Connect 4" BSP parallel (G4 / BSPP) to 3" BSP parallel (G3 / BSPP)

Stepping a G4 port down to a G3 fitting uses a parallel BSP (BSPP, ISO 228) reducing bush: a male G4 on the outside and a female G3 inside, both to the same parallel BSP form so they seal flat washer or bonded seal on the face. The major diameter goes from 113.03 mm at G4 to 87.884 mm at G3. This is a single stocked reducing part; the step is one nominal size.

Both ends are the same parallel 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.