Michael Benno

I am in the process of installing new disk brake rotors and pads on the front. I have all the new parts installed. However, the right disk seems out of true (runout). The runout specification for a GTS front disk is 0.1mm. I can see there is at least 2mm or more variance as I spin the wheel. This means the rotor drags on the pads in parts of the wheel rotation. The other rotor spins true.

My question is, where is the issue? Is it in the rotor? Do rotors occasionally arrive out of secification? This is an OEM Sebro rotor, so I would expect the quality to be very high. Is there some other area that I should be checking? I checked the runout of the hub and it is within the specification (0.05mm)

Zirconocene

I have a woodworking straightedge, if you want to test the rotor itself. The variation along the length was certified to WAY under that tolerance.

Have you double checked the area between the rotor hat and where it sits on the front axle? Is there anything on there that is causing for the rotor not to seat fully? Or, are there any paint/protection globs on the hat visible?

Cheers

Mrmerlin

You could have a out of tolerance rotor ,
put it on a lathe and see what you find or return it,
I have found a few bad New rotors on other cars not a 928

Michael Benno

Stan, thanks for the insight. I don't have access to a lathe

I did not. I will pull the rotor and inspect and clean the mating surfaces. Hopefully, this is the issue. I will pull the rotor and inspect it. If there are other WYAIT checks let me know.

Michael Benno

There must have been enough grit and debris on the hub from the old rotors that was preventing the new rotors from seating properly.

I removed the rotors and cleaned the hub with a razor blade and some solvent and made sure the rotor faces was clean as well



I decided to check the runout with a dial indicator. The runout was now 0.0078mm well within specifications

And to double check I verified the thickness was constant. These rotors are very high quality indeed.



lesson learned, clean hubs very well before installing rotors

Rob Edwards

Glad that this was the outcome- I have had the same thing happen and so now the SOP on any car is to take the dremel and use a cup brush to de-louse the hub face to make sure the new rotor will sit flat.

Petza914

Yep, I use a wire bush on a drill on the hub mating surfaces whenever I do a brake job so everything fits flush and true.


Battery drill and wire wheel brush




Mounting hub before



Mounting hub after

Mrmerlin

I suggest to put a thin smear of grease or DC111 on the hub face or the rotor mating face to prevent corrosion.
Also depending on the wheel cap you use, put a thin bead of silicone around the outer edge under the cap ,
This to prevent water from getting past the cap edge and filling up the holes in the hub rotor as is clearly the case from Petes pictures.

Gage

I would not expect an accurate runout measurement with only the two (off center) retaining screws securing the rotor to the hub.