Rick's Free Auto Repair Advice

When to replace brake rotors

When to replace brake rotors

How long do brake rotors last?

Carmakers design brake rotors to last through at least two sets of brake pads. But that’s a best-case scenario and doesn’t account for issues like corrosion, grooving, or disc thickness variation caused by lateral runout.

How to know when to replace rotors?

If you’re feeling pedal pulsation, chances are the rotor has uneven deposits of friction material that’s causing disc thickness variation. You restore the rotor by machining it if it meets the following thickness standards, is crack-free, contains no heat discoloration and the rust is cleaned from the cooling vanes.

Guidelines for resurfacing rotors

Nominal thickness: The thickness of a new rotor.

Discard thickness: When a rotor reaches discard thickness it must be removed from service and recycled. Most people think the discard thickness is based on the rotor’s ability to dissipate heat. It’s not. Instead, the discard thickness is based on maximum caliper piston travel limits. If the rotor is thinned beyond it’s discard thickness and the brake pad friction material wears down to the backing plate, the caliper piston will pop out of the bore. The average difference between nominal and discard thickness is .050”

Machine-to thickness

This refers to the thinnest the rotor can be machined to and still provide enough metal to withstand the wear that will be encountered with a new set of pads. In other words, after machining the rotor, it must be at least .015” thicker than the discard thickness.

Even if the rotor meets the above thickness requirements, you can’t reuse or resurface a rotor if:

• The groove thickness is .039” or deeper

• The rotor has heat discoloration spots (blue or purple). Overheated brake rotorThe metallurgy has changed and the rotor will never perform properly

• The rotor has cracks. It cannot be resurfaced or reused

• You haven’t cleaned the rust from the cooling vanes. Sandblasting the cooling vanes is the most effective way to remove rust.

• You can’t determine rotor thickness visually. You must use a micrometer and measure in at least three places

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