Originally Posted by
FBinNY
I haven't looked at a set of Shimano barcons in years, but if they still use 3 sectors, it may be easier to modify the sectors than the bar.
Remove the expander bolt, and take the sectors out from the spring. Be gentle so you don't destroy the spring. Then file or sand a bit off the mating ends of each sector. This allows them to fit a bit closer together making the minimum OD smaller.
Reassemble and you should be good to go.
I did this to fit some Shimano bar-ends in a thick wall aluminum handlebar. It look some filing and sanding, but it worked.
If you're near Portland, Oregon, talk to Terracycle. They custom-make handlebars for recumbents, and they know how to ream the handlebar for the shifter.