Rotor resurfacing is a pretty routine job in most undercar service shops, yet there are differences of opinion about whether or not brand new rotors should be resurfaced before they're installed.
Virtually every rotor supplier I've talked with says brand new rotors are ready to install right out of the box. Resurfacing is not necessary and only reduces the thickness of the rotor which ultimately shortens the rotor's useful service life.
Yet many technicians continue to routinely resurface brand new rotors anyway. Those that do say they take a light cut to make sure the rotor is true before it goes on a customer's vehicle. This, they say, reduces comebacks and improves customer satisfaction.
The manufacturers are right when they say new rotors are properly finished at the factory and are ready to install. But not every manufacturer in the world makes their rotors to the same level of quality. So technicians who do a light clean up cut on new rotors may be justified in doing so depending on the quality of the rotors in question.
We've seen brand new rotors from some offshore suppliers that have excessive runout and parallelism problems right out of the box. Some also do not meet OEM surface finish specifications. Put a set of these on a customer's vehicle "as is" and you'll likely have a comeback and a dissatisfied customer.
Rather than trying to compensate for a manufacturer's lack of quality control by resurfacing such rotors, I'd suggest buying a better brand of rotor even if it costs a little more. Time is money in this business, so don't just look at the cost of the rotors as the only factor. If you have to turn brand new rotors to make them right, are they really worth the hassle compared to ones that are ready to install right out of the box?
You also need to consider wear characteristics and noise control. All rotors look pretty much alike, but subtle differences in metallurgy can make a measurable difference in wear and noise. A quality rotor will usually score better on both counts.
As for resurfacing used rotors, you should always aim to achieve the best finish that's possible. Rotor mounting surfaces must be clean, flat and smooth so the rotors can be mounted squarely and securely on the lathe arbor. Composite rotors must be properly supported with adapters or bell caps. Once a rotor is mounted, check runout to make sure you're cutting it straight. If you don't and end up cutting runout into the rotor, it can create a pedal shudder.
The ideal technique for accurately resurfacing a rotor on a bench lathe is to use a dial indicator to measure rotor runout on the vehicle before the rotor is removed. Note the location, amount and direction of the run-out. After mounting the rotor on the lathe, check runout again with a dial indicator. If it isn't within .004 in. (and at the same location and direction) of what it was on the vehicle, adjust or shim the rotor mounting until the runout is duplicated. Or, you can use an on-car lathe to cut the rotors in place to minimize runout on vehicles that are especially sensitive (such as Honda and certain GM applications).
Sharp tools are extremely important for a good surface finish. The smoother the finish the better. For many applications, anything less than 80 microinches is acceptable. But on applications that tend to be noisy, a surface finish in the 20 to 50 microinch range is recommended. Dull bits don't produce a clean cut, leaving a rough finish that can cause noise with semi-metallic linings. For best results, you might consider upgrading to premium quality titanium nitride coated carbide tool bits. They don't cost much more than standard carbide bits, do a better job of cutting through rust and typically last twice as long.
To achieve a good surface finish, use a crossfeed rate of about .003 in. to .005 in. per revolution if your lathe has a fixed spindle speed (100 to 150 rpm). The best finish can be achieved with a cut limited to .002 to .004 in. in depth at a feed rate of no more than .002 in. per revolution.
On lathes with adjustable spindle speeds, the spindle speed should be slowed down for larger rotors to maintain a high quality finish: for 10-inch and smaller rotors, use 170 rpm; for 11- to 16-inch rotors, use 100 rpm; for 17-inch or larger rotors, use 60 rpm.
Applying a non-directional swirl or crosshatch finish to the rotors by sanding with #150 grit aluminum oxide sandpaper, an abrasive pad or flexible hone shouldn't be necessary if the rotors were finished correctly, but can be done to help the rotors run quieter. Sanding typically improves the surface finish by 15 to 20 microinches. Rotors should also be washed after turning and sanding to remove debris that can become embedded in the brake linings and cause noise.
Finally, consider using one of the new aerosol "treatments" that are sprayed on the rotor to suppress noise and aid lining break-in.
Brake roughness may occur on 1989-1992 Ford Thunderbird models after the lug nuts are installed and tightened with an impact wrench. The resulting torque difference from stud to stud can be as high as 60 lb.ft. (81 Nm). This torque difference stud to stud may cause an increase in rotor runout by as much as .003 in.-.005 in. (.076mm-.127mm). The increase in rotor runout may speed up the development of brake roughness.
To eliminate the brake roughness, tighten the wheel lug nuts in
the "final" torque in the sequence described in the
appropriate service manual. Use a torque wrench set to 95 +/-
10 lb.ft. (129 +/- 14 Nm). When performing brake service work
that involves replacing or removing the rotor, refer to the following
service procedure which minimizes hub/rotor "stacked"
runout, a cause of brake roughness and uneven rotor wear.
HUB/ROTOR STACKED RUNOUT CHECK
If the assembled system (new/turned rotor installed on the hub)
indicates more than .002 in. (.05mm) total runout (TIR), it will
cause excessive thickness variation in the rotor as mileage accumulates
by wearing the high spots off the rotor. Excessive thickness variation
in the rotor causes brake pulsation and steering wheel shake.
HIGH SYSTEM RUNOUT
High system runout is caused by:
ROTOR REPLACEMENT SERVICE TIPS
1. Completely clean the hub area that will contact the new/turned rotor. Remove rust build up.
2. Install the new/turned rotor on the hub and secure it with at least three (3) lug nuts. Tighten the lug nuts to 20 lb.ft. (27 Nm) for testing.
3. Using a dial indicator, determine the total runout of the system. The total system (rotor and hub) runout must not exceed .002 in. (.05mm).
4. If the runout exceeds .002 in. (.05mm), proceed as follows:
a. Remove the rotor and rotate it clockwise until the next hole lines up with the stud.
b. Secure the rotor in the new position, reindexing as described in step two.
c. Repeat the checking and rotation (reindexing process) until the system measures .002 in. (.05mm) or less total runout.
5. If .002 in. (0.5mm) or less runout cannot be reached, the hub should be replaced and steps two through four repeated when the new hub/rotor is installed.
Note: Ford recommends that new brake rotors not be machined. New OEM replacement rotors are well within the runout and thickness variation specifications.
Ford Thunderbird TSB courtesy of AllData Corporation.