Rear brake disc uneven wear
 

I have refurbished all brake calipers with new pistons and seals two years ago. New brake lines. New rotors rear. I have foolishly ignored the old adagio "always change rotors and pads at the same time" :o

The right rear doesn't look to good; left rear looks ok. I have maybe driven 2000 kms since changing out the rotors.

So what is the best course of action? Do I leave it and eventually the pads will form to the rotors, or do I change the pads?

Right
Attachment 555031

Left
Attachment 555032

turn the rotors, new pads.

+1 ... bare minimum off the rotors to true them and new pads.

Thanks guys.

Turning rotors is not common here, I'm not sure whether that is due to safety/regulations/insurance or just because it's more attractive to sell new rotors.

I am contemplating just installing new pads, or would I make the same mistake all over again?

Our rotors tend to have 2 mm from new to minimum (like 32 to 30 mm), and most places take a minimum of 1 mm when they refinish, so it's rare to be able to reuse a rotor.

As Tom said, there's little tolerance for machining, but as the rotors are almost new they should clean up very easily with an amount less than 1mm ground off them.

Rotors are commonly turned, but some engine rebuilding shops and brake shops here can grind the rotor, which allows a very small amount to be taken off to get rid of ridges and inconsistencies only, without removing more material as is common with turning. If you try to take less material off with turning, the tool tends to ride up on the surface and not cut in, whereas grinding can take minute amounts off. Grinding is commonly carried out on flywheels and gives a nice fine finished pattern to bed friction material in .. as it does with rotors.

It would be better/preferable if you can get someone to true the discs up, because those rusted areas are going to glaze or chop into the pads, and you may be in a similar situation all over again.

I had a flywheel ground recently and they removed hardly any material, but trued the surface and restored a good friction surface.

I am starting a quest for a shop that will do this type of work, if there are any.

So I've been checking the specs in the tolerances and dimensions book. Seems that minimul thinkness for remachined rear rotors is 22.6 mm where 22 mm is the wear limit.

Thickness tolerance is 0.02 mm: is that the max. difference in thickness measures in different places?

Lateral runout is max. 0.05 mm uninstalled and max. 0.1 mm installed. This is the "side to side movement" of the disc.

Then there is mention of max. surface finish after machining: 0.006 mm. How the heck is this measured??

......

Install a higher rear bias valve, and give it a few good hard stops to bed the pads.

What you see is not uncommon or dangerous. But the rear brakes in the 928 do very little unless the bias valve is changed.