I just received a pair of NOS Sun CR-18 36 hole rims that were advertised/auctioned as 20" BMX rims. It turns out that they aren't 406 but 451 rims and quite a bit narrower than I thought they would be. I want to be certain the rims can be safely used on the Twenty before I start spending more to build new wheels. After searching here and reading on Sheldon's page about rims I'm a little confused in trying to determine their width. Is the width measurement taken on the inside of the rim? These measure 18mm across the inside top of the hook. Also, will the brakes need anything more than an adjustment and re-alignment? Thanks for any information.