Looking at your pic I would say turning that down to 26.4 shouldn't be any problem.
Ok, how are you actually going to hold it in a perfectly concentric position, not misaligned like a spring washer, that would be easier than starting again?
secondly 0.6mm off a diameter with emery would take longer than starting again. Polishing 0.1mm is a lot of work on a bar, let alone a fiddly piece like that.
Right - correct me if I'm wrong but I'm thinking you're getting confused with posters here. I'm not the guy (integerspin) who was kindly offering his turning services...I'm the dummy who got himself into this situation in the first place. RE: it being perfectly concentric - it was going to be trial and error - hence the palm in face - very heath robinson - but then I don't profess to be a pro. As for the emery cloth, again I'm no pro, but with the right setip would it be that hard to reduce 0.6mm off an alloy shim with a bit of elbow grease and the right grit sandpaper??
Anyhow, this was highly hypothetical and as I said further down "but I'm sure there are much more conventional approaches available"
Still listening to sensible suggestions folks