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Reaming should really only be for getting rid of lumps, bumps and distortion from brazing or welding, not changing the internal diameter of the tubing. Reaming that tube back to 31.7mm at any point would be fine. Trying to ream it out further to fit a bigger seat post is asking for trouble though.
On Brommers Bike no. 3 I used a 31.7mm external seat tube, because that fitted the lugs I was using. I bonded a 30.6>27.2mm shim in place to allow me to use a 27.2mm seat post.
Makes sense. This one can't be reamed beyond the external taper, then?
http://www.henryjames.com/Cpdf/seat-tubes/ZON113550001.pdf