C2 & C3 Corvette Rotor Runout Measuring

When The Brakes Don’t: Secrets of Corvette rotor runout, a critical issue for 1965-1982 Corvettes

John Pfanstiehl May 13, 2016 0 Comment(s)
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Driving a Corvette is supposed to be exciting, but one thrill nobody wants to experience is when your brake pedal goes to the floor with a stop sign or a stopped car in front of you. That’s happened to many Corvette owners, especially after brake repairs due to a quirk in the design of 1965-’82 disc brakes.

The problem is caused by excessive rotor runout, which pumps air into the calipers that results in an unexpected loss of braking. Checking rotor runout is important for all disc brake cars but it is particularly needed for C2 and C3 Corvettes because their rotors, front hubs and rear spindles were produced differently than more modern designs.

The 1965 Corvette was GM’s first try at disc brakes for production cars and the engineers wanted these brakes to be great. Remember that Duntov wanted Corvettes to compete with Cobras and Jaguars at grueling endurance races such as Le Mans and Sebring … and win!

The result was impressive for a production car of that era: disc brakes with a large, vented rotor and four-piston, fixed caliper per wheel. (Even two decades later, most “disc-brake equipped” cars only had 2 pistons compared to Corvette’s 16.)

The large, vented rotors were riveted to the spindles in the rear and to the hubs in the front. There was no problem with this design (except for repairing the parking brakes) when the rotor/spindle assembly or rotor/hub assembly was replaced with another factory-riveted assembly. However, when only one part was replaced, a serious problem often occurred.

The rotor mounting surfaces on the rotors, hubs and spindles were not precisely machined. These surfaces were only rough cut because they were to be riveted together, mounted on the bearing surfaces and then precisely machined to produce a very low rotor runout.

Take the example of a factory spindle with a runout (wobble) of 0.005 inch on the rotor-mating surface. A rotor was riveted to it, and then machined to have a very low runout, let’s say a perfect 0.000 inch for this example. To obtain this low runout, the rotor has its brake pad surface out of parallel with the spindle-mating surface by an effective 0.005 inch to cancel the spindle’s runout. That was all fine and good until a rotor, spindle or hub was replaced, or even if the original rotor was reinstalled at a different alignment to the spindle or hub.

If the rotor was reinstalled with its alignment to the spindle almost 180 degrees off, the runout would be 0.010 inch or more because the 0.005 spindle runout is added to the 0.005 rotor runout. That’s the effect of tolerance stack-up in an assembly. In this case, the tolerances can either be added or subtracted depending on the alignment of the two parts. Also, if a different rotor that had perfectly parallel pad-to-spindle mating surfaces was installed on that spindle, the runout would be over 0.005 inch. Same thing if a perfectly machined spindle was installed to the original rotor. In either case, the runout would be dangerously high.

It’s slightly more complicated for two reasons. First, the rotor can only be aligned in five positions because it has only five lug nut stud holes. Secondly, the spindle or hub runout is usually measured just out from the lug nut circle at a 3-inch radius, while the rotor runout is typically measured on the pad surface at almost a 6-inch radius. It’s not uncommon to find an original spindle that has a runout of 0.005 inch or more. If a perfectly machined new rotor is used with that spindle, the assembly would have a runout of almost 0.010.

You don’t have to know the math, just be aware that C2 and C3 rotors should always be checked for runout. This is critical if rotors, spindles or hubs have been replaced. Changes in rotor runouts do not occur when replacing only brake pads or calipers.

How much runout is a problem? For front rotors, my goal was 0.001 or 0.002 inch because they did the majority of the braking. For the rear rotors, while less is better, I never had a problem with runouts as high as 0.003 inch. Those were the specs I used for calipers that had the original lip-type seals. VanSteel, which has specialized in Corvette suspension, steering and brakes for almost 40 years, now only sells calipers that use O-rings in place of the lip seals. Dan Dorsett said that their O-ring calipers can handle runouts as much as 0.005 inch.

By the time the C4 Corvettes came around, disc brakes had been around quite a while, and manufacturing rotors and spindles that had very low runouts was common practice for both OEM and aftermarket parts. Similarly, today’s replacement rotors, spindles and hubs for 1965-’82 Corvettes generally have very low runouts. If newly manufactured rotors are used with newly manufactured spindles or hubs, the chances of excessively high rotor runouts and brake loss is lessened.

However, parts and parts suppliers inevitably have variances, and poorly machined parts sometimes slip by, so it is wise to check runout whenever replacing brake parts on any disc brake vehicle. It’s quick, easy and provides peace of mind.

2 Rotors Riveted Front Hubs 2/23

01. Rotors were riveted to the front hubs and to the rear spindles on 1965-’82 Corvettes and then machined to a very low runout, creating a matched set. If a rotor, spindle or hub is replaced, the new runout can be dangerously high.

3 Front Rotor Runout 3/23

02. Front rotor runout is easy to measure. Just remove the bearing dust cap and cotter pin to temporarily tighten the wheel bearing nut. If the rotor is no longer riveted, install and tighten all five lug nuts.

4 Rear Rotor Runout 4/23

03. Rear rotor runout is more accurately measured off the car. On the car, rear wheel bearing axial clearance makes measurements difficult because the axleshaft moves and tilts the spindle as it is rotated. Unbolting the axleshaft from the T-arm eliminates this effect. Alternately, jacking the T-arm up until the axleshaft is parallel reduces the effect.

5 Dial Indicator 5/23
6 Dial Indicator Tip 6/23

04-05. The dial indicator tip should be positioned near the outer area of the rotor’s pad surface. Also, the tip on this type of indicator should be perpendicular to the surface being measured. Lever-type indicators should have the lever parallel to the surface.

7 Factory Installed Lip Seals 7/23

06. The factory-installed lip seals are fine unless rotor runout is high. Corvette calipers are bolted solidly to a bracket and therefore the caliper pistons are forced in and out with each revolution of a high runout rotor. This action pumps air into the brake system, causing the loss of brakes.

8 Aftermarket Caliper Piston 8/23

07. An aftermarket caliper piston design for 1965-’82 Corvettes uses O-ring seals in place of the OEM lip seals. Dan Dorsett of VanSteel says this design is less sensitive to rotor runout, but cautions that runout should still be below 0.005 inch.

9 Aftermarket Caliper Piston 9/23

08. The runout measurement on rear spindles is typically taken just outside of the lug nut studs, which is just under a 3-inch radius. Spindle runout causes rotor runout that is nearly twice as high because the outer rotor pad surface is nearly twice as far out.

10 Used Rotors 10/23

09. Whenever installing used rotors, surface rust and any scale or deposits should be cleaned from the inside surface that mates with the rear spindle or front hub. It’s equally important to clean the surface of the spindle that mates with the rotor.

11 Rotor Alignment 11/23

10. When a low rotor runout is achieved, the rotor’s alignment should be marked so that it can be re-installed with the same alignment upon future work. Left and Right also needs to be marked, same with Front and Rear when appropriate.

12 Front Rotors Aligned 12/23
13 Front Rotors Aligned 13/23

11-12. Front rotors can be aligned in five different positions corresponding to the five lug nut holes. However, rear rotors have only one alignment position where the larger parking brake adjustment holes line up with the corresponding holes in the spindle.

14 Aftermarket Rotors 14/23

13. Replacement and performance aftermarket rotors and spindles are usually made to closer tolerances than the original C2 and C3 factory parts. But it is still wise to check the runout, even if all new parts are used.

15 Magnetic Base 15/23

14. A magnetic base quickly mounts the indicator off-car to a table or vise or on-car to the frame or suspension. A flex arm makes positioning the indicator for runout even easier. The indicator is moved into position and then the flex arm is rigidly locked in place by turning a lever.

16 Shims Measured 16/23

15. Shims can be used to reduce rotor runout. Cut the shims and place them over and around the lug nut studs. Brass shims are available in various thicknesses at many hardware or industrial supply stores.

17 Directional Rotors 17/23

16. Directional rotors have an additional alignment issue. They are marked with an arrow to show the direction of rotation, and therefore can only be used on either the left side or the right side of the car. Pictured is VanSteel’s coilover system on their offset T-arm.

18 Measure Runout Wheels 18/23

17. If you’re ambitious, you can measure runout on wheels and tires at the same time. This is unrelated to braking but may help track down a vibration.

19 Lever Indicator 19/23

18. A lever-type indicator or a roller tip and use of duct tape on the tire treads helps when measuring tire axial runouts.

20 Rivets Installed 20/23

19. When the factory installed the rivets, the rivet heads were often considerably off-center from the rivet body (shaft). The rivet body diameter is 5/16-inch (0.312-inch).

21 Washer Rivet Counter 21/23

20. Placing a washer into the rivet counter bore of the rotor makes it easier to accurately center punch the center of the rivet body.

22 Rivet Hammered 22/23

21. Drilling the rivets is much easier off the car using a drill press. Tip: Use a 1/4-inch drill bit stamped HSS (High Speed Steel). Hardware store drill bits are often not up to the task. Cutting fluid and a slow speed are recommended.

23 Drilling Rotor 23/23

22. After drilling about 7/16-inch into the rivet with the 1/4-inch drill bit, a larger bit is used to drill just enough to pop the rivet head off. The rest of the rivet needs to be hammered or pressed out, but this task is much easier after much of the rivet body is drilled out.






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