BMW brake rotor facts and deductions
Here's the real answers to what seem to be the FAQ's. The facts are taken direct from the TIS, the deductions come from my engineering background. As always, please do not hesitate to identify and correct any errors I may have made.
Rotor sizes (in mm)323 - 286x22 front, 276x19 rear
325/328 - 300x22 front, 294x19 rear
330 - 325x25 front, 320x22 rear
Minimum Acceptable Thickness for reuse -
323/325/328 - 20.4mm front, 17.4mm rear
330 - 23.4mm front, 20.4mm rear (deduced)
Maximum Allowable Runout
Disc on it's own - 0.04mm
Disc installed - 0.2mm
Is it allowed to machine (turn) BMW rotors?
Yes, there is an actual BMW procedure in the TIS describing how to machine the rotors. You can machine them down to the minimum thickness if you need to. You do however, need to ensure that both sides are evenly machined and the appropiate runout and surface finish specifications are maintained.
Should you machine your BMW rotors?
Generally speaking, BMW rotors are cheap enough where it does not make economic sense to turn them. Plus, replacing them guarantees you will have a rotor with the correct runout and surface finish criteria.
When do you have to replace your rotors?
When they are below the specified minimum thickness.
What if my rotors are at, but not below the minimum thickness?
Then you can use them again, but only for one more set of pads, assuming you are using OEM pads or pads with equal or less rotor wear than the OEM pads.
What's the theory behind the minimum thickness criteria?
When BMW selected the pads and designed the rotor thickness, it was not done randomly. The expected range of rotor and pad wear is known and designed on purpose.
Your pads have a wear sensor which goes off when you have 3mm or less pad material. It is a safe assumption that the pad wear sensor is designed to go off before you wear your rotors down so thin that they are hazardous. So the deduction from this is that the wear rate of rotors with OEM pads is 1.6mm or less per set of pads. Which in turn means the absolute safety limit of rotor thickness is probably about 18.8mm front and 15.8mm rear (for 323/325/328 - add 3mm for 330). To put it another way, the BMW rotors and pads are designed in such a way that if you reuse the rotors at minimum thickness, your pad wear sensor will go off before your rotors get too thin.
So do should I replace the rotors everytime I replace the pads?
Assuming they are at or above the minimum thickness for reuse and are otherwise within spec, it's up to you to strike the best balance of tradeoffs between performance and money. It is an absolute certainty that you will get better braking performance out of new rotors than reused rotors. But depending upon the condition of the rotors, the difference in performance may be slight.
The one thing you do NOT want to do however, is reuse rotors with a non-OEM brake pad of unknown wear rate. By doing so, you run a very serious risk of having a higher rotor wear rate than was intended by design. As a result, your rotors may become dangerously thin and even fail before the pad wear sensor goes off.