Sprocket Spacer. Spider Adapter. Hat Washer. No matter what you call it, you probably need it, to make your sprocket fit your BB spindle properly. These go on the back side of the sprocket, and are made of steel. These are not actually needed for Profile Chainring Spiders, which fit flush on a 19mm BB spindle.

A. for most chainrings, including Profile Imperial, Whippit, Widowmaker, Ripsaw 2 OD= .975"(24.7mm) ID= .750"(19.05mm) .305"(7.7mm) thick

B. for old style 1-piece chainrings on profile cranksets. OD= .940"(23.8mm) ID= .750"(19.05mm) .240"(6.09mm) thick

C. for thick sprockets, such as Profile Blade, 7075 Imperials, and Nano Drive ImperialOD= .975"(24.7mm) ID= .750"(19.05mm) .240"(6.09mm) thick

D. for SS spindles OD= .940"(23.8mm) ID= .875"(22.2mm) .235"(5.95mm) thick