you are talking about the edge of the rotor. this is the thickness it was before wearing down. and if too tall. it will rub against metal on your pad well before your pad is worn out. hence a rumbling. metal to metal.

tom w

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.