<< OK, lets start the next story Ramon. Why could I not turn down the OD of a later more plantifull disc and use it on the "god awfull roundish pad early girling" caliper? I spied a late disc once, and the dimensions all looked OK save the OD of the disc. Enlighten me. >>
Because if you did that, then the inside edge of the pads would hang over the inside lip of the disk. Though the later rotors are larger in diameter, the inside edge of the disk also is moved outward.
Next time you compare early/late rotors, compare the diameters of the inside edges of the disks as well. The difference is about the same as the difference in the outside dimeters, and the "swept" distance from the inside to the outside of the disk is about the same on both. That's why you can't just turn down a later rotor and use it on an early car--not without doing something else, anyway.
Ramon