Connect 3/4" BSP tapered (R3/4 / BSPT) to 3/4" NPSH straight hose thread

R3/4 and 3/4-14NPSH are both called the same nominal size but are different thread forms: R3/4 is tapered BSP (BSPT) at a 55 degree flank angle and 3/4-14NPSH is straight NPSH hose at a 60 degree flank angle (26.441 mm versus 26.29 mm major diameter). They start a turn or two then jam without sealing, so a machined tapered BSP (BSPT)-to-straight NPSH hose adapter is required, not tape.

The matching 3/4 inch label is the trap. The 55 degree versus 60 degree flank angle means the flanks never fully mate; add the correct adapter rather than forcing the joint.