F31 2015 328iT xDrive - Brake rotors: to replace or not to replace?

[xx328xx]

Hi everyone, in Bentley service manual brake rotors thickness specked to 24 mm front and 22mm rear.

Part numbers for front rotors 312X24: 34-11-8-848-417 replaces 34-11-6-774-875, 34-11-6-792-219
Part numbers for rear rotors 300X20: 34-21-6-864-900 replaces 34-21-6-792-229

But when I go shopping for genuine new rotors with correct part numbers (checked with dealer), they are listed as front 24 mm thick, and rear 20mm thick. The front matches Bentley SM spec, but the rear is not. Again, the BSM spec for the rear is 22, while the part sold specked at 20 mm thick.

Is the Bentley service manual has an error? Or what am I missing?

I checked my actual rotor thickness with calipers and got even more confused. After 5 years and 70K of the millage, two sets of pads worn, my actual rotors thickness measures as follows:

Front: 24 mm
Rear: 20 mm

Does it mean that my rotors did not wear out at all after 5y/70k? Is that possible? Or what am I missing? Could it be that dealer did not give me the correct part number? Is the F31 wagon use different rotors than F30?

I'm preparing to replace brake pads and I want to see if my rotors need to be replaced or not. But these two issues confuse me and I can't make the decision:

1) Does Bentley SM contains an error?
2) How could rotors not suffer any wear after 5y/70k
3) Brake pedal feels a bit deeper, is that due to pads wear?
4) Do I need to replace rotors at this point? They do not produce any vibration at this point.

I would appreciate any input!

[328 Power 04]

1. Bentley is not the definitive voice in car repair, as they're simply a 3rd party. They do have mistakes. Often they're good for general reference, or as a door stop.
2. Yes, this is possible. But if you see a lip on them or cracks/uneven surface, heavy rust... replace them anyways.
3. Yes, pedal is deeper due to pads being thin. Also, you should be flushing brake fluid every 2 years. It greatly increases the feel of the pedal.
4. See #2. I would if you can.



Straight from BMW for the base brake system (not multi piston):
Fronts:
Nominal size (diameter x thickness) 312x24mm
(MIN TH) is stamped in brake disc chamber 22.4mm

Rears:
Nominal size (diameter x thickness) 300x20mm
(MIN TH) is stamped in brake disc chamber 18.4mm

[dworthy]

So the general rule of thumb with BMW is they don't turn rotors. That being said, the Minimum thickness is for turning and/or the thinnest they can be before you slap a last set of pads on them.

Even if you don't feel any shacking, if you want to ensure the new pads won't squeak, get the rotors turned. If there is a lip on the inside/outside of the braking surface, get them turned.