Not a direct fit. Joining 2-1/2" BSP parallel (G2-1/2 / BSPP) to 2" BSP parallel (G2 / BSPP) needs a G2-1/2 to G2 reducing bush. Sealing method, the real-world trap, and the exact adapter to buy.

Updated 05 September 2026.

Connect 2-1/2" BSP parallel (G2-1/2 / BSPP) to 2" BSP parallel (G2 / BSPP)

How to join G2-1/2 and G2, and what stops it leaking.

Needs adapter
End A
2-1/2" BSP parallel (G2-1/2 / BSPP) (G2-1/2)
Seals on flat washer or bonded seal on the face
via adapter
End B
2" BSP parallel (G2 / BSPP) (G2)
Seals on flat washer or bonded seal on the face
Adapter neededG2-1/2 to G2 reducing bush

Not a direct fit. Joining 2-1/2" BSP parallel (G2-1/2 / BSPP) to 2" BSP parallel (G2 / BSPP) needs a G2-1/2 to G2 reducing bush.

The adapter to buy

Stepping a G2-1/2 port down to a G2 fitting uses a parallel BSP (BSPP, ISO 228) reducing bush: a male G2-1/2 on the outside and a female G2 inside, both to the same parallel BSP form so they seal flat washer or bonded seal on the face. The major diameter goes from 75.184 mm at G2-1/2 to 59.614 mm at G2. 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.

Related connections

Questions and answers

Connect 2-1/2" BSP parallel (G2-1/2 / BSPP) to 2" BSP parallel (G2 / BSPP)?

Not a direct fit. Joining 2-1/2" BSP parallel (G2-1/2 / BSPP) to 2" BSP parallel (G2 / BSPP) needs a G2-1/2 to G2 reducing bush.