Connect 4" BSP tapered (R4 / BSPT) to 3" BSP tapered (R3 / BSPT)

Stepping a R4 port down to a R3 fitting uses a tapered BSP (BSPT, BS 21 / ISO 7) reducing bush: a male R4 on the outside and a female R3 inside, both to the same tapered BSP form so they seal on the thread flanks with PTFE tape or paste. The major diameter goes from 113.03 mm at R4 to 87.884 mm at R3. This is a single stocked reducing part; the step is one nominal size.

Both ends are the same tapered BSP standard, so the only thing that changes is the size; do not swap standards at the same time. If you need to cross to a different thread form as well, add the correct thread adapter rather than forcing a same-standard reducer to do two jobs.