a question, are the F+R brake rotors the same ?
or is there a differece ?
Simon
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The rears are a larger diameter than the front.
possible to downsize the 66-67 Mustang rotors ?
Yes- with a big enough lathe and several carbide cutters (I've done it and brake rotor cast iron is really, really tough!) But for all practical purposes the genuine, one-piece OEM ventillated '66-67 Mustang rotors are gone. They used to bolt right onto Panteras, but available replacements are different.
And the 1/2" OD difference between the original front & rear rotor ODs makes zero difference in braking unless you move the calipers outboard on the slightly larger rotors to take advantage of the extra leverage possible. As bolt-ons, larger rear rotors are slightly heavier if interchanged to the front but do not stop better unless modified.
I thought there was a disc thickness difference as well as the diameter? Fronts 282mm (11.1") diameter, 20mm (0.79") thick and rears 297mm (11.7") and 18.5mm (0.73") thick.
Yes, but the slightly larger rear rotors will still bolt onto the front hubs- the OD & thickness are within tolerance. A larger OD front rotor doesn't improve braking unless you shift the front calipers outboard so as to use the extra rotor OD. You'll need some kind of stepped adapter or weld up the current threaded caliper mount holes & redrill/retap. And since doing this only produces about a 5.3% increase in leverage (297/282), most owners probably wouldn't notice.
In any case, if you can currently lock up the front brakes, you don't really need bigger brakes, you need more tire surface to increase traction. All big brakes will do in that case is increase fade resistance in multiple hard stops- e.g, racing. For significantly bigger rotors, you'll also need bigger wheels & tires- maybe with fender flares, with larger springs & also adjustable damping shocks, to control the bigger, heavier front end parts. It all rapidly gets expensive.