Am changing brake pads & discs on my E 39 at the moment.

Am amazed at Brembo's clever wear indicators on their disc rotors - they have drilled 2 marks into the face of the rotor, 1 shallow and 1 a little deeper.

When the shallow one disappears the rotor is wearing and when the deep one disappears the rotor needs changing.

They didn't have that on the Brembo E34 discs I just replaced on my 525i, but hopefully soon it will be on all disc rotors.