By Larry Carley
Resurfacing or replacing brake rotors, and rebuilding or replacing brake calipers are two tasks that are often necessary when doing a brake job. These parts may not require much attention the first time the brakes are relined. But by the second or third brake job, it’s likely the vehicle will need rotors and calipers.
Regardless of the age or miles on a vehicle, always measure rotor thickness when you’re inspecting the rotors. As rotors wear and become thinner, they also become weaker and are less able to absorb and dissipate heat. This increases the risk of the brakes overheating as well as rotor failure.
All rotors have a minimum thickness and/or discard specification that’s usually stamped or cast onto the rotor itself. If a rotor is worn down to this specification (you must use a micrometer to measure the thickness), or it cannot be resurfaced without exceeding this dimension, the rotor must be replaced.
Rotors can also wear unevenly. This is often due to excessive runout in the rotor, or from hard spots in the surface of the rotor. In many instances, rotors can be distorted if the lug nuts are not evenly torqued. Not using a torque wrench or torque-limiting sticks to final-tighten the lug nuts when a wheel is mounted can unevenly load the center hat section of the rotor. This may produce enough distortion in the rotor to cause it to wear unevenly. Dirt or rust that’s trapped between the rotor and hub can also cause the same kind of distortion.
Uneven rotor wear will lead to variations in the thickness of the rotor over time, which will eventually cause a noticeable pedal pulsation when the brakes are applied. So if a customer is complaining about a pedal pulsation, you can bet the rotors are worn unevenly or have too much run-out. The question is, which condition does the vehicle have? If you’re going to fix the problem, don’t guess. Take some diagnostic measurements.
DIAGNOSTIC ROTOR MEASUREMENTS
Busy technicians don’t have time to fool around with micrometers and dial indicators. Most of the time, these diagnostic tools are not used because it’s faster and easier to simply replace the rotors if the vehicle has a pedal pulsation problem. But what if you’ve replaced the rotors and the problem is still there? That’s when you need to open the toolbox, blow the dust off your gauges and actually take some measurements.
Measuring rotor thickness variation is tricky because you have to make at least 12 to 18 measurements around the circumference of the rotor with a micrometer to find the minimum and maximum thickness. After you’ve taken all the measurements and written them down, subtract the smallest thickness reading from the largest thickness reading to determine how much total thickness variation is in the rotor. A difference of only 0.001 inches (0.025 mm) in rotor thickness can usually be felt as a pedal pulsation.
On some cars, you can even feel variations as small as 0.0005 inches (0.01 mm) when braking hard. New rotors and resurfaced rotors should have almost no detectable variation in thickness. If you find any variation, you either have a problem with your rotor supplier (poor quality) or your brake lathe.
Measuring rotor runout is easier, but it does require a dial indicator. The dial indicator must be rigidly attached to the caliper support bracket, steering knuckle, strut or control arm so you can take an accurate reading. Place the tip of the indicator against the outside face of the rotor, zero-out the indicator, then rotate the rotor one full turn while watching the reading. If the total movement of the needle on the dial indicator is more than about 0.003 inches (0.076 mm), you may have a problem. It all depends on the year, make and model of the vehicle because some manufacturers allow much less runout than others (which is why you should always look up the exact specifications).
On a 2003 Lexus ES300, for example, the maximum allowable runout at the rotors is 0.002 inches (0.05 mm). On a 2003 Toyota Tundra truck, the maximum allowable limit is 0.0028 inches (0.07 mm). On a 2003 Honda Civic, the limit is 0.004 inches (0.10 mm). And on a 2003 BMW 325i, there can be as much as 0.0078 inches (0.20 mm) of installed rotor runout.
Regardless of what the factory runout spec calls for, always aim for the least amount of runout as possible. If you can keep runout to 0.002 inches (0.05 mm) or less on most cars, there shouldn’t be any noticeable problems.
MINIMIZING ROTOR RUNOUT
If you find that a rotor has too much runout, the problem could be in the hub, the rotor or a stack up of tolerances between the two. The factory assembly tolerances for some hubs allows up to 0.0015 inches (0.04 mm) of runout. If you have some runout in the hub, as well as in the rotor, or any rust or dirt trapped between the rotor and hub, the tolerances can stack up and create too much total runout when runout is measured at the rotor.
Runout can sometimes be minimized by simply reindexing the position of the rotor on the hub. Pull the rotor off the hub, thoroughly clean the inside of the rotor hat and the face of the hub with an abrasive brush so you get a flat, clean mounting surface. Then remount the rotor on the hub with the position of the rotor moved over one or two bolt holes from its original position. Remeasure runout with your dial indicator and see if you made runout better or worse. Repeat as needed to get the least possible runout. If runout still exceeds the factory specifications, you may have to recut the rotor with an on-car lathe (or replace the hub, rotor or both).
RESURFACING AND REPLACING ROTORS
Resurfacing the rotors can correct uneven wear, and on-car resurfacing will usually eliminate runout issues whether the runout is in the rotor or the hub. But if a rotor has hard spots (discolored, blue areas on the rotor face), replacing it is the smartest thing to do. Hard spots are the result of metallurgical changes in the rotor itself that extend beneath the surface. Resurfacing just shaves off the top, so the hard spots almost always come back within a few thousand miles. Why risk a comeback? Just replace the rotors.
Cracks are another reason to replace rotors. Some minor surface cracking is normal as a result of heat and wear. But deep cracks or ones that are more than one-inch long are dangerous because they could cause the rotor to fail.
Unless rotors are in very good condition (minimal wear and grooving only), they should usually be resurfaced when new pads are installed. This will provide the proper surface finish for pad break-in and braking. Some vehicle manufacturers recommend resurfacing the rotors any time the rotors are badly scored and the depth of the grooves exceeds 0.060 inches (1.5 mm), if there is excessive corrosion on the rotor faces, if rotor runout exceeds maximum specifications or if there is more than 0.001 inch (0.025 mm) in thickness variation.
If you’re resurfacing your own rotors with a bench lathe, make sure your lathe is in good shape, the arbor is running true, the lathe bits are sharp, and the rotor adapters are clean and undamaged so the rotor can be mounted squarely on your lathe with minimal runout. Use the proper rpm and feed rate, and don’t rush the job. If the rotor looks like an old phonograph record after you’ve turned it, the surface is too rough and will likely cause noise. You want a nice, smooth, flat finish when you’re done.
If you want to apply a nondirectional finish, you can buff both sides of the rotor with a flexible abrasive rotor brush in a drill, or you can block-sand both sides of the rotor while it’s turning on the lathe with #120 or #150 grit aluminum oxide sandpaper. Don’t forget to wash off the rotors with warm soapy water afterward to remove all the debris from the surface (aerosol brake cleaner won’t do a good enough job).
If you’re replacing the old rotors with new ones, they should be ready to install right out of the box. There should be no need to resurface brand new rotors because it just removes metal unnecessarily, wastes your time, shortens the life of the rotor, and may increase the risk of a comeback if you end up cutting runout into a new rotor that wasn’t there in the first place.
When replacing rotors, keep in mind any possible upgrade opportunities for customers who may want increased braking performance. The stock rotors can be replaced with aftermarket performance rotors.
Slotted rotors are often used on race cars as well as street cars to deliver maximum braking performance. Slotted rotors typically have four to six shallow grooves machined into both faces of the rotor. The slots may be straight or curved, and usually radiate out at an angle from the center. The slots wipe the surface of the pads when the brakes are applied to expel brake dust and gas away from the brakes. Slots also improve wet-weather braking by preventing road splash from getting between the pads and rotors.
Some performance rotors have drilled or cast holes to provide additional cooling. The holes increase airflow and help vent gases from hot pads during hard braking. On some rotors, the holes do not extend all the way through the rotor face but are shallow pits in the surface. This is done to reduce the risk of cracking which can occur if drilled holes are not chamfered properly to reduce stress.
Some performance rotors are also cryogenically treated by chilling them down to -300° below zero F in special nitrogen refrigeration equipment. The freezing process is done gradually, and can take 36 to 72 hours to complete. At such extremely cold temperatures, changes occur in the crystal structure of the metal that relieve stress and improve fatigue resistance, tensile strength, abrasion resistance and hardness, allowing the rotors to run cooler and last longer than untreated rotors.
WHY CALIPERS NEED TO BE REBUILT OR REPLACED
Most calipers do not need to be rebuilt or replaced the first time the brakes are relined. But after 75,000 miles, or seven to 10 years of service, the calipers may be reaching the end of the road. As the rubber seals age and harden, the risk of sticking and leaking goes up.
Few shops rebuild their own calipers these days because it takes too long and increases the risk of a comeback if the caliper sticks or leaks. It’s easier, faster and safer to simply replace the old calipers with new or remanufactured calipers. Loaded calipers are even better because they give you everything you need in one box, and there’s much less chance of mismatched parts.
Caliper replacement is required if a caliper is leaking brake fluid, if a piston is sticking, or the caliper is worn or damaged. Leaks are very dangerous and should never be ignored because loss of fluid can cause brake failure.
Leaks eventually occur as the caliper ages and the piston seals become hard and lose their elasticity. Corrosion inside the piston bores or on the pistons themselves can also accelerate seal wear. When you push the pistons back into the caliper to install new pads, it forces the piston seal to scrape across areas of the bore that may be rough with corrosion. If the caliper was not leaking before, it may soon start.
The condition of the caliper slides, pins and bushings on floating calipers should also be carefully inspected for wear, rust, cracks or other damage. If you’re replacing a caliper, use new slides, bushings and mounting hardware, and be sure to grease these parts with a high-temperature, moisture-resistant brake lubricant.
If the slides or bushings on a floating caliper are rusty, worn or damaged, the caliper may not move freely causing the pads to wear unevenly. A “sticky” or “frozen” caliper may also cause the brakes to drag if the caliper does not slide so the outer pad can kick away from the rotor. This may cause increased wear on the outer pad as well as a steering pull to one side because of the constant drag.
Fixed calipers don’t suffer from this type of problem because the position of the caliper is fixed and the caliper does not move. But both fixed and floating calipers alike can sometimes “freeze up” or lock if a piston becomes jammed or stuck in its bore due to corrosion or swelling.
When a piston sticks, one of two things can happen: the brake may not apply or it may not release. The uneven braking that results may produce a hard pull toward the “good” side when the brakes are applied, and/or a steering pull toward the “bad” side when the brakes are released.
If you find uneven pad wear between the inner and outer disc brake pads, it’s a sure indication the caliper (or caliper piston) is sticking.
Calipers should usually be replaced in pairs, especially on high-mileage vehicles. But if only one caliper needs to be replaced because of a leak or problem, the replacement caliper should have the same type of piston (steel or phenolic) as the one on the opposite side, as well as the same type of friction material (nonasbestos organic or semi-metallic). Mismatches side-to-side may cause uneven braking.
Other items that may be needed when replacing calipers include new brake hoses (replace if leaking, cracked, brittle or worn), and certainly brake fluid. Always flush the brakes to get rid of the old fluid, and bleed all of the lines to get rid of any trapped air that could cause a soft pedal.