At 11:05 AM 5/9/00 -0400, Debbie3074@aol.com
Should rotors on our '85 AT 928S be turned or replaced? We found a ridge and are not sure what that means. The brakes do squeal.. but what else is new?
The pads normally do not come quite to the outside of the disc on a 928, which does result in a wear ridge at the outer edge of the disc.
You will need to measure the thickness of the discs to determine if they are worn too badly to use, assuming that they are reasonably smooth and the brakes do not "pulse" or shake upon application.
On the '85, new front discs are 32 mm thick, and the minimum thickness after machining should be 31.2 mm.
Rear discs start at 20 mm, and the minimum after machining should be 19.2 mm.
In actual practice, the discs will need replacing when they warp, causing pulsing upon brake application, or when they get scarred and scored because you ran out of friction material on the pads. If the discs get too thin, they will usually warp. Racing or very hard street usage will be a little more critical, and the thickness and smoothness must be checked more carefully.