Good grief.
Turning the rotor trues the rotor (if necessary) AND provides the proper surface to bed in the new pads. It is how you get the best performance out of your brakes over the life of the pads.
If the rotor isn't warped or gouged and you just put in new pads, the pads will eventually wear to the rotors and create the surface they want and the braking will be fine. But in the time it takes to get to that point the brakes will not be as effective and that process will wear the pads faster. Not the end of the world but definitely not best practice or best for performance.
If you're not measuring the rotor thickness before you change the pads and run them, you're an idiot. Even reproduction rotors for these cars usually won't be in spec for a 2nd pad change. Not with any performance brake pads anyway.
Most shops just buy new rotors now because most new rotors are not thick enough to turn and run again. The rotors don't come with a ton of extra material on them, so if you've gone through a set of pads you've also worn down the rotor, and not a lot of them would remain in spec after you turned them if something is visibly wrong with the rotors. So they get replaced instead of turned.
If you know how to turn a rotor, you know you don't need to take much off. If the rotors are true and smooth you're not losing much thickness, just providing the proper surface. Using a sander on them reduces the thickness too, and won't necessarily provide a flat surface. If you're gonna blast them with a sander, roloc, etc you should just have them turned, you're not saving metal, just introducing the possibility of a non-flat surface.
If a set of rotors won't survive being turned and remain in spec, they're not true or flat anyway. If a rotor has visibly "deep" gouges or is warped enough to cause a vibration, it likely won't true up without going out of spec anymore.
Yes, I've absolutely changed my own brake pads without turning or replacing my rotors. No, I didn't die. No, it's not best practice. And yes, after that 2nd set of pads wore out the rotors were out of spec, so they didn't last any longer than if I'd turned them at the previous change.
What a bunch of over cautious Nancys here.
No, I don't turn a rotor or drum unless they are scored, grooved, went metal to metal or had a shimmy to them.
Each time you resurface a drum or rotor, you shorten it's life and leave LESS metal to absorb heat which accelerates the rate at which they develop warping issues.
Rick Ehrenberg, the Mopar Action tech editor is one of the most meticulous, do it by the book types you'd ever meet and even HE advises against resurfacing anything until it meets the criteria I listed above.
If a rotor is visibly scored, grooved, went metal on metal or has a vibration they're most likely
junk, and need replaced instead of turned anyway. Most modern rotors aren't thick enough anymore to clean up with obvious damage like that. Maybe 50 years ago, not anymore.
Any rotor within spec should work just fine. That's why there's a spec, some poor bastard actually had to test that. Nowadays if you're able to turn a set of rotors once you're doing good, they wear down with use so it's not like they last forever if you don't turn them. Realistically if you've changed the pads once the rotor will be out of spec when the next set of pads needs to go on regardless of them being turned or not.
Ehrenberg is also a cheap purist. He doesn't advise turning rotors because he doesn't want to buy a new set and when he wrote that article the rotors/drums he was using probably weren't widely reproduced. It's the same argument from people that use less aggressive brake pads. That's fine if you care more about not buying a new set of rotors than how well your car stops. People that actually
drive their cars should care more about how well they stop than about saving their rotors.
I'd rather chew up a set of rotors with aggressive pads and stop better than make my rotors last forever. Rotors are wearing parts, stopping is more important than rotor longevity. A set of rotors is a lot cheaper than new bodywork.
There are some real cautious, scared of their own shadow types that will disagree but I'd bet a weeks pay that they were the types that also believed the "science" in other matters.
Anyone that puts science in quotes is just demonstrating they don't know enough science to have an informed opinion on the topic. Clearly that's the case with you on this one. And probably lots of others.
