He's talking about the riser part of the bars. They bolt to upper triple-clamp and cover top of fork tubes. Ex500 has 37mm tubes while pre-gen has 36mm tubes. To fit the shorter/lower risers from 250 onto EX500 requires boring out big hole in bottom from 36 to 37mm so they fit over EX500's larger fork tubes. To prevent loading bolts in shear (engineering no-no), you want them to fit snuggly over fork tubes.
