Connect 3/8" BSP tapered (R3/8 / BSPT) to 1/8" BSP tapered (R1/8 / BSPT)

Stepping a R3/8 port down to a R1/8 fitting uses a tapered BSP (BSPT, BS 21 / ISO 7) reducing bush: a male R3/8 on the outside and a female R1/8 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 16.662 mm at R3/8 to 9.728 mm at R1/8. This is a single stocked reducing part; the step is two nominal sizes.

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.