Dont think I've ever seen 27mm before, but:
I've opened a couple of 1" sets to 26mm without issue, maybe you could do the same for 27mm.
I put .015 shim stock between the ring halves and clamp them good, then setup in the mill and helical them out.
I would imagin you could also do it with an adjustable reamer and a simple jig to keep the reamer strait.
Cheers, YV