rotor question

I'm helping a friend this weekend with brake pads on her 1998 E328i sedan. I believe there are about 90,000 miles on the car. Some have recommended
hanging the rotors.
How do you tell if a rotor is bad and needs changing?
Which rotors tend to fail first, front or rear?
Thank you.
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Deep scratches such that rotor can't be resurfaced.

Jim
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You have to measure them with a micrometer or suitable calipers. The Minimum thickness specification is stamped on the rotors, and you must be thicker than the min spec after the machine work is complete.

Front, by a wide margin. With 90k on the car, there is a good chance the rear brakes need attention too, but the rotors ought not be worn too badly.
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Since they only last for two sets of pads, the chances of turning them and having enough left for the second set without going below minimum is remote. Just change them for new - they're not expensive.
--
*Wrinkled was not one of the things I wanted to be when I grew up

Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Jeff Strickland wrote:

Further, BMW does not recommend resurfacing rotors at all (ever). If the rotors are still thick enough you should just put on replacement pads and leave the uneven surface of the rotor alone. The pads will take on the shape of the rotors in very little time, you'll actually have slightly greater contact area with the pads(though probably insignificant) and you will save a lot of mileage on the old rotors. After machining the rotors tend to warp due to loss of mass (heat dissipation).
Machining old rotors flat is highly over-rated.
--
-Fred W

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Seems to be a US thing as I doubt you'd find anywhere that offered this service in the UK. Drums, on the other hand, often benefitted from a skim to get them round again.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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Here's a discussion on measuring brake rotor runout:
http://web.archive.org/web/20001215134700/www.ultimategarage.com/bbrunout.html
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Franz Fripplfrappl wrote:

There is a minimum thickness specification. It's probably stamped in the rotor. If it's worn beyond the spec or it's deeply grooved, it needs to be replaced. (BMW rotors generally cannot be resurfaced and stay within the minimum spec.)
JRE
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IMO it's never worth having the rotors skimmed, even if they would still be in spec, as the cost of grinding them is likely to be near the cost of replacing them with new. Mike.
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Mike G wrote:

<snip>
I rarely have rotors ground either.
Oddly enough, my wife's brake indicator came on today, so she stopped for the parts on the way home and I did them this afternoon after work.
The rotors, after one set of pads' worth of wear, were .010" from the machining limit (not counting the depressed parts of the grooves, which I couldn't measure with the micrometer I used, but which I estimate were close to .005-.010" deep or so per side). While they were not deeply grooved, and though they were still pretty smooth, I replaced them anyway.
Incidentally, neither the old nor new rotors, despite what the TIS says, were stamped with the minimum thickness.
Oh, the car? 2002 E46 325i.
JRE
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