Connect 1-1/4" BSP parallel (G1-1/4 / BSPP) to 1/2" BSP parallel (G1/2 / BSPP)

Stepping a G1-1/4 port down to a G1/2 fitting uses a parallel BSP (BSPP, ISO 228) reducing bush: a male G1-1/4 on the outside and a female G1/2 inside, both to the same parallel BSP form so they seal flat washer or bonded seal on the face. The major diameter goes from 41.91 mm at G1-1/4 to 20.955 mm at G1/2. This is a single stocked reducing part; the step is three nominal sizes.

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.