[Irish Rich]

You're right, there is another group of Harley rotors, too. Up front the '77-'83 dual disc FX's, Sportys, and FXR Shovels, and the '79-'83 FLH's front and rear use rotors with a 2 3/8" dia. center hole.

"Generally" speaking.....as there are always exceptions to every rule. Harley-Davidson rotors are separated into two distinct groups. Those that are designed wheels with sealed bearings and those rotors for wheels with tapered Timken style bearings. The sealed bearing rotor have a larger center register (about 2.25") while the Timken bearinged hubs have a smaller center (~2"). The other major difference(s) is the the size of the mounting hardware. Fronts use a 5/16' bolt/screw and rear use 3/8". BTW...those are two rear rotors in your picture. 1 "early" on the left, and 1 "late' (2000+) on the right. FWIW, rotor ODs were 11.5" until 2008 when they went to 11.8"