Connect 2-1/2" BSP parallel (G2-1/2 / BSPP) to 2-1/2" NH fire hose thread (NST)

G2-1/2 and 2-1/2-7.5NH are both called the same nominal size but are different thread forms: G2-1/2 is parallel BSP (BSPP) at a 55 degree flank angle and 2-1/2-7.5NH is gasket-sealed NH hose at a 60 degree flank angle (75.184 mm versus 77.94 mm major diameter). They start a turn or two then jam without sealing, so a machined parallel BSP (BSPP)-to-gasket-sealed NH 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.