishwasher inlet 3/4 BSP connector data