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When to Replace Brake Discs
Brake discs should be replaced before they reach the minimum disc thickness recommended by the vehicle manufacturer or the supplier of the disc. This figure will be in the workshop manual and should be marked on the disc itself (often on the outside diameter of the disc).
The brake disc may show some evidence of surface crazing, radial cracks, scoring and heat spotting without the need for rectification or replacement.
Disc crazing
Short, light surface crazing or cracking seen randomly on disc faces.
These are a result of thermal events.
The disc will not need replacing or rectifying.
Radial cracking
Light cracks that have width and depth.
These cracks can extend to the outside diameter of the disc and cause a full open crack.
Cracks that are greater than 0.5 mm wide and/or 1 mm deep that extend radially over more than 75% of the disc friction surface should be replaced or cracks removed by machining.*
Scoring
The disc friction surface may be scored through use at high temperature. These are seen tangentially around the surface.
If the depth of the groove is less than 0.5 mm, the disc will not need replacing or rectifying.
Discs with grooves deeper than 0.5 mm should be replaced or grooves removed by machining.*
Heat spotting
Very high temperatures can cause permanent disc distortion. Initial evidence may be seen as heat spotting or discoloration.
The distortion can then be measured as axial and radial runout.
If either the maximum axial runout exceeds 0.3 mm, the radial runout exceeds 0.8 mm or the disc thickness variation exceeds 0.05 mm, the disc should be replaced or machined to specification.*
*Disc skimming
If allowable, according to the directions above, the disc may be machined to remove scoring, cracking or to reduce runout. This must be carried out equally on both friction surfaces to maintain equal friction ring thickness on both sides of the air gap.
The disc must not be modified below its minimum thickness.
Technical article provided by Alcon Components Ltd. Published by AlconKits with permission.