This adapter is required if you want to install Shimano 24mm HollowTech II cranks on a BB30 frame. In case you are wondering, a BB30 spec frame has a 42 inner diameter BB shell and a Pressfit 30 frame has a 46mm inner diameter BB shell. Made from lightweight Delrin. Does not include bearings so order those if you don't already have them.
Works on BB30 frames only - (i.e does not work on Pressfit30 frames)