I would do as Kimmo suggested. You can also measure the shell faces for parallelism and be fairly certain it's close enough for OBs. There was a case posted on this forum (can't find it now) where they had the BB faced and ended up with horrible chatter marks that couldn't be undone. I'm of the opinion that the cutting angles on the cutter was not proper for aluminum. Not saying this is the case with every tool, and every operator, but I'd not chance it.