Sci-fi_Wasabi

I am changing my rear brake pads on my 09 A8L 4.2:
The rotors mic at 20.6 and 20.5 mm on either size. They look pretty smooth no gouges or unusual rust, just regular circular striations. I'm pretty sure they have never been changed, and the car is at 86,000 miles now. I think this is the 1st time the pads have been changed too, as the dealership where the cars previous owner used to take it only has a history of front pad changes
So they are within the 20 mm minimum thickness but close, so I'm not sure if that means change them or not, or is there a lot of leeway in letting it go down a little less than min thickness.

Another thing I note is that both inner pads are almost gone, but the outer pads are at 4mm-5 mm or so, is that pretty common? I noticed ltooz_a6_a8_q7 (aka louis) has a DIY rear pad change vid that shows the same.

1 more thing, on these 310mm rear rotors, can they be removed without taking out the caliper mounting bracket, like on the W12's?

If anyone has any insight on those 3 questions it would be appreciated, particularly if they would change smooth rotors at 20.5 mm thickness.

Thanks

jamm220

I would change them the rears are cheap, did mine 2 years ago using the centric brand which can be found on rockauto, auto parts warehouse etc. I'm sure the bracket comes off like I said it's been two years don't remember doing anything different from a normal rotor removal just needed vag to do the rear brakes

Mister Bally

If you have money to burn, replace them. If you were having the job done at an honest shop, they would say OK to reuse.

I would not replace them at this time. Save the money for the next necessary part. I just spent about $300.00 for my lower front sway bar. All because of the rubber bushings are bad.

As for the uneven wear of the pads, look for a seized caliper pin. Remove, wire brush (on a bench grinder if possible) and lube them so they freely move.

1Wasserwerks

+1 on not replacing them at this time. I just changed rear pads on my S8, 71,200+mi. Passenger side inner was worn more than the outer pad. Left side had even wear, pins were smooth and grease was ok, no boots torn, I would attribute the uneven wear possibly to dirty stainless sliders. Probably had 10K more miles left on the pads, but I'd rather change them now than in February, in the snow.
Anyhow, if you have concerns about the wavy grooves on the rotors, any decent garage or machine shop could resurface them for you if you can deal with some down time. If not I would just install the new pads and at next interval replace both pads and rotors.
If you are using VCDS to open the calipers, do hook up a battery charger during your replacement procedure.

Sci-fi_Wasabi

Thanks for the various suggestion. I suppose, if i only lost 1.5 mm of thickness in 86,000 miles, then, I'll lose 3 mm total by 170,000 miles, which is just 1 more mm than the minimum loss of 2 mm. I just wonder if there is any noticeable degradation of braking function as your rotor wears down a few mm's. Thanks

mishar

OEM rear rotors are under $100 a piece. Thinner rotors have lower mass and heat up faster for the same braking power. So it is more likely to have brake fading. No real effect in daily driving unless...

MP4.2+6.0

Coin toss on replace rotors or not. On back I would probably run them one more pad cycle. FWIW, even though it doesn't show in service records you have, my instinct is the pads were probably actually done once already. It's not consistent to have that much rotor wear to me with just one set of pads, especially with them wearing somewhat unevenly. With two sets of pads, easier to believe. If they were front rotors I would more likely just change them out, but rears don't take as much load or tend to be the ones that fade easily. Brakes are really sized literally for Autobahnen too, and that's just not North America real world.

Having found similar wear on my recent W12 rear pad and rotor change--much more worn on inside and a bit off angle--I would carefully clean the caliper sliding pins and regrease, and replace the stainless clips on which the brake pads slide. My rotors had gone .2mm under spec BTW so and easy call. Braking was always very strong BTW even w/ the W12 weight and power as I went under the wear spec in back and I am now close on the fronts. You can get both a rubber boot + grease kit and those clips from places like Rock Auto. The dealer price is a good amount more, and they are pretty generic underlying parts.

Not sure on whether you have to pull bracket or not to pull the rotor with the 4.2 size, but my guess is not (same as my W12). Not a big deal even if you had to. The front brackets are more of a pain with the fairly high bolt torques. The only real hassle in the rear is the VCDS work with the parking brake and resetting the pad thickness to new.

Panelhead

Mine were severely corroded, the ribs could be broken off by hand. But the lip was huge. The fronts about 1/8" and rears 1/16". This was outside, inner may have been more.
I would get a set of Brembo's. Less than 200.00 and will last the life of the car. Turn the fronts while in the shop.

mishar

Your rotors are way under the minimum (front A8 31 mm, S8 33 mm, rears 20 mm). Where are you going to buy Brembos and why do you think they will last the life of the car? Turning rotors is definitely not recommended and with price of new rotors has no sense at all.

Sci-fi_Wasabi

Thanks for the thorough response MP4.2. from what you are saying then maybe this rotor has seen two sets of pads... I'll take a closer look at the rotors and pads and look for any wear that may suggest that the rotor surfaces are no longer parallel, i.e. the thickness towards the hub is more than the thickness towards the outside, or that a singe pad may be worn more towards the center/hub side than the outside... if so I'll replace. Unfortunately my mic gauge is not deep enough to get a sampling of the thickness towards the inside. But if it has seen 2 sets of pads that means I'm averaging 0.75 mm thickness wear per pad replacement in the rear, which would bring me down to about where you were when you had your rotors changed (0.2mm less then min). Thanks