Jadz928

I have a new-to-me set of used rotors, all around.

Both front and rear spec out beyond the wear limit.
Front limit: 30.6mm (reads 28.5mm)
Rear limit: 22.6mm (reads 22mm)

The rotor surfaces look really good, and smooth/flat.

Could I run these, or is this a no-no?
Keep in mind, I'm not asking if I should use these rotors.
Just asking if its possible, and if so, what are the caveats?

Thanks in advance for your help!

SeanR

Jim, mine fronts are a tad bit less than that and I don't feel bad about driving. I'd not recommend it to someone else, but for mine, I'm ok with it.

Hilton

The wear limits for S4 rotors are 30mm front and 22mm rear. The numbers you quote are for the minimum brake disk thickness after machining (see page 46-07 in WSM).

If it was me, for street use, I'd change the fronts as they do most of your braking, but the rears should be good for a little while yet.

Speedtoys

One set of pads, one set of rotors...always been my thing.

You can turn em (to limits)..but as a heat sink, they're compromised compared to new..but thats how I drive to, it matters.

Big, fat, new, unblemished (no slot/drilling nonsense) and you got good stuff.


Lose mass in the rotor or remove surface area (where friction happens) and its not as good as could be stuff.

hacker-pschorr

If the pads are rubbing the cooling vanes, time for new rotors

Speedtoys

Nono...that just means they run EXTRA cool that way.

dr bob

Wear limits for rotors are defined based on the predicted mass of metal left at the end of life on pads installed, and therefore the ability to absorb and disspate heat in a worst-case stopping situation. For our cars that might be something like a certain number of high-speed stops from top speed, without brake fluid temps exceeding 450ºF (boiling point of wet fluid). You might argue that you don't drive that fast therefore you don't need that margin. Same rationale used to justify running H-rated tires, since we never really drive Z-rated fast in the U.S. Never. Meanwhile, back at the rotors, how many/how hard/how fast/how scary are you willing to compromise on stops?

Wear limits are also defined based on some pad thickness dimensions. Thin rotors and worn pads mean the piston is sticking out of the caliper a lot more. The primary sealing of the piston in the bore is done by a pressure sealing ring at the back of the piston, and that should stay back in the bore. The front of the piston is guided in the bore by a wiper ring, which also serves the purpose of keeping dirt and crud from gettiong near that pressure seal. There's usually a dust boot of some kind at the pad end of the piston, and sometimes the mounting of the dust boot is also part of the wiper ring (or vice-versa) for manufacturing ease. Overextending the piston risks that wiper coming clear of the bore, and also risks damaging that dust boot. If the wiper ring comes clear of the ore, the piston may not stay square in the bore. If dirt gets behind the wiper, it's just a matter of time before the pressure seal is damaged and you have a leak. Last time I looked, a set of new pistons and seals was in the $30-$40 range each. For one wheel on my car, I need four kits for the four-piston S4 caliper. It's in the same price range as a rotor. Rotors are easier to change, too.

Good news is that mine are due this time around on pads.

James Bailey

You could probably run them with little problem IF BIG IF you do not let the new pads wear down below about 1/2 thickness. A shop cannot legally allow a car to run a rotor if turned/worn under the wear limit.

Jadz928

This is all good stuff. Thanks Hilton for the clarification. Dr Bob, wow, very elaborate and understood, thank you.
New rear pads are going on.
All four rotors were $50, so I'm not feelin' the love for these. Plus, they are crazed from track duty, so... bubye!
I'm with ya Speedtoy, the no-nonsense OE style is just fine by me. Oh Roger....?