Connect 2" camlock (cam-and-groove) coupling to 2-1/2" BSP parallel (G2-1/2 / BSPP)

A 2 inch cam-and-groove coupling reduces up to the larger G2-1/2 port with a reducing threaded camlock adapter: the cam end stays 2 inch while the back thread is G2-1/2 (parallel BSP (BSPP)), a parallel, 55 degree thread of 75.184 mm major diameter at 11 threads per inch that seals flat washer or bonded seal on the face. Because the camlock is one nominal size away from the thread, this is the reducing version of the adapter, carried as one part rather than a same-size adapter stacked on a separate G2-1/2 reducer.

The cam end mates only to its opposite 2 inch camlock half, so the G2-1/2 back thread does not change what fits the front. Match the cam type (A to F) to the half you are joining and confirm the back really is G2-1/2 and not the same size in a different standard before buying.