Part 2 -- Refinishing and Lateral Run-Out (LRO) Correction
There are five steps which must be performed to complete a successful brake service.
1. Measure and document rotor thickness and thickness variation (fig. 1).Â
2. Clean hub, rotor and wheel mating surfaces of all rust and debris to shiny surfaces.
3. Perform and document proper rotor refinish using either on-car or off-car lathe following the manufacturer's directions.
4. Measure, document and correct existing lateral run-out (LRO) with Brake-Align correction plates.
5. Document final rotor thickness, reassemble wheel to hub with proper torque, and burnish the refinished rotors to the pads.
The first two steps were covered in Brake Service, Part 1 (TechLink, December 2004). The present article explains the remaining steps: rotor finishing and measuring, correcting LRO, final assembly and documentation.
TIP:Â Refer to bulletin 00-05-22-002F for GM's standard brake service procedures. Another excellent information resource is Service Know-How Course 5040.01B -- Brake Pulsation Due to Lateral Runout.
IMPORTANT:Â These procedures and discussion are highlights of proper brake rotor service and are presented for general information. They are not comprehensive or specific to a certain model/year vehicle and are not intended as such. Always refer to procedures in SI, applicable factory service manuals and applicable Service Bulletins for complete procedures and specifications for the year and model vehicle you are working on.
ROTOR FINISHING
Critical characteristics of a brake rotor include:
- Surface finish
- Parallelism
- Perpendicularity to hub centerline
- Flatness
- LRO
Of these, only LRO and flatness can be measured; the brake lathe must provide the other characteristics.
On-Car vs. Off-Car Lathe
The purpose of machining a rotor is to produce two parallel surfaces with minimum LRO. Both types of lathe will do this, and both methods have considerations.
On-Car (fig. 2)Â
- Portable
- Compensates for LRO
- Cuts captured rotors (TechLink Jan. 2004)
- Non-directional finish
- Requires more training
Off-Car (fig. 3)Â
- Easier to set up
- Stationary location
- Substantial disassembly (or impossible to use) on captured rotor vehicles
- Does not compensate for LRO
TIP:Â Before machining a rotor on either kind of equipment, the hub and wheel sides of the rotor mounting flange must be cleaned to shiny metal on both sides.
Lathe Use and Inspection
A later article will present much greater detail about the critical care of both types of lathes. But before turning a rotor on any type of lathe, take the following into account.
Lathe Safety
- Lathe's electrical cord, motor, switches, light(s) and belt(s) (as applicable) in good working condition
- Lathe clean and free of debris, rust and chips
- Safety shields, vibration dampers, other attachments clean and in good working order
- Cutting tools sharp and in good condition
- Operator wearing proper personal protection equipment
Correct any shortcomings before proceeding.
Lathe Inspection
Carefully inspect the bell clamps or adapters you are going to use. Their machined surfaces should be clean, free of rust and grease and not have any nicks or damage present. Any dirt or rust should be cleaned off and any nicks or damage should be removed with a stone. If the damage or rust is significant, the bell clamp or adapter should be verified and/or corrected before use. See TSB 00-05-22-002F or the Brake Pulsation Due to Lateral Runout Service Know-How video and booklet for details.
Inspect the lathe arbor (Off-Car) and lathe arbor shoulder (both On and Off-car) for any nicks, dirt or damage. Clean or repair as necessary as indicated above.
Verify that the adjustment and locking features of the lathe work smoothly and correctly. Verify the cutting head (Off-Car) moves freely in its ways without excessive slop. If necessary, disassemble and remove any chips, dirt or debris which prevent this and perform adjustments as indicated by the lathe manufacturer.
Vehicle/Hoist Inspection (On-Car Only)
Insure that the hoist supporting the vehicle is in good condition and supports the vehicle solidly
Make sure the vehicle's wheel bearings, drive axle and suspension components (as appropriate) are in good condition without excessive play, damage or wear. Repair/replace components as necessary BEFORE attempting to turn a rotor on an On-Car lathe.
TIP:Â Failure to support a vehicle on a sturdy hoist may result in the On-Car lathe not being able to self compensate itself. If this occurs, move the vehicle to a suitable hoist or secure it fore/aft with jackstands as appropriate.
Rotor Cutting
Once the lathe equipment is verified to be in good operating order, cut the rotor to achieve a good finish according to the lathe manufacturer's instructions.
Always do the math ahead of time, BEFORE beginning to cut. Properly measure the rotor with a brake micrometer to determine what your bottom depth is and how much rotor surface to remove. Remove only as much material as needed to obtain a smooth, even rotor surface. Make the cut in one pass, if it is within the lathe's capability.
TIP:Â Equal amounts of material do not have to be removed from both sides of the rotor on a floating caliper system.
TIP:Â On a rotor surface with rust buildup, insure that the cut is deep enough to cut metal. Dragging the tool across rust will dramatically shorten its useful life.
Rotors generally can be turned and should NOT automatically be replaced for:
- Low mileage rust conditions (lot rot)
- Brake pulsation due to rust buildup or thickness variation induced from excessive LRO
- Minor rust accumulation which is flaking off
Refer to Brake Service, Part 1 (TechLink, Dec. 2004) as well as Bulletin 00-05-22-002F for more information.
Final Rotor Finishing and Cleaning
The rotor surface must have a non-directional finish. Follow the lathe manufacturer's recommendation for applying a non-directional finish using moderate pressure. If the lathe is equipped with a non-directional finishing tool, use 120 grit aluminum oxide sandpaper. If the lathe is not equipped with a non-directional finishing tool, use a sanding block and 150 grit aluminum oxide sandpaper.
Rotor Measurement
Before removing the rotor from the lathe (or on-car lathe from the rotor), be sure the rotor thickness is at or above the Minimum Thickness After Machining (sometimes called Minimum Machine) thickness specification in SI. (fig. 4)Â
Discard a rotor that is not at or above minimum machine thickness. It cannot safely be placed back on a vehicle.
Remove all machining debris from the rotor surfaces to insure quiet brake operation. Failure to remove all minute metallic machining residue will result in significant brake squeal with some brake pad compounds.
The best method is thorough scrubbing with hot, soapy water. If you use an environmentally friendly hot tank washer, use brake cleaning spray to remove residual oil from the rotor surfaces.
MEASURING AND CORRECTING LATERAL RUN-OUT
TIP:Â Before installing the rotor to the hub, be sure there is no debris on the rotor or hub mating surfaces.
TIP:Â When installing a rotor on a hub, hold the rotor at the 5 o'clock and 8 o'clock positions to avoid dropping any residue from inside the rotor cooling vanes into the hub/rotor interface area.
Hold the rotor flat to the hub. Install J 45101-100 conical washers (A) and lug nuts (B) and finger tighten (fig. 5). Torque the lug nuts to SI specifications using a star pattern.Â
A Conical washer
B Lug nut
TIP:Â Conical washers simulate the clamping load of the wheel on the rotor/hub interface to make an accurate LRO measurement.
TIP:Â Print the brake specifications from SI as handy reference during the repair. Attach it to the repair order as a permanent part of the vehicle history file to substantiate the specifications you were working with.
LATERAL RUNOUT (LRO) CORRECTION
IMPORTANT:Â Over time, excessive LRO causes thickness variation which causes brake pulsation. See the December 2004 TechLink as well as 00-05-22-002F.
This procedure involves placing a machined, tapered correction plate between the rotor and hub.
Install a dial indicator J 45101 (or equivalent) to the steering knuckle and position the indicator button so it contacts the brake rotor friction surface at a 90° angle, approximately 0.5 inch (13 mm) from the outer edge of the rotor (fig. 6).
With the dial indicator installed, rotate the rotor until the lowest reading is displayed on the indicator dial and set the dial to zero. Rotate the rotor until the highest reading is displayed on the dial. Mark the location of the high spot relative to the nearest wheel stud, or studs. Mark the studs and rotor (fig. 7) to allow you to reinstall the rotor to the hub in the same index location. Record the amount of measured LRO on the repair order for the corner of the vehicle you are working on.Â
Refer to LRO measurement procedures in SI. On most passenger cars, if LRO is greater than 0.002 inch (0.055 mm), correction is required. P-90/N-car specification is 0.0015 inch (0.038 mm). Always consult SI for the specifications for the model/year vehicle your are working on.
Select and document the appropriate correction plate. Use the Brake-Align application chart supplied with the plates. Brake-Align plates are numbered XXX-XX, where the first three numbers represent the type of hub they fit and the last two numbers represent the correction value in thousandths of an inch (03 gives 0.003-inch correction, 06 gives 0.006-inch correction and 09 gives 0.009-inch correction). Brake-Align plates come in only these three thicknesses. Select the appropriate correction plate as indicated below.
You are trying to get the total LRO as close to zero as possible. Because you are compensating a rotating device, it doesn't matter if the values go positive or negative.
EXAMPLE:Â If the runout is 0.005-inch, the 0.006-inch plate would give you -0.001-inch LRO after installation. The 0.003-inch plate would bring you to +0.002-inch (which might be within spec) but the 0.006-inch plate minimizes the total LRO and gives a better repair.
With the rotor removed, install the plate (A) on the hub with the V-notch (B) at the high point previously marked (C)Â (fig. 8).Â
A Plate
B V-notch
C High point mark
TIP:Â Brake-Align plates have a double wheel lug pattern of mounting holes to allow positioning the V-notch at a lug location or in between lugs, depending on the location of the high spot mark.
Install the rotor, observing the index marks you made earlier.
Hold the rotor flat to the hub, add a conical washer and finger-tighten the first lug nut. Repeat with the other lug nuts. Tighten to the specified torque, in a star pattern.
Confirm that LRO is within specification using the measurement procedure above.
Measure the final rotor thickness for that rotor in that position on that vehicle and record on the repair order.
Install the caliper and pads. Pad inspection and replacement guidelines were covered in part 1 and bulletin 00-05-22-002F.
Remove the lug nuts and conical washers; re-install the wheels, using the recommended lug torque and procedure.
Depress the brake pedal several times to take up changes in component clearance due to rotor refinishing and to secure the rotor in place.
Verify and adjust the brake fluid level in the master cylinder reservoir.
Brake-Align Tips
Brake-Align plates save you time. Using them is an approved repair procedure and they should be used to correct brake rotor LRO when it exceeds the specification. Brake-Align plates, properly used, give you repeatable, exact LRO correction results quickly.
TIP:Â The LRO Indexing Correction procedure in SI is valid to correct minor LRO variation. But you may need to bolt and un-bolt the rotor from the hub 4 or 5 times to get the lowest LRO measurement, which still may or may not be within the vehicle's maximum LRO specification.
The most commonly used plates are the 0.003-inch and 0.006-inch. If rotor LRO measurements indicate the need for a 0.009-inch plate, determine EXACTLY where that LRO is coming from. First, remove the marked/indexed rotor from the hub and check the hub lateral surface for LRO with a dial indicator in the same method as above for checking the rotor. Place the indicator tip just outboard of the lug circle. Generally, most hub outer edges are approximately half the distance from the centerline of the axle to the area you were checking on the rotor. Using some geometry, it would be logical to expect this value to be approximately half of what you were seeing at the rotor, if 100% of the runout is in the hub.
EXAMPLE:Â You measured 0.011-inch LRO on the rotor. You disassembled the rotor from the hub and measured the outer edge of the hub and found 0.001-inch LRO. This would indicate that approximately 0.002-inch of your 0.011-inch total LRO is due to hub LRO.The rest is LRO in the rotor itself. If you have just cut that rotor, you need to take a very close look at your cleaning process and your lathe equipment.
TIP:Â Never re-use a Brake-Align correction plate. Never stack more than one Brake-Align correction plate on an individual hub.
FINAL DOCUMENTATION
At this point you should have performed the following measurements and recorded them on the repair order (or diagnostic worksheet as appropriate):
- Initial rotor thickness
- Rotor thickness variation measured in at least four equal places around the circumference of the rotor (if customer concern was brake pulsation)
- Final rotor thickness after machining (if rotor was turned)
- Assembled rotor lateral runout measured and recorded in thousandths of an inch so it can be coded for warranty purposes
- Brake-Align correction plate used (XXX-XX format)
- Final rotor thickness installed on the vehicle in its final position (left/right/front/rear)
BURNISHING PADS AND ROTORS
CAUTION:Â Road test a vehicle under safe conditions and while obeying all traffic laws. Do not attempt any maneuvers that could jeopardize vehicle control. Failure to adhere to these precautions could lead to serious personal injury and vehicle damage.
Burnishing the brake pads and brake rotors is necessary to ensure that the braking surfaces are properly prepared after disc brake system service.
Perform this procedure whenever the disc brake rotors have been refinished or replaced, and/or whenever the disc brake pads have been replaced.
Select a smooth road with little or no traffic. Accelerate the vehicle to 48 km/h (30 mph).
Use care to avoid overheating the brakes while performing this step.
Using moderate to firm pressure, apply the brakes to bring the vehicle to a stop. Do not allow the brakes to lock. Repeat until approximately 20 stops have been completed. Allow sufficient cooling periods between stops to properly burnish the brake pads and rotors.
- Thanks to North Central Region Service Engineers and Field Warranty Specialists
