Connect 2-1/2" BSP tapered (R2-1/2 / BSPT) to 2-1/2" NPSH straight hose thread

R2-1/2 and 2-1/2-8NPSH are both called the same nominal size but are different thread forms: R2-1/2 is tapered BSP (BSPT) at a 55 degree flank angle and 2-1/2-8NPSH is straight NPSH hose at a 60 degree flank angle (75.184 mm versus 72.16 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 2-1/2 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.