Mouthpiece shank bores are not standardized and some are straight and some are tapered. If you have several mouthpieces you like to use you can get a new cork to fit the most common size. If you have an odd ball tight one, the bore can be sanded out larger. If you have a large bore, you can use paper or Teflon tape to get a temporary good for. The bore can be made smaller by coating the inside of the bore with epoxy and sanding it until the size is right. The sanding can take a long time by hand. Using a high speed rotary tool, like a Dremmel, with burs and sanding drums makes the job easier.