GM Prizm/Nova 1985-1993 Repair Guide

Brake Rotor

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REMOVAL & INSTALLATION



  1. Elevate and safely support the car. If only the front end is supported, set the parking brake and block the rear wheels.
  2.  
  3. Remove the wheel.
  4.  
  5. Remove the brake caliper from its mount and suspend it out of the way. Don't disconnect the hose and don't let the caliper hang by the hose. Remove the brake pads with all the clips, shims, etc.
  6.  
  7. Install all the lug nuts to hold the rotor in place. If the nuts are open at both ends, it is helpful to install them backwards (tapered end out) to secure the disc. Tighten the nuts a bit tighter than finger tight, but make sure all are at approximately the same tightness.
  8.  
  9. Perform the run-out and thickness measurements explained in Inspection. Run-out must be measured with the rotor mounted on the car. Thickness measurements can be made either on or off the car.
  10.  
  11. Remove the 2 bolts holding the caliper mounting bracket to the steering knuckle. These bolts will be tight. Remove the 4 lug nuts holding the rotor.
  12.  
  13. Remove the bracket from the knuckle. Before removing the rotor, make a mark on the rotor indexing 1 wheel stud to 1 hole in the rotor. This assures the rotor will be re-installed in its original position, serving to eliminate minor vibrations in the brake system.
  14.  
  15. When reinstalling, make certain the rotor is clean and free of any particles of rust or metal from resurfacing. Observe the index mark made earlier and fit the rotor over the wheel lugs. Install 2 lug nuts to hold it in place.
  16.  
  17. Install the caliper mounting bracket in position and tighten its bolts to 65 ft. lbs. (88 Nm).
  18.  
  19. Install the brake pads and the hardware.
  20.  
  21. Install the caliper. Tighten the mounting bolts to 18 ft. lbs. (25 Nm) for 1985-92 and 25 ft. lbs. (34 Nm) for 1993.
  22.  
  23. Install the wheel and lower the car to the ground.
  24.  

INSPECTION



Run-out

See Figure 1

Before measuring the run-out on the front discs, confirm that the front wheel bearing play is within specification.

  1. Elevate and safely support the car. If only the front end is supported, set the parking brake and block the rear wheels.
  2.  
  3. Remove the wheel.
  4.  
  5. Remove the brake caliper from its mount and suspend it out of the way. Don't disconnect the hose and don't let the caliper hang by the hose. Remove the brake pads with all the clips, shims, etc.
  6.  
  7. Install all the lug nuts to hold the rotor in place. If the nuts are open at both ends, it is helpful to install them backwards (tapered end out) to secure the disc. Tighten the nuts a bit tighter than finger tight, but make sure all are at approximately the same tightness.
  8.  
  9. Mount a dial indicator with a magnetic or universal base on the strut so that the tip of the indicator contacts the rotor about 1 / 2 in. from the outer edge.
  10.  



Click image to see an enlarged view

Fig. Fig. 1: Measuring the rotor run-out

  1. Zero the dial indicator. Turn the rotor 1 complete revolution and observe the total indicated run-out.
  2.  
  3. If the run-out exceeds 0.15mm (Nova) or 0.09mm (Prizm), clean the wheel hub and rotor mating surfaces and remeasure. If the run-out still exceeds maximum, remove the rotor and remount it so that the wheel studs now run through different holes. If this re-indexing does not provide correct run-out measurements, the rotor should be considered warped beyond use and either resurfaced or replaced.
  4.  

Thickness

See Figure 2

The thickness of the rotor partially determines its ability to withstand heat and provide adequate stopping force. Every rotor has a minimum thickness established by the manufacturer. This minimum measurement must not be exceeded. A rotor which is too thin may crack under braking; if this occurs the wheel can lock instantly, resulting in sudden loss of control.



Click image to see an enlarged view

Fig. Fig. 2: Measuring the rotor thickness

If any part of the rotor measures below minimum thickness, the disc must be replaced. Additionally, a rotor which needs to be resurfaced may not allow sufficient cutting before reaching minimum. Since the allowable wear from new to minimum is about 1mm, it is wise to replace the rotor rather than resurface it.

Thickness and thickness variation can be measured with a micrometer capable of reading to one ten-thousandth inch. All measurements must be made at the same distance in from the edge of the rotor. Measure at 4 equally spaced points around the disc and record the measurements. Compare each measurement to the minimum thickness specifications in the chart at the end of this Section.

Compare the 4 measurements to each other and find the difference between each pair. A rotor varying by more than 0.013mm can cause pedal vibration and/or front end vibration during stops. A rotor which does not meet these specifications should be resurfaced or replaced.

Condition

A new rotor will have a smooth even surface which rapidly changes during use. It is not uncommon for a rotor to develop very fine concentric scoring (like the grooves on a record) due to dust and grit being trapped by the brake pad.

This slight irregularity is normal, but as the grooves deepen, wear and noise increase and stopping may be affected. As a general rule, any groove deep enough to snag a fingernail during inspection is cause for action or replacement.

Any sign of blue spots, discoloration, heavy rusting or outright gouges require replacement of the rotor. If you are checking the disc on the car (such as during pad replacement or tire rotation) remember to turn the disc and check both the inner and outer faces completely. If anything looks questionable or requires consideration, choose the safer option and replace the rotor. The front brakes are a critical system and must be maintained at 100% reliability.

Any time a rotor is replaced, the pads should also be replaced so that the surfaces mate properly. Since brake pads should be replaced in axle sets (both front or rear wheels), consider replacing both rotors instead of just one. The restored feel and accurate stopping make the extra investment worthwhile.

 
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