I have not worked on a twin with a bush for many years. When I last worked on my A10 the main bearing bush was a steel housing with a steel backed bush of similar composition to a big end shell. Putting a reamer anywhere near it would have ruined it.
My Aprilia has plain bush main bearings, these are split shells like big ends pressed into the crankcases
Has the idea of reaming come from after market solid bronze bushes?