Seems like you’re perfectly clear on how to be sure a rotor needs to be turned...

Buy a Runout gauge.

Bonus: you get a new tool.

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My training

(not a mechanic)

Unless your brakes shudder it's probably unnecessary.
It would help get rid of any lip on the inner or outer edge of the contact area, but you can similarly speed up the mating process by just filing/sanding back the edges of the pads to suit.
Rotor turning is (properly) done with two cutting tools simultaneously to control the thickness, not just the runout on each side independently.

Thanks MattyK, that's kind of what I'm thinking. There is no indication of brake shudder and the rotors only have 40k on them, as long as they mic out for thickness I think I'm good to go.

A mini Cooper dealer quotes $1200 for brake job – they don't even turn the rotors they just replace them every time. That's how they sell their extended service plan, which would cover the brake job. Uh, no.

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I miss YaHey

Just did all 4 wheels on my wifes mini with a premium kit that included rotors, pad, sensors, clips etc. for I think $280. I never find it worth turning commonly found rotors. I can have it done for $15-$20 apiece at a local parts shop but it is almost never worth it unless you have something rare. I will say that turned or new rotors does make for a much better job but is not always needed.

To OP. Run your fingernail along the surface if there are any notches at all just replace the rotors in pairs.

First off the rotors need to be checked while torqued in position, there's equipment for that. Then, they should be turned while stil on the the car, again there's equipment for that. Also, I'm pretty sure you can replace the rotors with OEM quality rotors for the cost of turning. Also, as mentioned you need to check run-out, it's not enough to check minimum thickness. Back yard way of checking run out? Go for a drive on an empty road and brake hard. Outside of ABS vibration is your braking foot moving up & down? If yes, change rotors, if no, check pad thickness against new spec and replace if needed.

IIRR the rotors on the Mini are pretty thin to begin with, if there's much wear they just pitch them.... which is why I either do stuff myself or have an independent shop do the service.

One thing to think about - is there anyplace to turn rotors locally? I only have a few places that do it anymore. Lots of people have gone to rotors-as-maintenance items.

Ford put some weak pads on the Fiesta STs. Very dirty and don't last, but decent stopping power. My OE set lasted about 26k miles. I didn't change the rotors - even though the rotors and calipers were made by Lucas.

Like Tom races Miatas, I race an old BMW. Screw turning the rotors. Just put the new pads on and drive on.

Ask for your old pads to be given to you. I bet they aren't really at 2mm, because all modern cars have pad wear indicators built in to the design, so when a pad gets thin, you get a light on your dash. That light gives you plenty of notice. It makes me wonder if it's really "2mm until warning light" and if that's the case, I wouldn't bother changing them yet unless it's far more convenient to do it now rather than in a couple months.

Re. shuddering brakes. As above a solution to this "can be" turning the rotors, or even scuffing them with sandpaper. This issue causes confusion among enthusiasts because our father's all told us about "warped" brake rotors. In real life tho, rotors almost never warp. What happens is "uneven pad deposits" on the rotors. The fix is to go do some hard braking to clean up the "non-homogenous" invisibly thin layer of brake pad material on the rotors.

Hard braking means getting out on a lonely freeway and doing 3-4x 80mph to 10mph braking efforts in rapid succession. Don't come to a complete stop, brake as hard as you can w/o ABS engaging, and do them in rapid succession so your brakes don't get time to cool between sessions.

http://www.stoptech.com/...disc-and-other-myths

Re. OEM rotors. Get the cheapest rotors you can find. Enthusiasts get all goofy eyed over cryo treated slotted and drilled silliness and that's all a waste of money. Be strong. Don't give in to marketing foolishness. See if you can find rotors for <$20 ea.

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"If only he had used his genius for niceness, instead of Evil." M. Smart

I like clean brakes on cars that have open rims and you can see the caliper and rotor. If that’s the case, I always paint my calipers black and if the rotors are rusted to hell, just replace them with new rotors. Buy the rotors that are painted, they are only like $10 more. I wouldn’t be surprised if a new rotor for an ultima is only $40. I wouldn’t bother turning the, at that price.

If you can’t see the caliper and rotor and don’t care, I would just put pads in and call it a day.

On my last BMW, I put pads in the first time. Second time I replaced all rotors and pads. On the Porsche Cayenne, rotors checked out fine so I just stuffed in pads. Brakes work perfectly. On my Mazda CX-7 I’m driving now, I found all 4 painted rotors and pads for under $200. Just hilarious... at that price I went new for everything and wire brushed my calipers and hit them with black paint. I do all of my own work though, it’s a lot cheaper and for whatever reason, it’s a break from life’s daily stresses for me.

I don't replace the rotors unless they have gouges, are out of spec, super rusty or have a vibration. Most shops I've encountered price the turning whether its actually required or not.

My daughter drives an Altima, I did her brakes front/rear with new pads and rotors for a little over $100 from memory.

If you can do brakes, you're certainly going to save some $$$ over paying a shop.

 
I'm surprised you have already worn out the rear pads at 40k miles, typically the rear last quite a while compared to the front. Brake shops always try to sell a rotor turn job, I doubt they even do it most of the time and I highly doubt that you need it.

I just finally replaced the front rotors on my wife's BMW 330ci. Plenty of thickness and they still stopped great, but they had a significant lip going. Since it needed new pads I decided to just do the whole thing. But that was at 140k miles on the original FRONT rotors. The rears are still in great shape.

Thanks everyone for the responses and great info. This is what I was looking for. And thanks for the input from the race car guys about removing pad residue with the series of hard braking. I'd never heard that and it makes sense.

I was also surprised, A-A-Ron, that they need to be replaced in the rear first at only 40k. I'll take a good look at them once they're off. And it was good to hear others that say that they just turn the rotors automatically--whether needed or not. They can charge more that way, I guess. I even ran across a couple of articles that argued that if the rotor really doesn't need to be turned, they're removing perfectly good braking surface material.

It was also good to hear firsthand experiences of multiple pad changes without turning the rotors--that's what I'm hoping for.

I will be saving a ton of money. I found the pads at Napa for $28 and they wanted almost $300 at the repair shop.

Most car rotors (especially the front ones) are beefy such that they can last 60-80k miles before they need to be replaced. Unless you do a lot of heavy braking then it could be less. Rears will wear quicker because they are typically smaller than the fronts.

FYI at AutoZone, they will rent you tools where you pay a deposit that is refunded to you after you return it (so in essence doesn't cost you anything but time and gas to pick it up and return it). I also learned from the Autozone guy if I didn't return the tool then they'd just not return my deposit and then I'd own it instead. They would just order a new set for rentals when somebody doesn't return it and carry on like normal. At the time the rentals were $30 so I specifically told the AutoZone guy who rang up my tool rental that I'm not returning it and to just go and order a replacement right away (he didn't care or bother to pretend to try to stop me). This was for the specialty brake caliper piston tool.

They typically offer a good, better, best sort of thing with brake pads. Often the price difference between them is not great. I would recommend the best if the price isn't crazy. I bought the cheap pads once, between the noise, the dust, and the crappy brake feel, I ended up replacing them long before they were used up.

Thanks Ron, on the pad material. I'm just learning about that as well, so good stuff. Interestingly, the first place I stopped was Auto Zone and they had both the cheaper, I believe referred to as organic, and the more expensive ceramic pads.

Napa only carried ceramic for my model, and they were considerably cheaper than Auto Zone so it looks like Napa unless I can find ceramic pads cheaper than $28. Oh, and the ones at Napa came with the new hardware clips in a bag and the ones at Auto Zone didn't.

FYI ceramic pads while they do offer increased braking power are a harder substance and tend to squeal under low braking situations (like coming to a smooth stop). There really isn't much you can do about it either as no amount of "bedding in" technique has ever worked for me on one of my previous cars. They also eat up rotors quicker because they are a more abrasive material than organic pads. The initial bite of the ceramics is much "grippier" than most stock pads you've probably driven on.

Shop around, I once had a mechanic try to sell my wife (then my GF) some $90 ceramic brake pads for her Honda CRV (front brakes only). I told him no way it is that expensive as I could get for $80 better and bigger brake pads for my car (which had giant racing Brembo calipers 3x the size of my GF's Honda). I told him to go organic $40 pads instead as that was sufficient (those incompetent fools at that shop never checked the ABS sensor like I asked them and it was out of place tripping the ABS warning light). That was a recommendation of a co-worker of my wife (co-worker's uncle). Yeah, no thanks to those shoddy mechanics, I'm no expert mechanic but I do know my way around machinery well enough.