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What's the story with turning the brake rotors?

I found this answer in the Z Car & Classic Datsun (September/October 1998) in the Mr. Goodtech Section:

Q. My late model 300ZX (90+) may have a rotor warpage problem, but I can't seem to get it fixed. I've got a shimmying in the wheel that my dealer told me was due to warped rotors. We tried turning them, but the problem came right back. They then told me I needed new rotors, that mine were too thin and that is why they warped again. However, even after they were replaced, the problems came right back. So far I've had my rotors turned, had them replaced, had new brake pads installed, had the alignment checked several times, replaced several suspension components, and still the problem continues. What's the solution?

Washington, USA

A. Well, its funny you should ask, because this is a common topic on these cars. This same topic was discussed on the Internet Z car list recently, and someone on the list had the best reply I've ever seen. Back in the "old" days when I worked on British cars, we used to balance wire wheels and tires on the car because that was the only way to really get them to balance effectively. Same sort of things applies to the Z32s brakes it seems. Anyway, Steve Mitchel had a great answer for this problem. He knew about this because he used to work for Nissan Motor Corp/ and has worked very closely with the engineer who worked on this problem. Here is the solution:

There are many reasons for warped rotors on cars. The specific problem on the Z32 is due to the fact that the Z32 uses a fixed caliper and there is excessive runout in the hub and/or the rotor. This leads to thickness variations in the rotor, thick and thin spots. This happens because the caliper is fixed, but the rotor has a wobble to its rotation due to the runout of the rotor or the hub. The brake pads begin to wear down the high spots, and this is when the thickness variation starts.

In the factory service bulletin on this issue, it clearly states that the technician is supposed to measure the runout of the rotor and the hub independently. If the hub has more than about .003" of runout, then the hub need to be replaced. Clean the mounting face of the hub and the rotor with emery cloth, then brake cleaner to remove all contaminants. Measure the rotor to see if it is at minimum thickness, and replace if needed.

Next try the rotor on each of the five lug holes to find the position with the lowest amount of runout. Then machine the rotors ON THE CAR ONLY. Do not turn the rotors off the car, ever! Then measure the runout once more - it should be zero. The dealers all have on-car brake lathes made by Ammco; they were required to buy them by Nissan. Most top-notch aftermarket shops have on-car equipment, too.

Other factors that can aggravate the brake shimmy are bent wheels, out-of-balance wheels and uneven torqueing of the wheels. The main culprit, though, is the runout of the hub and/or rotor, causing the thickness variation. If the shimmy still occurs after doing all the above, but you get it only after driving for 20-30 miles on the highway, then replace the front hub/bearing assemblies as they are going out of specs when hot.

I have yet to come across a Z32 I could not fix using the above methods. I fixed quite a few at the store I worked at and only a few comebacks that needed to have the hubs replaced. Many new dealer techs do not know about the on-car process and will cut the rotors off the car. This will put the problem off for anywhrer from 500 to 5,000 miles, depending on how much runout and how you drive. Demand the on-car lathe, or suffer the consequences! A tip of the hat to Steve Mitchell for this solution to a nagging problem for some Z32 owners.

Arturo C.

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